text:
perlfunc - Perl builtin functions
The functions in this section can serve as terms in an expression. They fall into two major categories: list operators and named unary operators. These differ in their precedence relationship with a following comma. (See the precedence table in perlop.) List operators take more than one argument, while unary operators can never take more than one argument. Thus, a comma terminates the argument of a unary operator, but merely separates the arguments of a list operator. A unary operator generally provides scalar context to its argument, while a list operator may provide either scalar or list contexts for its arguments. If it does both, scalar arguments come first and list argument follow, and there can only ever be one such list argument. For instance, splice
has three scalar arguments followed by a list, whereas gethostbyname
has four scalar arguments.
In the syntax descriptions that follow, list operators that expect a list (and provide list context for elements of the list) are shown with LIST as an argument. Such a list may consist of any combination of scalar arguments or list values; the list values will be included in the list as if each individual element were interpolated at that point in the list, forming a longer single-dimensional list value. Commas should separate literal elements of the LIST.
Any function in the list below may be used either with or without parentheses around its arguments. (The syntax descriptions omit the parentheses.) If you use parentheses, the simple but occasionally surprising rule is this: It looks like a function, therefore it is a function, and precedence doesn't matter. Otherwise it's a list operator or unary operator, and precedence does matter. Whitespace between the function and left parenthesis doesn't count, so sometimes you need to be careful:
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
print (1+2)+4; # Also prints 3!
print +(1+2)+4; # Prints 7.
print ((1+2)+4); # Prints 7.
If you run Perl with the use warnings
pragma, it can warn you about this. For example, the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
A few functions take no arguments at all, and therefore work as neither unary nor list operators. These include such functions as time
and endpwent
. For example, time+86_400
always means time() + 86_400
For functions that can be used in either a scalar or list context, nonabortive failure is generally indicated in scalar context by returning the undefined value, and in list context by returning the empty list.
Remember the following important rule: There is no rule that relates the behavior of an expression in list context to its behavior in scalar context, or vice versa. It might do two totally different things. Each operator and function decides which sort of value would be most appropriate to return in scalar context. Some operators return the length of the list that would have been returned in list context. Some operators return the first value in the list. Some operators return the last value in the list. Some operators return a count of successful operations. In general, they do what you want, unless you want consistency.
A named array in scalar context is quite different from what would at first glance appear to be a list in scalar context. You can't get a list like (1,2,3)
into being in scalar context, because the compiler knows the context at compile time. It would generate the scalar comma operator there, not the list concatenation version of the comma. That means it was never a list to start with.
In general, functions in Perl that serve as wrappers for system calls ("syscalls") of the same name (like chown(2), fork(2), closedir(2), etc.) return true when they succeed and undef
otherwise, as is usually mentioned in the descriptions below. This is different from the C interfaces, which return -1
on failure. Exceptions to this rule include wait
, waitpid
, and syscall
. System calls also set the special $!
variable on failure. Other functions do not, except accidentally.
Extension modules can also hook into the Perl parser to define new kinds of keyword-headed expression. These may look like functions, but may also look completely different. The syntax following the keyword is defined entirely by the extension. If you are an implementor, see "PL_keyword_plugin" in perlapi for the mechanism. If you are using such a module, see the module's documentation for details of the syntax that it defines.
Perl Functions by Category
Here are Perl's functions (including things that look like functions, like some keywords and named operators) arranged by category. Some functions appear in more than one place. Any warnings, including those produced by keywords, are described in perldiag and warnings.
- Functions for SCALARs or strings
is available only if the"fc"
feature is enabled or if it is prefixed withCORE::
. The"fc"
feature is enabled automatically with ause v5.16
(or higher) declaration in the current scope. - Regular expressions and pattern matching
- Numeric functions
- Functions for real @ARRAYs
- Functions for list data
- Functions for real %HASHes
- Input and output functions
is available only if the"say"
feature is enabled or if it is prefixed withCORE::
. The"say"
feature is enabled automatically with ause v5.10
(or higher) declaration in the current scope. - Functions for fixed-length data or records
- Functions for filehandles, files, or directories
is available only if you enable the experimental"switch"
feature or use theCORE::
prefix. The"switch"
feature also enables thedefault
statements, which are documented in "Switch Statements" in perlsyn. The"switch"
feature is enabled automatically with ause v5.10
(or higher) declaration in the current scope. In Perl v5.14 and earlier,continue
required the"switch"
feature, like the other keywords.evalbytes
is only available with the"evalbytes"
feature (see feature) or if prefixed withCORE::
is only available with the"current_sub"
feature or if prefixed withCORE::
. Both the"evalbytes"
features are enabled automatically with ause v5.16
(or higher) declaration in the current scope. -
is available only if the"state"
feature is enabled or if it is prefixed withCORE::
. The"state"
feature is enabled automatically with ause v5.10
(or higher) declaration in the current scope. - Miscellaneous functions
- Functions for processes and process groups
- Low-level socket functions
- System V interprocess communication functions
- Fetching user and group info
- Fetching network info
- Non-function keywords
Perl was born in Unix and can therefore access all common Unix system calls. In non-Unix environments, the functionality of some Unix system calls may not be available or details of the available functionality may differ slightly. The Perl functions affected by this are:
, binmode
, chmod
, chown
, chroot
, crypt
, dbmclose
, dbmopen
, dump
, endgrent
, endhostent
, endnetent
, endprotoent
, endpwent
, endservent
, exec
, fcntl
, flock
, fork
, getgrent
, getgrgid
, gethostbyname
, gethostent
, getlogin
, getnetbyaddr
, getnetbyname
, getnetent
, getppid
, getpgrp
, getpriority
, getprotobynumber
, getprotoent
, getpwent
, getpwnam
, getpwuid
, getservbyport
, getservent
, getsockopt
, glob
, ioctl
, kill
, link
, lstat
, msgctl
, msgget
, msgrcv
, msgsnd
, open
, pipe
, readlink
, rename
, select
, semctl
, semget
, semop
, setgrent
, sethostent
, setnetent
, setpgrp
, setpriority
, setprotoent
, setpwent
, setservent
, setsockopt
, shmctl
, shmget
, shmread
, shmwrite
, socket
, socketpair
, stat
, symlink
, syscall
, sysopen
, system
, times
, truncate
, umask
, unlink
, utime
, wait
, waitpid
For more information about the portability of these functions, see perlport and other available platform-specific documentation.
Alphabetical Listing of Perl Functions
A file test, where X is one of the letters listed below. This unary operator takes one argument, either a filename, a filehandle, or a dirhandle, and tests the associated file to see if something is true about it. If the argument is omitted, tests
, except for-t
, which tests STDIN. Unless otherwise documented, it returns1
for true and''
for false. If the file doesn't exist or can't be examined, it returnsundef
and sets$!
(errno). With the exception of the-l
test they all follow symbolic links because they usestat()
and notlstat()
(so dangling symlinks can't be examined and will therefore report failure).Despite the funny names, precedence is the same as any other named unary operator. The operator may be any of:
-r File is readable by effective uid/gid. -w File is writable by effective uid/gid. -x File is executable by effective uid/gid. -o File is owned by effective uid. -R File is readable by real uid/gid. -W File is writable by real uid/gid. -X File is executable by real uid/gid. -O File is owned by real uid. -e File exists. -z File has zero size (is empty). -s File has nonzero size (returns size in bytes). -f File is a plain file. -d File is a directory. -l File is a symbolic link (false if symlinks aren't supported by the file system). -p File is a named pipe (FIFO), or Filehandle is a pipe. -S File is a socket. -b File is a block special file. -c File is a character special file. -t Filehandle is opened to a tty. -u File has setuid bit set. -g File has setgid bit set. -k File has sticky bit set. -T File is an ASCII or UTF-8 text file (heuristic guess). -B File is a "binary" file (opposite of -T). -M Script start time minus file modification time, in days. -A Same for access time. -C Same for inode change time (Unix, may differ for other platforms)
while (<>) { chomp; next unless -f $_; # ignore specials #... }
Note that
does not do a negated substitution. Saying-exp($foo)
still works as expected, however: only single letters following a minus are interpreted as file tests.These operators are exempt from the "looks like a function rule" described above. That is, an opening parenthesis after the operator does not affect how much of the following code constitutes the argument. Put the opening parentheses before the operator to separate it from code that follows (this applies only to operators with higher precedence than unary operators, of course):
-s($file) + 1024 # probably wrong; same as -s($file + 1024) (-s $file) + 1024 # correct
The interpretation of the file permission operators
, and-X
is by default based solely on the mode of the file and the uids and gids of the user. There may be other reasons you can't actually read, write, or execute the file: for example network filesystem access controls, ACLs (access control lists), read-only filesystems, and unrecognized executable formats. Note that the use of these six specific operators to verify if some operation is possible is usually a mistake, because it may be open to race conditions.Also note that, for the superuser on the local filesystems, the
, and-W
tests always return 1, and-x
return 1 if any execute bit is set in the mode. Scripts run by the superuser may thus need to do astat
to determine the actual mode of the file, or temporarily set their effective uid to something else.If you are using ACLs, there is a pragma called
that may produce more accurate results than the barestat
mode bits. When underuse filetest 'access'
, the above-mentioned filetests test whether the permission can(not) be granted using the access(2) family of system calls. Also note that the-x
tests may under this pragma return true even if there are no execute permission bits set (nor any extra execute permission ACLs). This strangeness is due to the underlying system calls' definitions. Note also that, due to the implementation ofuse filetest 'access'
, the_
special filehandle won't cache the results of the file tests when this pragma is in effect. Read the documentation for thefiletest
pragma for more information.The
tests work as follows. The first block or so of the file is examined to see if it is valid UTF-8 that includes non-ASCII characters. If so, it's a-T
file. Otherwise, that same portion of the file is examined for odd characters such as strange control codes or characters with the high bit set. If more than a third of the characters are strange, it's a-B
file; otherwise it's a-T
file. Also, any file containing a zero byte in the examined portion is considered a binary file. (If executed within the scope of a use locale which includesLC_CTYPE
, odd characters are anything that isn't a printable nor space in the current locale.) If-T
is used on a filehandle, the current IO buffer is examined rather than the first block. Both-T
return true on an empty file, or a file at EOF when testing a filehandle. Because you have to read a file to do the-T
test, on most occasions you want to use a-f
against the file first, as innext unless -f $file && -T $file
.If any of the file tests (or either the
operator) is given the special filehandle consisting of a solitary underline, then the stat structure of the previous file test (orstat
operator) is used, saving a system call. (This doesn't work with-t
, and you need to remember thatlstat
leave values in the stat structure for the symbolic link, not the real file.) (Also, if the stat buffer was filled by anlstat
will reset it with the results ofstat _
). Example:print "Can do.\n" if -r $x || -w _ || -x _; stat($filename); print "Readable\n" if -r _; print "Writable\n" if -w _; print "Executable\n" if -x _; print "Setuid\n" if -u _; print "Setgid\n" if -g _; print "Sticky\n" if -k _; print "Text\n" if -T _; print "Binary\n" if -B _;
As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file test operators, in a way that
-f -w -x $file
is equivalent to-x $file && -w _ && -f _
. (This is only fancy syntax: if you use the return value of-f $file
as an argument to another filetest operator, no special magic will happen.)Portability issues: "-X" in perlport.
To avoid confusing would-be users of your code with mysterious syntax errors, put something like this at the top of your script:
use v5.10; # so filetest ops can stack
- abs VALUE
- abs
Returns the absolute value of its argument. If VALUE is omitted, uses
. - accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as accept(2) does. Returns the packed address if it succeeded, false otherwise. See the example in "Sockets: Client/Server Communication" in perlipc.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of
. See "$^F" in perlvar. - alarm SECONDS
- alarm
Arranges to have a SIGALRM delivered to this process after the specified number of wallclock seconds has elapsed. If SECONDS is not specified, the value stored in
is used. (On some machines, unfortunately, the elapsed time may be up to one second less or more than you specified because of how seconds are counted, and process scheduling may delay the delivery of the signal even further.)Only one timer may be counting at once. Each call disables the previous timer, and an argument of
may be supplied to cancel the previous timer without starting a new one. The returned value is the amount of time remaining on the previous timer.For delays of finer granularity than one second, the Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard distribution) provides
. You may also use Perl's four-argument version ofselect
leaving the first three arguments undefined, or you might be able to use thesyscall
interface to access setitimer(2) if your system supports it. See perlfaq8 for details.It is usually a mistake to intermix
calls, becausesleep
may be internally implemented on your system withalarm
.If you want to use
to time out a system call you need to use aneval
pair. You can't rely on the alarm causing the system call to fail with$!
set toEINTR
because Perl sets up signal handlers to restart system calls on some systems. Usingeval
always works, modulo the caveats given in "Signals" in perlipc.eval { local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required alarm $timeout; my $nread = sysread $socket, $buffer, $size; alarm 0; }; if ($@) { die unless $@ eq "alarm\n"; # propagate unexpected errors # timed out } else { # didn't }
For more information see perlipc.
Portability issues: "alarm" in perlport.
- atan2 Y,X
Returns the arctangent of Y/X in the range -PI to PI.
For the tangent operation, you may use the
function, or use the familiar relation:sub tan { sin($_[0]) / cos($_[0]) }
The return value for
is implementation-defined; consult your atan2(3) manpage for more information.Portability issues: "atan2" in perlport.
- bind SOCKET,NAME
Binds a network address to a socket, just as bind(2) does. Returns true if it succeeded, false otherwise. NAME should be a packed address of the appropriate type for the socket. See the examples in "Sockets: Client/Server Communication" in perlipc.
- binmode FILEHANDLE, LAYER
- binmode FILEHANDLE
Arranges for FILEHANDLE to be read or written in "binary" or "text" mode on systems where the run-time libraries distinguish between binary and text files. If FILEHANDLE is an expression, the value is taken as the name of the filehandle. Returns true on success, otherwise it returns
and sets$!
(errno).On some systems (in general, DOS- and Windows-based systems)
is necessary when you're not working with a text file. For the sake of portability it is a good idea always to use it when appropriate, and never to use it when it isn't appropriate. Also, people can set their I/O to be by default UTF8-encoded Unicode, not bytes.In other words: regardless of platform, use
on binary data, like images, for example.If LAYER is present it is a single string, but may contain multiple directives. The directives alter the behaviour of the filehandle. When LAYER is present, using binmode on a text file makes sense.
If LAYER is omitted or specified as
the filehandle is made suitable for passing binary data. This includes turning off possible CRLF translation and marking it as bytes (as opposed to Unicode characters). Note that, despite what may be implied in "Programming Perl" (the Camel, 3rd edition) or elsewhere,:raw
is not simply the inverse of:crlf
. Other layers that would affect the binary nature of the stream are also disabled. See PerlIO, and the discussion about the PERLIO environment variable in perlrun.The
, and any other directives of the form:...
, are called I/O layers. The open pragma can be used to establish default I/O layers.The LAYER parameter of the
function is described as "DISCIPLINE" in "Programming Perl, 3rd Edition". However, since the publishing of this book, by many known as "Camel III", the consensus of the naming of this functionality has moved from "discipline" to "layer". All documentation of this version of Perl therefore refers to "layers" rather than to "disciplines". Now back to the regularly scheduled documentation...To mark FILEHANDLE as UTF-8, use
just marks the data as UTF-8 without further checking, while:encoding(UTF-8)
checks the data for actually being valid UTF-8. More details can be found in PerlIO::encoding.In general,
should be called afteropen
but before any I/O is done on the filehandle. Callingbinmode
normally flushes any pending buffered output data (and perhaps pending input data) on the handle. An exception to this is the:encoding
layer that changes the default character encoding of the handle. The:encoding
layer sometimes needs to be called in mid-stream, and it doesn't flush the stream.:encoding
also implicitly pushes on top of itself the:utf8
layer because internally Perl operates on UTF8-encoded Unicode characters.The operating system, device drivers, C libraries, and Perl run-time system all conspire to let the programmer treat a single character (
) as the line terminator, irrespective of external representation. On many operating systems, the native text file representation matches the internal representation, but on some platforms the external representation of\n
is made up of more than one character.All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use a single character to end each line in the external representation of text (even though that single character is CARRIAGE RETURN on old, pre-Darwin flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other systems like OS/2, DOS, and the various flavors of MS-Windows, your program sees a
as a simple\cJ
, but what's stored in text files are the two characters\cM\cJ
. That means that if you don't usebinmode
on these systems,\cM\cJ
sequences on disk will be converted to\n
on input, and any\n
in your program will be converted back to\cM\cJ
on output. This is what you want for text files, but it can be disastrous for binary files.Another consequence of using
(on some systems) is that special end-of-file markers will be seen as part of the data stream. For systems from the Microsoft family this means that, if your binary data contain\cZ
, the I/O subsystem will regard it as the end of the file, unless you usebinmode
is important not only forreadline
operations, but also when usingread
(see perlport for more details). See the$/
variables in perlvar for how to manually set your input and output line-termination sequences.Portability issues: "binmode" in perlport.
- bless REF,CLASSNAME
- bless REF
tells Perl to mark the item referred to byREF
as an object in a package. The two-argument version ofbless
is always preferable unless there is a specific reason to not use it.Bless the referred-to item into a specific package (recommended form):
bless $ref, $package;
The two-argument form adds the object to the package specified as the second argument.
Bless the referred-to item into package
:bless $ref, "";
If the second argument is an empty string,
adds the object to packagemain
.Bless the referred-to item into the current package (not inheritable):
bless $ref;
is used without its second argument, the object is created in the current package. The second argument should always be supplied if a derived class might inherit a method executingbless
. Because it is a potential source of bugs, one-argumentbless
is discouraged.
See perlobj for more about the blessing (and blessings) of objects.
returns its first argument, the supplied reference, as the value of the function; sincebless
is commonly the last thing executed in constructors, this means that the reference to the object is returned as the constructor's value and allows the caller to immediately use this returned object in method calls.CLASSNAME
should always be a mixed-case name, as all-uppercase and all-lowercase names are meant to be used only for Perl builtin types and pragmas, respectively. Avoid creating all-uppercase or all-lowercase package names to prevent confusion.Also avoid
ing things into the class name0
; this will cause code which (erroneously) checks the result ofref
to see if a reference isbless
ed to fail, as "0", a falsy value, is returned.See "Perl Modules" in perlmod for more details.
- break
Break out of a
is available only if the"switch"
feature is enabled or if it is prefixed withCORE::
. The"switch"
feature is enabled automatically with ause v5.10
(or higher) declaration in the current scope. - caller EXPR
- caller
Returns the context of the current pure perl subroutine call. In scalar context, returns the caller's package name if there is a caller (that is, if we're in a subroutine or
) and the undefined value otherwise.caller
never returns XS subs and they are skipped. The next pure perl sub will appear instead of the XS sub in caller's return values. In list context, caller returns# 0 1 2 my ($package, $filename, $line) = caller;
, the filename and line number returned here may be altered by the mechanism described at "Plain Old Comments (Not!)" in perlsyn.With EXPR, it returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call frames to go back before the current one.
# 0 1 2 3 4 my ($package, $filename, $line, $subroutine, $hasargs, # 5 6 7 8 9 10 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash) = caller($i);
Here, $subroutine is the function that the caller called (rather than the function containing the caller). Note that $subroutine may be
if the frame is not a subroutine call, but aneval
. In such a case additional elements $evaltext and$is_require
are set:$is_require
is true if the frame is created by arequire
statement, $evaltext contains the text of theeval EXPR
statement. In particular, for aneval BLOCK
statement, $subroutine is(eval)
, but $evaltext is undefined. (Note also that eachuse
statement creates arequire
frame inside aneval EXPR
frame.) $subroutine may also be(unknown)
if this particular subroutine happens to have been deleted from the symbol table.$hasargs
is true if a new instance of@_
was set up for the frame.$hints
contain pragmatic hints that the caller was compiled with.$hints
corresponds to$^H
, and$bitmask
corresponds to${^WARNING_BITS}
. The$hints
values are subject to change between versions of Perl, and are not meant for external use.$hinthash
is a reference to a hash containing the value of%^H
when the caller was compiled, orundef
was empty. Do not modify the values of this hash, as they are the actual values stored in the optree.Note that the only types of call frames that are visible are subroutine calls and
. Other forms of context, such aswhile
loops ortry
blocks are not considered interesting tocaller
, as they do not alter the behaviour of thereturn
expression.Furthermore, when called from within the DB package in list context, and with an argument, caller returns more detailed information: it sets the list variable
to be the arguments with which the subroutine was invoked.Be aware that the optimizer might have optimized call frames away before
had a chance to get the information. That means thatcaller(N)
might not return information about the call frame you expect it to, forN > 1
. In particular,@DB::args
might have information from the previous timecaller
was called.Be aware that setting
is best effort, intended for debugging or generating backtraces, and should not be relied upon. In particular, as@_
contains aliases to the caller's arguments, Perl does not take a copy of@_
, so@DB::args
will contain modifications the subroutine makes to@_
or its contents, not the original values at call time.@DB::args
, like@_
, does not hold explicit references to its elements, so under certain cases its elements may have become freed and reallocated for other variables or temporary values. Finally, a side effect of the current implementation is that the effects ofshift @_
can normally be undone (but notpop @_
or other splicing, and not if a reference to@_
has been taken, and subject to the caveat about reallocated elements), so@DB::args
is actually a hybrid of the current state and initial state of@_
. Buyer beware. - chdir EXPR
- chdir FILEHANDLE
- chdir DIRHANDLE
- chdir
Changes the working directory to EXPR, if possible. If EXPR is omitted, changes to the directory specified by
, if set; if not, changes to the directory specified by$ENV{LOGDIR}
. (Under VMS, the variable$ENV{'SYS$LOGIN'}
is also checked, and used if it is set.) If neither is set,chdir
does nothing and fails. It returns true on success, false otherwise. See the example underdie
.On systems that support fchdir(2), you may pass a filehandle or directory handle as the argument. On systems that don't support fchdir(2), passing handles raises an exception.
- chmod LIST
Changes the permissions of a list of files. The first element of the list must be the numeric mode, which should probably be an octal number, and which definitely should not be a string of octal digits:
is okay, but"0644"
is not. Returns the number of files successfully changed. See alsooct
if all you have is a string.my $cnt = chmod 0755, "foo", "bar"; chmod 0755, @executables; my $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to # --w----r-T my $mode = "0644"; chmod oct($mode), "foo"; # this is better my $mode = 0644; chmod $mode, "foo"; # this is best
On systems that support fchmod(2), you may pass filehandles among the files. On systems that don't support fchmod(2), passing filehandles raises an exception. Filehandles must be passed as globs or glob references to be recognized; barewords are considered filenames.
open(my $fh, "<", "foo"); my $perm = (stat $fh)[2] & 07777; chmod($perm | 0600, $fh);
You can also import the symbolic
constants from theFcntl
module:use Fcntl qw( :mode ); chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; # Identical to the chmod 0755 of the example above.
Portability issues: "chmod" in perlport.
- chomp VARIABLE
- chomp( LIST )
- chomp
This safer version of
removes any trailing string that corresponds to the current value of$/
(also known as$INPUT_RECORD_SEPARATOR
in theEnglish
module). It returns the total number of characters removed from all its arguments. It's often used to remove the newline from the end of an input record when you're worried that the final record may be missing its newline. When in paragraph mode ($/ = ''
), it removes all trailing newlines from the string. When in slurp mode ($/ = undef
) or fixed-length record mode ($/
is a reference to an integer or the like; see perlvar),chomp
won't remove anything. If VARIABLE is omitted, it chomps$_
. Example:while (<>) { chomp; # avoid \n on last field my @array = split(/:/); # ... }
If VARIABLE is a hash, it chomps the hash's values, but not its keys, resetting the
iterator in the process.You can actually chomp anything that's an lvalue, including an assignment:
chomp(my $cwd = `pwd`); chomp(my $answer = <STDIN>);
If you chomp a list, each element is chomped, and the total number of characters removed is returned.
Note that parentheses are necessary when you're chomping anything that is not a simple variable. This is because
chomp $cwd = `pwd`;
is interpreted as(chomp $cwd) = `pwd`;
, rather than aschomp( $cwd = `pwd` )
which you might expect. Similarly,chomp $x, $y
is interpreted aschomp($x), $y
rather than aschomp($x, $y)
. - chop VARIABLE
- chop( LIST )
- chop
Chops off the last character of a string and returns the character chopped. It is much more efficient than
because it neither scans nor copies the string. If VARIABLE is omitted, chops$_
. If VARIABLE is a hash, it chops the hash's values, but not its keys, resetting theeach
iterator in the process.You can actually chop anything that's an lvalue, including an assignment.
If you chop a list, each element is chopped. Only the value of the last
is returned.Note that
returns the last character. To return all but the last character, usesubstr($string, 0, -1)
.See also
. - chown LIST
Changes the owner (and group) of a list of files. The first two elements of the list must be the numeric uid and gid, in that order. A value of -1 in either position is interpreted by most systems to leave that value unchanged. Returns the number of files successfully changed.
my $cnt = chown $uid, $gid, 'foo', 'bar'; chown $uid, $gid, @filenames;
On systems that support fchown(2), you may pass filehandles among the files. On systems that don't support fchown(2), passing filehandles raises an exception. Filehandles must be passed as globs or glob references to be recognized; barewords are considered filenames.
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: "; chomp(my $user = <STDIN>); print "Files: "; chomp(my $pattern = <STDIN>); my ($login,$pass,$uid,$gid) = getpwnam($user) or die "$user not in passwd file"; my @ary = glob($pattern); # expand filenames chown $uid, $gid, @ary;
On most systems, you are not allowed to change the ownership of the file unless you're the superuser, although you should be able to change the group to any of your secondary groups. On insecure systems, these restrictions may be relaxed, but this is not a portable assumption. On POSIX systems, you can detect this condition this way:
use POSIX qw(pathconf _PC_CHOWN_RESTRICTED); my $can_chown_giveaway = ! pathconf($path_of_interest, _PC_CHOWN_RESTRICTED);
Portability issues: "chown" in perlport.
- chr NUMBER
- chr
Returns the character represented by that NUMBER in the character set. For example,
in either ASCII or Unicode, andchr(0x263a)
is a Unicode smiley face.Negative values give the Unicode replacement character (
), except under the bytes pragma, where the low eight bits of the value (truncated to an integer) are used.If NUMBER is omitted, uses
.For the reverse, use
.Note that characters from 128 to 255 (inclusive) are by default internally not encoded as UTF-8 for backward compatibility reasons.
See perlunicode for more about Unicode.
- chroot FILENAME
- chroot
This function works like the system call by the same name: it makes the named directory the new root directory for all further pathnames that begin with a
by your process and all its children. (It doesn't change your current working directory, which is unaffected.) For security reasons, this call is restricted to the superuser. If FILENAME is omitted, does achroot
.NOTE: It is mandatory for security to
to the root directory) immediately after achroot
, otherwise the current working directory may be outside of the new root.Portability issues: "chroot" in perlport.
- class NAMESPACE
- class NAMESPACE VERSION
- class NAMESPACE BLOCK
- class NAMESPACE VERSION BLOCK
Declares the BLOCK or the rest of the compilation unit as being in the given namespace, which implements an object class. This behaves similarly to
, except that the newly-created package behaves as a class. - close FILEHANDLE
- close
Closes the file or pipe associated with the filehandle, flushes the IO buffers, and closes the system file descriptor. Returns true if those operations succeed, and if no error was reported by any PerlIO layer, and there was no existing error on the filehandle.
If there was an existing error on the filehandle, close will return false and
will be set to the error from the failing operation, so you can safely use its value when reporting the error.Closes the currently selected filehandle if the argument is omitted.
You don't have to close FILEHANDLE if you are immediately going to do another
on it, becauseopen
closes it for you. (Seeopen
.) However, an explicitclose
on an input file resets the line counter ($.
), while the implicit close done byopen
does not.If the filehandle came from a piped open,
returns false if one of the other syscalls involved fails or if its program exits with non-zero status. If the only problem was that the program exited non-zero,$!
will be set to0
. Closing a pipe also waits for the process executing on the pipe to exit--in case you wish to look at the output of the pipe afterwards--and implicitly puts the exit status value of that command into$?
.If there are multiple threads running,
on a filehandle from a piped open returns true without waiting for the child process to terminate, if the filehandle is still open in another thread.Closing the read end of a pipe before the process writing to it at the other end is done writing results in the writer receiving a SIGPIPE. If the other end can't handle that, be sure to read all the data before closing the pipe.
open(OUTPUT, '|sort >foo') # pipe to sort or die "Can't start sort: $!"; #... # print stuff to output close OUTPUT # wait for sort to finish or warn $! ? "Error closing sort pipe: $!" : "Exit status $? from sort"; open(INPUT, 'foo') # get sort's results or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value can be used as an indirect filehandle, usually the real filehandle name or an autovivified handle.
If an error occurs when perl implicitly closes a handle, perl will produce a warning. Explicitly calling close on the handle prevents that warning.
- closedir DIRHANDLE
Closes a directory opened by
and returns the success of that system call. - connect SOCKET,NAME
Attempts to connect to a remote socket, just like connect(2). Returns true if it succeeded, false otherwise. NAME should be a packed address of the appropriate type for the socket. See the examples in "Sockets: Client/Server Communication" in perlipc.
- continue BLOCK
- continue
When followed by a BLOCK,
is actually a flow control statement rather than a function. If there is acontinue
BLOCK attached to a BLOCK (typically in awhile
), it is always executed just before the conditional is about to be evaluated again, just like the third part of afor
loop in C. Thus it can be used to increment a loop variable, even when the loop has been continued via thenext
statement (which is similar to the Ccontinue
, orredo
may appear within acontinue
behave as if they had been executed within the main block. So willnext
, but since it will execute acontinue
block, it may be more entertaining.while (EXPR) { ### redo always comes here do_something; } continue { ### next always comes here do_something_else; # then back to the top to re-check EXPR } ### last always comes here
Omitting the
section is equivalent to using an empty one, logically enough, sonext
goes directly back to check the condition at the top of the loop.When there is no BLOCK,
is a function that falls through the currentwhen
block instead of iterating a dynamically enclosingforeach
or exiting a lexically enclosinggiven
. In Perl 5.14 and earlier, this form ofcontinue
was only available when the"switch"
feature was enabled. See feature and "Switch Statements" in perlsyn for more information. - cos EXPR
- cos
Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, takes the cosine of
.For the inverse cosine operation, you may use the
function, or use this relation:sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
- crypt PLAINTEXT,SALT
Creates a digest string exactly like the crypt(3) function in the C library (assuming that you actually have a version there that has not been extirpated as a potential munition).
is a one-way hash function. The PLAINTEXT and SALT are turned into a short string, called a digest, which is returned. The same PLAINTEXT and SALT will always return the same string, but there is no (known) way to get the original PLAINTEXT from the hash. Small changes in the PLAINTEXT or SALT will result in large changes in the digest.There is no decrypt function. This function isn't all that useful for cryptography (for that, look for Crypt modules on your nearby CPAN mirror) and the name "crypt" is a bit of a misnomer. Instead it is primarily used to check if two pieces of text are the same without having to transmit or store the text itself. An example is checking if a correct password is given. The digest of the password is stored, not the password itself. The user types in a password that is
'd with the same salt as the stored digest. If the two digests match, the password is correct.When verifying an existing digest string you should use the digest as the salt (like
crypt($plain, $digest) eq $digest
). The SALT used to create the digest is visible as part of the digest. This ensurescrypt
will hash the new string with the same salt as the digest. This allows your code to work with the standardcrypt
and with more exotic implementations. In other words, assume nothing about the returned string itself nor about how many bytes of SALT may matter.Traditionally the result is a string of 13 bytes: two first bytes of the salt, followed by 11 bytes from the set
, and only the first eight bytes of PLAINTEXT mattered. But alternative hashing schemes (like MD5), higher level security schemes (like C2), and implementations on non-Unix platforms may produce different strings.When choosing a new salt create a random two character string whose characters come from the set
(likejoin '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]
). This set of characters is just a recommendation; the characters allowed in the salt depend solely on your system's crypt library, and Perl can't restrict what saltscrypt
accepts.Here's an example that makes sure that whoever runs this program knows their password:
my $pwd = (getpwuid($<))[1]; system "stty -echo"; print "Password: "; chomp(my $word = <STDIN>); print "\n"; system "stty echo"; if (crypt($word, $pwd) ne $pwd) { die "Sorry...\n"; } else { print "ok\n"; }
Of course, typing in your own password to whoever asks you for it is unwise.
function is unsuitable for hashing large quantities of data, not least of all because you can't get the information back. Look at the Digest module for more robust algorithms.If using
on a Unicode string (which potentially has characters with codepoints above 255), Perl tries to make sense of the situation by trying to downgrade (a copy of) the string back to an eight-bit byte string before callingcrypt
(on that copy). If that works, good. If not,crypt
dies withWide character in crypt
.Portability issues: "crypt" in perlport.
- dbmclose HASH
[This function has been largely superseded by the
function.]Breaks the binding between a DBM file and a hash.
Portability issues: "dbmclose" in perlport.
- dbmopen HASH,DBNAME,MASK
[This function has been largely superseded by the
function.]This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a hash. HASH is the name of the hash. (Unlike normal
, the first argument is not a filehandle, even though it looks like one). DBNAME is the name of the database (without the .dir or .pag extension if any). If the database does not exist, it is created with protection specified by MASK (as modified by theumask
). To prevent creation of the database if it doesn't exist, you may specify a MASK of 0, and the function will return a false value if it can't find an existing database. If your system supports only the older DBM functions, you may make only onedbmopen
call in your program. In older versions of Perl, if your system had neither DBM nor ndbm, callingdbmopen
produced a fatal error; it now falls back to sdbm(3).If you don't have write access to the DBM file, you can only read hash variables, not set them. If you want to test whether you can write, either use file tests or try setting a dummy hash entry inside an
to trap the error.Note that functions such as
may return huge lists when used on large DBM files. You may prefer to use theeach
function to iterate over large DBM files. Example:# print out history file offsets dbmopen(%HIST,'/usr/lib/news/history',0666); while (($key,$val) = each %HIST) { print $key, ' = ', unpack('L',$val), "\n"; } dbmclose(%HIST);
See also AnyDBM_File for a more general description of the pros and cons of the various dbm approaches, as well as DB_File for a particularly rich implementation.
You can control which DBM library you use by loading that library before you call
:use DB_File; dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") or die "Can't open netscape history file: $!";
Portability issues: "dbmopen" in perlport.
- defined EXPR
- defined
Returns a Boolean value telling whether EXPR has a value other than the undefined value
. If EXPR is not present,$_
is checked.Many operations return
to indicate failure, end of file, system error, uninitialized variable, and other exceptional conditions. This function allows you to distinguishundef
from other values. (A simple Boolean test will not distinguish amongundef
, zero, the empty string, and"0"
, which are all equally false.) Note that sinceundef
is a valid scalar, its presence doesn't necessarily indicate an exceptional condition:pop
when its argument is an empty array, or when the element to return happens to beundef
.You may also use
to check whether subroutinefunc
has ever been defined. The return value is unaffected by any forward declarations offunc
. A subroutine that is not defined may still be callable: its package may have anAUTOLOAD
method that makes it spring into existence the first time that it is called; see perlsub.Use of
on aggregates (hashes and arrays) is no longer supported. It used to report whether memory for that aggregate had ever been allocated. You should instead use a simple test for size:if (@an_array) { print "has array elements\n" } if (%a_hash) { print "has hash members\n" }
When used on a hash element, it tells you whether the value is defined, not whether the key exists in the hash. Use
for the latter purpose.Examples:
print if defined $switch{D}; print "$val\n" while defined($val = pop(@ary)); die "Can't readlink $sym: $!" unless defined($value = readlink $sym); sub foo { defined &$bar ? $bar->(@_) : die "No bar"; } $debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse
and are then surprised to discover that the number0
(the zero-length string) are, in fact, defined values. For example, if you say"ab" =~ /a(.*)b/;
The pattern match succeeds and
is defined, although it matched "nothing". It didn't really fail to match anything. Rather, it matched something that happened to be zero characters long. This is all very above-board and honest. When a function returns an undefined value, it's an admission that it couldn't give you an honest answer. So you should usedefined
only when questioning the integrity of what you're trying to do. At other times, a simple comparison to0
is what you want. - delete EXPR
Given an expression that specifies an element or slice of a hash,
deletes the specified elements from that hash so thatexists
on that element no longer returns true. Setting a hash element to the undefined value does not remove its key, but deleting it does; seeexists
.In list context, usually returns the value or values deleted, or the last such element in scalar context. The return list's length corresponds to that of the argument list: deleting non-existent elements returns the undefined value in their corresponding positions. Since Perl 5.28, a key/value hash slice can be passed to
, and the return value is a list of key/value pairs (two elements for each item deleted from the hash).delete
may also be used on arrays and array slices, but its behavior is less straightforward. Althoughexists
will return false for deleted entries, deleting array elements never changes indices of existing values; useshift
for that. However, if any deleted elements fall at the end of an array, the array's size shrinks to the position of the highest element that still tests true forexists
, or to 0 if none do. In other words, an array won't have trailing nonexistent elements after a delete.WARNING: Calling
on array values is strongly discouraged. The notion of deleting or checking the existence of Perl array elements is not conceptually coherent, and can lead to surprising behavior.Deleting from
modifies the environment. Deleting from a hash tied to a DBM file deletes the entry from the DBM file. Deleting from atied
hash or array may not necessarily return anything; it depends on the implementation of thetied
package's DELETE method, which may do whatever it pleases.The
delete local EXPR
construct localizes the deletion to the current block at run time. Until the block exits, elements locally deleted temporarily no longer exist. See "Localized deletion of elements of composite types" in perlsub.my %hash = (foo => 11, bar => 22, baz => 33); my $scalar = delete $hash{foo}; # $scalar is 11 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22 my @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
The following (inefficiently) deletes all the values of %HASH and @ARRAY:
foreach my $key (keys %HASH) { delete $HASH{$key}; } foreach my $index (0 .. $#ARRAY) { delete $ARRAY[$index]; }
And so do these:
delete @HASH{keys %HASH}; delete @ARRAY[0 .. $#ARRAY];
But both are slower than assigning the empty list or undefining %HASH or @ARRAY, which is the customary way to empty out an aggregate:
%HASH = (); # completely empty %HASH undef %HASH; # forget %HASH ever existed @ARRAY = (); # completely empty @ARRAY undef @ARRAY; # forget @ARRAY ever existed
The EXPR can be arbitrarily complicated provided its final operation is an element or slice of an aggregate:
delete $ref->[$x][$y]{$key}; delete $ref->[$x][$y]->@{$key1, $key2, @morekeys}; delete $ref->[$x][$y][$index]; delete $ref->[$x][$y]->@[$index1, $index2, @moreindices];
- die LIST
raises an exception. Inside aneval
the exception is stuffed into$@
and theeval
is terminated with the undefined value. If the exception is outside of all enclosingeval
s, then the uncaught exception is printed toSTDERR
and perl exits with an exit code indicating failure. If you need to exit the process with a specific exit code, seeexit
.Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
Most of the time,
is called with a string to use as the exception. You may either give a single non-reference operand to serve as the exception, or a list of two or more items, which will be stringified and concatenated to make the exception.If the string exception does not end in a newline, the current script line number and input line number (if any) and a newline are appended to it. Note that the "input line number" (also known as "chunk") is subject to whatever notion of "line" happens to be currently in effect, and is also available as the special variable
. See "$/" in perlvar and "$." in perlvar.Hint: sometimes appending
", stopped"
to your message will cause it to make better sense when the string"at foo line 123"
is appended. Suppose you are running script "canasta".die "/etc/games is no good"; die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123. /etc/games is no good, stopped at canasta line 123.
If LIST was empty or made an empty string, and
already contains an exception value (typically from a previouseval
), then that value is reused after appending"\t...propagated"
. This is useful for propagating exceptions:eval { ... }; die unless $@ =~ /Expected exception/;
If LIST was empty or made an empty string, and
contains an object reference that has aPROPAGATE
method, that method will be called with additional file and line number parameters. The return value replaces the value in$@
; i.e., as if$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };
were called.If LIST was empty or made an empty string, and
is also empty, then the string"Died"
is used.You can also call
with a reference argument, and if this is trapped within aneval
contains that reference. This permits more elaborate exception handling using objects that maintain arbitrary state about the exception. Such a scheme is sometimes preferable to matching particular string values of$@
with regular expressions.Because Perl stringifies uncaught exception messages before display, you'll probably want to overload stringification operations on exception objects. See overload for details about that. The stringified message should be non-empty, and should end in a newline, in order to fit in with the treatment of string exceptions. Also, because an exception object reference cannot be stringified without destroying it, Perl doesn't attempt to append location or other information to a reference exception. If you want location information with a complex exception object, you'll have to arrange to put the location information into the object yourself.
is a global variable, be careful that analyzing an exception caught byeval
doesn't replace the reference in the global variable. It's easiest to make a local copy of the reference before any manipulations. Here's an example:use Scalar::Util "blessed"; eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; if (my $ev_err = $@) { if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) { # handle Some::Module::Exception } else { # handle all other possible exceptions } }
If an uncaught exception results in interpreter exit, the exit code is determined from the values of
with this pseudocode:exit $! if $!; # errno exit $? >> 8 if $? >> 8; # child exit status exit 255; # last resort
As with
is set prior to unwinding the call stack; anyDESTROY
handlers can then alter this value, and thus Perl's exit code.The intent is to squeeze as much possible information about the likely cause into the limited space of the system exit code. However, as
is the value of C'serrno
, which can be set by any system call, this means that the value of the exit code used bydie
can be non-predictable, so should not be relied upon, other than to be non-zero.You can arrange for a callback to be run just before the
does its deed, by setting the$SIG{__DIE__}
hook. The associated handler is called with the exception as an argument, and can change the exception, if it sees fit, by callingdie
again. See "%SIG" in perlvar for details on setting%SIG
entries, andeval
for some examples. Although this feature was to be run only right before your program was to exit, this is not currently so: the$SIG{__DIE__}
hook is currently called even insideeval
ed blocks/strings! If one wants the hook to do nothing in such situations, putdie @_ if $^S;
as the first line of the handler (see "$^S" in perlvar). Because this promotes strange action at a distance, this counterintuitive behavior may be fixed in a future release.
- do BLOCK
Not really a function. Returns the value of the last command in the sequence of commands indicated by BLOCK. When modified by the
loop modifier, executes the BLOCK once before testing the loop condition. (On other statements the loop modifiers test the conditional first.)do BLOCK
does not count as a loop, so the loop control statementsnext
, orredo
cannot be used to leave or restart the block. See perlsyn for alternative strategies. - do EXPR
Uses the value of EXPR as a filename and executes the contents of the file as a Perl script:
# load the exact specified file (./ and ../ special-cased) do '/foo/stat.pl'; do './stat.pl'; do '../foo/stat.pl'; # search for the named file within @INC do 'stat.pl'; do 'foo/stat.pl';
do './stat.pl'
is largely likeeval `cat stat.pl`;
except that it's more concise, runs no external processes, and keeps track of the current filename for error messages. It also differs in that code evaluated with
do FILE
cannot see lexicals in the enclosing scope;eval STRING
does. It's the same, however, in that it does reparse the file every time you call it, so you probably don't want to do this inside a loop.Using
with a relative path (except for ./ and ../), likedo 'foo/stat.pl';
will search the
directories, and update%INC
if the file is found. See "@INC" in perlvar and "%INC" in perlvar for these variables. In particular, note that whilst historically@INC
contained '.' (the current directory) making these two cases equivalent, that is no longer necessarily the case, as '.' is not included in@INC
by default in perl versions 5.26.0 onwards. Instead, perl will now warn:do "stat.pl" failed, '.' is no longer in @INC; did you mean do "./stat.pl"?
can read the file but cannot compile it, it returnsundef
and sets an error message in$@
. Ifdo
cannot read the file, it returns undef and sets$!
to the error. Always check$@
first, as compilation could fail in a way that also sets$!
. If the file is successfully compiled,do
returns the value of the last expression evaluated.Inclusion of library modules is better done with the
operators, which also do automatic error checking and raise an exception if there's a problem.You might like to use
to read in a program configuration file. Manual error checking can be done this way:# Read in config files: system first, then user. # Beware of using relative pathnames here. for $file ("/share/prog/defaults.rc", "$ENV{HOME}/.someprogrc") { unless ($return = do $file) { warn "couldn't parse $file: $@" if $@; warn "couldn't do $file: $!" unless defined $return; warn "couldn't run $file" unless $return; } }
- dump LABEL
- dump EXPR
- dump
This function causes an immediate core dump. See also the -u command-line switch in perlrun, which does the same thing. Primarily this is so that you can use the undump program (not supplied) to turn your core dump into an executable binary after having initialized all your variables at the beginning of the program. When the new binary is executed it will begin by executing a
goto LABEL
(with all the restrictions thatgoto
suffers). Think of it as a goto with an intervening core dump and reincarnation. IfLABEL
is omitted, restarts the program from the top. Thedump EXPR
form, available starting in Perl 5.18.0, allows a name to be computed at run time, being otherwise identical todump LABEL
.WARNING: Any files opened at the time of the dump will not be open any more when the program is reincarnated, with possible resulting confusion by Perl.
This function is now largely obsolete, mostly because it's very hard to convert a core file into an executable. As of Perl 5.30, it must be invoked as
.Unlike most named operators, this has the same precedence as assignment. It is also exempt from the looks-like-a-function rule, so
dump ("foo")."bar"
will cause "bar" to be part of the argument todump
.Portability issues: "dump" in perlport.
- each HASH
- each ARRAY
When called on a hash in list context, returns a 2-element list consisting of the key and value for the next element of a hash. In Perl 5.12 and later only, it will also return the index and value for the next element of an array so that you can iterate over it; older Perls consider this a syntax error. When called in scalar context, returns only the key (not the value) in a hash, or the index in an array.
Hash entries are returned in an apparently random order. The actual random order is specific to a given hash; the exact same series of operations on two hashes may result in a different order for each hash. Any insertion into the hash may change the order, as will any deletion, with the exception that the most recent key returned by
may be deleted without changing the order. So long as a given hash is unmodified you may rely onkeys
to repeatedly return the same order as each other. See "Algorithmic Complexity Attacks" in perlsec for details on why hash order is randomized. Aside from the guarantees provided here the exact details of Perl's hash algorithm and the hash traversal order are subject to change in any release of Perl.After
has returned all entries from the hash or array, the next call toeach
returns the empty list in list context andundef
in scalar context; the next call following that one restarts iteration. Each hash or array has its own internal iterator, accessed byeach
, andvalues
. The iterator is implicitly reset wheneach
has reached the end as just described; it can be explicitly reset by callingkeys
on the hash or array, or by referencing the hash (but not array) in list context. If you add or delete a hash's elements while iterating over it, the effect on the iterator is unspecified; for example, entries may be skipped or duplicated--so don't do that. Exception: It is always safe to delete the item most recently returned byeach
, so the following code works properly:while (my ($key, $value) = each %hash) { print $key, "\n"; delete $hash{$key}; # This is safe }
Tied hashes may have a different ordering behaviour to perl's hash implementation.
The iterator used by
is attached to the hash or array, and is shared between all iteration operations applied to the same hash or array. Thus all uses ofeach
on a single hash or array advance the same iterator location. All uses ofeach
are also subject to having the iterator reset by any use ofkeys
on the same hash or array, or by the hash (but not array) being referenced in list context. This makeseach
-based loops quite fragile: it is easy to arrive at such a loop with the iterator already part way through the object, or to accidentally clobber the iterator state during execution of the loop body. It's easy enough to explicitly reset the iterator before starting a loop, but there is no way to insulate the iterator state used by a loop from the iterator state used by anything else that might execute during the loop body. To avoid these problems, use aforeach
loop rather thanwhile
.This extends to using
on the result of an anonymous hash or array constructor. A new underlying array or hash is created each time so each will always start iterating from scratch, eg:# loops forever while (my ($key, $value) = each @{ +{ a => 1 } }) { print "$key=$value\n"; }
This prints out your environment like the printenv(1) program, but in a different order:
while (my ($key,$value) = each %ENV) { print "$key=$value\n"; }
Starting with Perl 5.14, an experimental feature allowed
to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.As of Perl 5.18 you can use a bare
in awhile
loop, which will set$_
on every iteration. If either aneach
expression or an explicit assignment of aneach
expression to a scalar is used as awhile
condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value.while (each %ENV) { print "$_=$ENV{$_}\n"; }
To avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious syntax errors, put this sort of thing at the top of your file to signal that your code will work only on Perls of a recent vintage:
use v5.12; # so keys/values/each work on arrays use v5.18; # so each assigns to $_ in a lone while test
- eof FILEHANDLE
- eof ()
- eof
Returns 1 if the next read on FILEHANDLE will return end of file or if FILEHANDLE is not open. FILEHANDLE may be an expression whose value gives the real filehandle. (Note that this function actually reads a character and then
s it, so isn't useful in an interactive context.) Do not read from a terminal file (or calleof(FILEHANDLE)
on it) after end-of-file is reached. File types such as terminals may lose the end-of-file condition if you do.An
without an argument uses the last file read. Usingeof()
with empty parentheses is different. It refers to the pseudo file formed from the files listed on the command line and accessed via the<>
operator. Since<>
isn't explicitly opened, as a normal filehandle is, aneof()
has been used will cause@ARGV
to be examined to determine if input is available. Similarly, aneof()
has returned end-of-file will assume you are processing another@ARGV
list, and if you haven't set@ARGV
, will read input fromSTDIN
; see "I/O Operators" in perlop.In a
while (<>)
can be used to detect the end of each file, whereaseof()
will detect the end of the very last file only. Examples:# reset line numbering on each input file while (<>) { next if /^\s*#/; # skip comments print "$.\t$_"; } continue { close ARGV if eof; # Not eof()! } # insert dashes just before last line of last file while (<>) { if (eof()) { # check for end of last file print "--------------\n"; } print; last if eof(); # needed if we're reading from a terminal }
Practical hint: you almost never need to use
in Perl, because the input operators typically returnundef
when they run out of data or encounter an error. - eval EXPR
- eval BLOCK
- eval
in all its forms is used to execute a little Perl program, trapping any errors encountered so they don't crash the calling program.Plain
with no argument is justeval EXPR
, where the expression is understood to be contained in$_
. Thus there are only two realeval
forms; the one with an EXPR is often called "string eval". In a string eval, the value of the expression (which is itself determined within scalar context) is first parsed, and if there were no errors, executed as a block within the lexical context of the current Perl program. This form is typically used to delay parsing and subsequent execution of the text of EXPR until run time. Note that the value is parsed every time theeval
executes.The other form is called "block eval". It is less general than string eval, but the code within the BLOCK is parsed only once (at the same time the code surrounding the
itself was parsed) and executed within the context of the current Perl program. This form is typically used to trap exceptions more efficiently than the first, while also providing the benefit of checking the code within BLOCK at compile time. BLOCK is parsed and compiled just once. Since errors are trapped, it often is used to check if a given feature is available.In both forms, the value returned is the value of the last expression evaluated inside the mini-program; a return statement may also be used, just as with subroutines. The expression providing the return value is evaluated in void, scalar, or list context, depending on the context of the
itself. Seewantarray
for more on how the evaluation context can be determined.If there is a syntax error or runtime error, or a
statement is executed,eval
in scalar context, or an empty list in list context, and$@
is set to the error message. (Prior to 5.16, a bug causedundef
to be returned in list context for syntax errors, but not for runtime errors.) If there was no error,$@
is set to the empty string. A control flow operator likelast
can bypass the setting of$@
. Beware that usingeval
neither silences Perl from printing warnings to STDERR, nor does it stuff the text of warning messages into$@
. To do either of those, you have to use the$SIG{__WARN__}
facility, or turn off warnings inside the BLOCK or EXPR usingno warnings 'all'
. Seewarn
, perlvar, and warnings.Note that, because
traps otherwise-fatal errors, it is useful for determining whether a particular feature (such assocket
) is implemented. It is also Perl's exception-trapping mechanism, where thedie
operator is used to raise exceptions.Before Perl 5.14, the assignment to
occurred before restoration of localized variables, which means that for your code to run on older versions, a temporary is required if you want to mask some, but not all errors:# alter $@ on nefarious repugnancy only { my $e; { local $@; # protect existing $@ eval { test_repugnancy() }; # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only $@ =~ /nefarious/ and $e = $@; } die $e if defined $e }
There are some different considerations for each form:
- String eval
Since the return value of EXPR is executed as a block within the lexical context of the current Perl program, any outer lexical variables are visible to it, and any package variable settings or subroutine and format definitions remain afterwards.
Note that when
blocks are embedded inside of an eval block the contents of the block will be executed immediately and before the rest of the eval code is executed. You can disable this entirely bylocal ${^MAX_NESTED_EVAL_BEGIN_BLOCKS} = 0; eval $string;
which will cause any embedded
blocks in$string
to throw an exception.- Under the
feature -
If this feature is enabled (which is the default under a
use 5.16
or higher declaration), Perl assumes that EXPR is a character string. Anyuse utf8
orno utf8
declarations within the string thus have no effect. Source filters are forbidden as well. (unicode_strings
, however, can appear within the string.)See also the
operator, which works properly with source filters. - Outside the
feature -
In this case, the behavior is problematic and is not so easily described. Here are two bugs that cannot easily be fixed without breaking existing programs:
Perl's internal storage of EXPR affects the behavior of the executed code. For example:
my $v = eval "use utf8; '$expr'";
If $expr is
(U+0100 in UTF-8), then the value stored in$v
will depend on whether Perl stores $expr "upgraded" (cf. utf8) or not:If upgraded,
will be"\xc4\x80"
(i.e., theuse utf8
has no effect.)If non-upgraded,
will be"\x{100}"
This is undesirable since being upgraded or not should not affect a string's behavior.
Source filters activated within
leak out into whichever file scope is currently being compiled. To give an example with the CPAN module Semi::Semicolons:BEGIN { eval "use Semi::Semicolons; # not filtered" } # filtered here!
fixes that to work the way one would expect:use feature "evalbytes"; BEGIN { evalbytes "use Semi::Semicolons; # filtered" } # not filtered
Problems can arise if the string expands a scalar containing a floating point number. That scalar can expand to letters, such as
; or, within the scope of ause locale
, the decimal point character may be something other than a dot (such as a comma). None of these are likely to parse as you are likely expecting.You should be especially careful to remember what's being looked at when:
eval $x; # CASE 1 eval "$x"; # CASE 2 eval '$x'; # CASE 3 eval { $x }; # CASE 4 eval "\$$x++"; # CASE 5 $$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the code contained in the variable $x. (Although case 2 has misleading double quotes making the reader wonder what else might be happening (nothing is).) Cases 3 and 4 likewise behave in the same way: they run the code
, which does nothing but return the value of $x. (Case 4 is preferred for purely visual reasons, but it also has the advantage of compiling at compile-time instead of at run-time.) Case 5 is a place where normally you would like to use double quotes, except that in this particular situation, you can just use symbolic references instead, as in case 6.An
eval ''
executed within a subroutine defined in theDB
package doesn't see the usual surrounding lexical scope, but rather the scope of the first non-DB piece of code that called it. You don't normally need to worry about this unless you are writing a Perl debugger.The final semicolon, if any, may be omitted from the value of EXPR.
- Under the
- Block eval
If the code to be executed doesn't vary, you may use the eval-BLOCK form to trap run-time errors without incurring the penalty of recompiling each time. The error, if any, is still returned in
. Examples:# make divide-by-zero nonfatal eval { $answer = $x / $y; }; warn $@ if $@; # same thing, but less efficient eval '$answer = $x / $y'; warn $@ if $@; # a compile-time error eval { $answer = }; # WRONG # a run-time error eval '$answer ='; # sets $@
If you want to trap errors when loading an XS module, some problems with the binary interface (such as Perl version skew) may be fatal even with
is set. See perlrun.Using the
eval {}
form as an exception trap in libraries does have some issues. Due to the current arguably broken state of__DIE__
hooks, you may wish not to trigger any__DIE__
hooks that user code may have installed. You can use thelocal $SIG{__DIE__}
construct for this purpose, as this example shows:# a private exception trap for divide-by-zero eval { local $SIG{'__DIE__'}; $answer = $x / $y; }; warn $@ if $@;
This is especially significant, given that
hooks can calldie
again, which has the effect of changing their error messages:# __DIE__ hooks may modify error messages { local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; eval { die "foo lives here" }; print $@ if $@; # prints "bar lives here" }
Because this promotes action at a distance, this counterintuitive behavior may be fixed in a future release.
eval BLOCK
does not count as a loop, so the loop control statementsnext
, orredo
cannot be used to leave or restart the block.The final semicolon, if any, may be omitted from within the BLOCK.
- evalbytes EXPR
- evalbytes
This function is similar to a string eval, except it always parses its argument (or
if EXPR is omitted) as a byte string. If the string contains any code points above 255, then it cannot be a byte string, and theevalbytes
will fail with the error stored in$@
.use utf8
andno utf8
within the string have their usual effect.Source filters activated within the evaluated code apply to the code itself.
is available starting in Perl v5.16. To access it, you must sayCORE::evalbytes
, but you can omit theCORE::
if the"evalbytes"
feature is enabled. This is enabled automatically with ause v5.16
(or higher) declaration in the current scope. - exec LIST
- exec PROGRAM LIST
function executes a system command and never returns; usesystem
instead ofexec
if you want it to return. It fails and returns false only if the command does not exist and it is executed directly instead of via your system's command shell (see below).Since it's a common mistake to use
instead ofsystem
, Perl warns you ifexec
is called in void context and if there is a following statement that isn'tdie
, orexit
(if warnings are enabled--but you always do that, right?). If you really want to follow anexec
with some other statement, you can use one of these styles to avoid the warning:exec ('foo') or print STDERR "couldn't exec foo: $!"; { exec ('foo') }; print STDERR "couldn't exec foo: $!";
If there is more than one argument in LIST, this calls execvp(3) with the arguments in LIST. If there is only one element in LIST, the argument is checked for shell metacharacters, and if there are any, the entire argument is passed to the system's command shell for parsing (this is
/bin/sh -c
on Unix platforms, but varies on other platforms). If there are no shell metacharacters in the argument, it is split into words and passed directly toexecvp
, which is more efficient. Examples:exec '/bin/echo', 'Your arguments are: ', @ARGV; exec "sort $outfile | uniq";
If you don't really want to execute the first argument, but want to lie to the program you are executing about its own name, you can specify the program you actually want to run as an "indirect object" (without a comma) in front of the LIST, as in
exec PROGRAM LIST
. (This always forces interpretation of the LIST as a multivalued list, even if there is only a single scalar in the list.) Example:my $shell = '/bin/csh'; exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system shell, results are subject to its quirks and capabilities. See "`STRING`" in perlop for details.
Using an indirect object with
is also more secure. This usage (which also works fine withsystem
) forces interpretation of the arguments as a multivalued list, even if the list had just one argument. That way you're safe from the shell expanding wildcards or splitting up words with whitespace in them.my @args = ( "echo surprise" ); exec @args; # subject to shell escapes # if @args == 1 exec { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect object, ran the echo program, passing it
an argument. The second version didn't; it tried to run a program named "echo surprise", didn't find it, and set$?
to a non-zero value indicating failure.On Windows, only the
exec PROGRAM LIST
indirect object syntax will reliably avoid using the shell;exec LIST
, even with more than one element, will fall back to the shell if the first spawn fails.Perl attempts to flush all files opened for output before the exec, but this may not be supported on some platforms (see perlport). To be safe, you may need to set
in English) or call theautoflush
method ofIO::Handle
on any open handles to avoid lost output.Note that
will not call yourEND
blocks, nor will it invokeDESTROY
methods on your objects.Portability issues: "exec" in perlport.
- exists EXPR
Given an expression that specifies an element of a hash, returns true if the specified element in the hash has ever been initialized, even if the corresponding value is undefined.
print "Exists\n" if exists $hash{$key}; print "Defined\n" if defined $hash{$key}; print "True\n" if $hash{$key};
exists may also be called on array elements, but its behavior is much less obvious and is strongly tied to the use of
on arrays.WARNING: Calling
on array values is strongly discouraged. The notion of deleting or checking the existence of Perl array elements is not conceptually coherent, and can lead to surprising behavior.print "Exists\n" if exists $array[$index]; print "Defined\n" if defined $array[$index]; print "True\n" if $array[$index];
A hash or array element can be true only if it's defined and defined only if it exists, but the reverse doesn't necessarily hold true.
Given an expression that specifies the name of a subroutine, returns true if the specified subroutine has ever been declared, even if it is undefined. Mentioning a subroutine name for exists or defined does not count as declaring it. Note that a subroutine that does not exist may still be callable: its package may have an
method that makes it spring into existence the first time that it is called; see perlsub.print "Exists\n" if exists &subroutine; print "Defined\n" if defined &subroutine;
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash or array key lookup or subroutine name:
if (exists $ref->{A}->{B}->{$key}) { } if (exists $hash{A}{B}{$key}) { } if (exists $ref->{A}->{B}->[$ix]) { } if (exists $hash{A}{B}[$ix]) { } if (exists &{$ref->{A}{B}{$key}}) { }
Although the most deeply nested array or hash element will not spring into existence just because its existence was tested, any intervening ones will. Thus
will spring into existence due to the existence test for the$key
element above. This happens anywhere the arrow operator is used, including even here:undef $ref; if (exists $ref->{"Some key"}) { } print $ref; # prints HASH(0x80d3d5c)
Use of a subroutine call, rather than a subroutine name, as an argument to
is an error.exists ⊂ # OK exists &sub(); # Error
- exit EXPR
- exit
Evaluates EXPR and exits immediately with that value. Example:
my $ans = <STDIN>; exit 0 if $ans =~ /^[Xx]/;
See also
. If EXPR is omitted, exits with0
status. The only universally recognized values for EXPR are0
for success and1
for error; other values are subject to interpretation depending on the environment in which the Perl program is running. For example, exiting 69 (EX_UNAVAILABLE) from a sendmail incoming-mail filter will cause the mailer to return the item undelivered, but that's not true everywhere.Don't use
to abort a subroutine if there's any chance that someone might want to trap whatever error happened. Usedie
instead, which can be trapped by aneval
function does not always exit immediately. It calls any definedEND
routines first, but theseEND
routines may not themselves abort the exit. Likewise any object destructors that need to be called are called before the real exit.END
routines and destructors can change the exit status by modifying$?
. If this is a problem, you can callPOSIX::_exit($status)
to avoidEND
and destructor processing. See perlmod for details.Portability issues: "exit" in perlport.
- exp EXPR
- exp
Returns e (the natural logarithm base) to the power of EXPR. If EXPR is omitted, gives
. - fc EXPR
- fc
Returns the casefolded version of EXPR. This is the internal function implementing the
escape in double-quoted strings.Casefolding is the process of mapping strings to a form where case differences are erased; comparing two strings in their casefolded form is effectively a way of asking if two strings are equal, regardless of case.
Roughly, if you ever found yourself writing this
lc($this) eq lc($that) # Wrong! # or uc($this) eq uc($that) # Also wrong! # or $this =~ /^\Q$that\E\z/i # Right!
Now you can write
fc($this) eq fc($that)
And get the correct results.
Perl only implements the full form of casefolding, but you can access the simple folds using "casefold()" in Unicode::UCD and "prop_invmap()" in Unicode::UCD. For further information on casefolding, refer to the Unicode Standard, specifically sections 3.13
Default Case Operations
, 4.2Case-Normative
, and 5.18Case Mappings
, available at https://www.unicode.org/versions/latest/, as well as the Case Charts available at https://www.unicode.org/charts/case/.If EXPR is omitted, uses
.This function behaves the same way under various pragmas, such as within
"use feature 'unicode_strings"
, aslc
does, with the single exception offc
of LATIN CAPITAL LETTER SHARP S (U+1E9E) within the scope ofuse locale
. The foldcase of this character would normally be"ss"
, but as explained in thelc
section, case changes that cross the 255/256 boundary are problematic under locales, and are hence prohibited. Therefore, this function under locale returns instead the string"\x{17F}\x{17F}"
, which is the LATIN SMALL LETTER LONG S. Since that character itself folds to"s"
, the string of two of them together should be equivalent to a single U+1E9E when foldcased.While the Unicode Standard defines two additional forms of casefolding, one for Turkic languages and one that never maps one character into multiple characters, these are not provided by the Perl core. However, the CPAN module
may be used to provide an implementation.fc
is available only if the"fc"
feature is enabled or if it is prefixed withCORE::
. The"fc"
feature is enabled automatically with ause v5.16
(or higher) declaration in the current scope. - fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the fcntl(2) function. You'll probably have to say
use Fcntl;
first to get the correct constant definitions. Argument processing and value returned work just like
below. For example:use Fcntl; my $flags = fcntl($filehandle, F_GETFL, 0) or die "Can't fcntl F_GETFL: $!";
You don't have to check for
on the return fromfcntl
. Likeioctl
, it maps a0
return from the system call into"0 but true"
in Perl. This string is true in boolean context and0
in numeric context. It is also exempt from the normalArgument "..." isn't numeric
warnings on improper numeric conversions.Note that
raises an exception if used on a machine that doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) manpage to learn what functions are available on your system.Here's an example of setting a filehandle named
to be non-blocking at the system level. You'll have to negotiate$|
on your own, though.use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); my $flags = fcntl($REMOTE, F_GETFL, 0) or die "Can't get flags for the socket: $!\n"; fcntl($REMOTE, F_SETFL, $flags | O_NONBLOCK) or die "Can't set flags for the socket: $!\n";
Portability issues: "fcntl" in perlport.
A special token that returns the name of the file in which it occurs. It can be altered by the mechanism described at "Plain Old Comments (Not!)" in perlsyn.
- field VARNAME
Declares a new field variable within the current class. Methods and
blocks of the class will have access to this variable as if it was a lexical in scope at that point. - fileno FILEHANDLE
- fileno DIRHANDLE
Returns the file descriptor for a filehandle or directory handle, or undefined if the filehandle is not open. If there is no real file descriptor at the OS level, as can happen with filehandles connected to memory objects via
with a reference for the third argument, -1 is returned.This is mainly useful for constructing bitmaps for
and low-level POSIX tty-handling operations. If FILEHANDLE is an expression, the value is taken as an indirect filehandle, generally its name.You can use this to find out whether two handles refer to the same underlying descriptor:
if (fileno($this) != -1 && fileno($this) == fileno($that)) { print "\$this and \$that are dups\n"; } elsif (fileno($this) != -1 && fileno($that) != -1) { print "\$this and \$that have different " . "underlying file descriptors\n"; } else { print "At least one of \$this and \$that does " . "not have a real file descriptor\n"; }
The behavior of
on a directory handle depends on the operating system. On a system with dirfd(3) or similar,fileno
on a directory handle returns the underlying file descriptor associated with the handle; on systems with no such support, it returns the undefined value, and sets$!
(errno). - flock FILEHANDLE,OPERATION
Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true for success, false on failure. Produces a fatal error if used on a machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
is Perl's portable file-locking interface, although it locks entire files only, not records.Two potentially non-obvious but traditional
semantics are that it waits indefinitely until the lock is granted, and that its locks are merely advisory. Such discretionary locks are more flexible, but offer fewer guarantees. This means that programs that do not also useflock
may modify files locked withflock
. See perlport, your port's specific documentation, and your system-specific local manpages for details. It's best to assume traditional behavior if you're writing portable programs. (But if you're not, you should as always feel perfectly free to write for your own system's idiosyncrasies (sometimes called "features"). Slavish adherence to portability concerns shouldn't get in the way of your getting your job done.)OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but you can use the symbolic names if you import them from the Fcntl module, either individually, or as a group using the
tag. LOCK_SH requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN releases a previously requested lock. If LOCK_NB is bitwise-or'ed with LOCK_SH or LOCK_EX, thenflock
returns immediately rather than blocking waiting for the lock; check the return status to see if you got it.To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE before locking or unlocking it.
Note that the emulation built with lockf(3) doesn't provide shared locks, and it requires that FILEHANDLE be open with write intent. These are the semantics that lockf(3) implements. Most if not all systems implement lockf(3) in terms of fcntl(2) locking, though, so the differing semantics shouldn't bite too many people.
Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE be open with read intent to use LOCK_SH and requires that it be open with write intent to use LOCK_EX.
Note also that some versions of
cannot lock things over the network; you would need to use the more system-specificfcntl
for that. If you like you can force Perl to ignore your system's flock(2) function, and so provide its own fcntl(2)-based emulation, by passing the switch-Ud_flock
to the Configure program when you configure and build a new Perl.Here's a mailbox appender for BSD systems.
# import LOCK_* and SEEK_END constants use Fcntl qw(:flock SEEK_END); sub lock { my ($fh) = @_; flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n"; # and, in case we're running on a very old UNIX # variant without the modern O_APPEND semantics... seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n"; } sub unlock { my ($fh) = @_; flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n"; } open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}") or die "Can't open mailbox: $!"; lock($mbox); print $mbox $msg,"\n\n"; unlock($mbox);
On systems that support a real flock(2), locks are inherited across
calls, whereas those that must resort to the more capricious fcntl(2) function lose their locks, making it seriously harder to write servers.See also DB_File for other
examples.Portability issues: "flock" in perlport.
- fork
Does a fork(2) system call to create a new process running the same program at the same point. It returns the child pid to the parent process,
to the child process, orundef
if the fork is unsuccessful. File descriptors (and sometimes locks on those descriptors) are shared, while everything else is copied. On most systems supporting fork(2), great care has gone into making it extremely efficient (for example, using copy-on-write technology on data pages), making it the dominant paradigm for multitasking over the last few decades.Perl attempts to flush all files opened for output before forking the child process, but this may not be supported on some platforms (see perlport). To be safe, you may need to set
in English) or call theautoflush
method ofIO::Handle
on any open handles to avoid duplicate output.If you
without ever waiting on your children, you will accumulate zombies. On some systems, you can avoid this by setting$SIG{CHLD}
. See also perlipc for more examples of forking and reaping moribund children.Note that if your forked child inherits system file descriptors like STDIN and STDOUT that are actually connected by a pipe or socket, even if you exit, then the remote server (such as, say, a CGI script or a backgrounded job launched from a remote shell) won't think you're done. You should reopen those to /dev/null if it's any issue.
On some platforms such as Windows, where the fork(2) system call is not available, Perl can be built to emulate
in the Perl interpreter. The emulation is designed, at the level of the Perl program, to be as compatible as possible with the "Unix" fork(2). However it has limitations that have to be considered in code intended to be portable. See perlfork for more details.Portability issues: "fork" in perlport.
- format
Declare a picture format for use by the
function. For example:format Something = Test: @<<<<<<<< @||||| @>>>>> $str, $%, '$' . int($num) . $str = "widget"; $num = $cost/$quantity; $~ = 'Something'; write;
See perlform for many details and examples.
- formline PICTURE,LIST
This is an internal function used by
s, though you may call it, too. It formats (see perlform) a list of values according to the contents of PICTURE, placing the output into the format output accumulator,$^A
in English). Eventually, when awrite
is done, the contents of$^A
are written to some filehandle. You could also read$^A
and then set$^A
back to""
. Note that a format typically does oneformline
per line of form, but theformline
function itself doesn't care how many newlines are embedded in the PICTURE. This means that the~
tokens treat the entire PICTURE as a single line. You may therefore need to use multiple formlines to implement a single record format, just like theformat
compiler.Be careful if you put double quotes around the picture, because an
character may be taken to mean the beginning of an array name.formline
always returns true. See perlform for other examples.If you are trying to use this instead of
to capture the output, you may find it easier to open a filehandle to a scalar (open my $fh, ">", \$output
) and write to that instead. - getc FILEHANDLE
- getc
Returns the next character from the input file attached to FILEHANDLE, or the undefined value at end of file or if there was an error (in the latter case
is set). If FILEHANDLE is omitted, reads from STDIN. This is not particularly efficient. However, it cannot be used by itself to fetch single characters without waiting for the user to hit enter. For that, try something more like:if ($BSD_STYLE) { system "stty cbreak </dev/tty >/dev/tty 2>&1"; } else { system "stty", '-icanon', 'eol', "\001"; } my $key = getc(STDIN); if ($BSD_STYLE) { system "stty -cbreak </dev/tty >/dev/tty 2>&1"; } else { system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL } print "\n";
Determination of whether
should be set is left as an exercise to the reader.The
function can do this more portably on systems purporting POSIX compliance. See also theTerm::ReadKey
module on CPAN. - getlogin
This implements the C library function of the same name, which on most systems returns the current login from /etc/utmp, if any. If it returns the empty string, use
.my $login = getlogin || getpwuid($<) || "Kilroy";
Do not consider
for authentication: it is not as secure asgetpwuid
.Portability issues: "getlogin" in perlport.
- getpeername SOCKET
Returns the packed sockaddr address of the other end of the SOCKET connection.
use Socket; my $hersockaddr = getpeername($sock); my ($port, $iaddr) = sockaddr_in($hersockaddr); my $herhostname = gethostbyaddr($iaddr, AF_INET); my $herstraddr = inet_ntoa($iaddr);
- getpgrp PID
Returns the current process group for the specified PID. Use a PID of
to get the current process group for the current process. Will raise an exception if used on a machine that doesn't implement getpgrp(2). If PID is omitted, returns the process group of the current process.Some very old machines may not support
and will throw an exception ifPID != 0
.Portability issues: "getpgrp" in perlport.
- getppid
Returns the process id of the parent process.
Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work around non-POSIX thread semantics the minority of Linux systems (and Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation has since been removed. See the documentation for $$ for details.
Portability issues: "getppid" in perlport.
- getpriority WHICH,WHO
Returns the current priority for a process, a process group, or a user. (See getpriority(2).) Will raise a fatal exception if used on a machine that doesn't implement getpriority(2).
can be any ofPRIO_PROCESS
imported from "RESOURCE CONSTANTS" in POSIX.Portability issues: "getpriority" in perlport.
- getpwnam NAME
- getgrnam NAME
- gethostbyname NAME
- getnetbyname NAME
- getprotobyname NAME
- getpwuid UID
- getgrgid GID
- getservbyname NAME,PROTO
- gethostbyaddr ADDR,ADDRTYPE
- getnetbyaddr ADDR,ADDRTYPE
- getprotobynumber NUMBER
- getservbyport PORT,PROTO
- getpwent
- getgrent
- gethostent
- getnetent
- getprotoent
- getservent
- setpwent
- setgrent
- sethostent STAYOPEN
- setnetent STAYOPEN
- setprotoent STAYOPEN
- setservent STAYOPEN
- endpwent
- endgrent
- endhostent
- endnetent
- endprotoent
- endservent
These routines are the same as their counterparts in the system C library. In list context, the return values from the various get routines are as follows:
# 0 1 2 3 4 my ( $name, $passwd, $gid, $members ) = getgr* my ( $name, $aliases, $addrtype, $net ) = getnet* my ( $name, $aliases, $port, $proto ) = getserv* my ( $name, $aliases, $proto ) = getproto* my ( $name, $aliases, $addrtype, $length, @addrs ) = gethost* my ( $name, $passwd, $uid, $gid, $quota, $comment, $gcos, $dir, $shell, $expire ) = getpw* # 5 6 7 8 9
(If the entry doesn't exist, the return value is a single meaningless true value.)
The exact meaning of the $gcos field varies but it usually contains the real name of the user (as opposed to the login name) and other information pertaining to the user. Beware, however, that in many system users are able to change this information and therefore it cannot be trusted and therefore the $gcos is tainted (see perlsec). The $passwd and $shell, user's encrypted password and login shell, are also tainted, for the same reason.
In scalar context, you get the name, unless the function was a lookup by name, in which case you get the other thing, whatever it is. (If the entry doesn't exist you get the undefined value.) For example:
my $uid = getpwnam($name); my $name = getpwuid($num); my $name = getpwent(); my $gid = getgrnam($name); my $name = getgrgid($num); my $name = getgrent(); # etc.
In getpw*() the fields $quota, $comment, and $expire are special in that they are unsupported on many systems. If the $quota is unsupported, it is an empty scalar. If it is supported, it usually encodes the disk quota. If the $comment field is unsupported, it is an empty scalar. If it is supported it usually encodes some administrative comment about the user. In some systems the $quota field may be $change or $age, fields that have to do with password aging. In some systems the $comment field may be $class. The $expire field, if present, encodes the expiration period of the account or the password. For the availability and the exact meaning of these fields in your system, please consult getpwnam(3) and your system's pwd.h file. You can also find out from within Perl what your $quota and $comment fields mean and whether you have the $expire field by using the
module and the valuesd_pwquota
, andd_pwexpire
. Shadow password files are supported only if your vendor has implemented them in the intuitive fashion that calling the regular C library routines gets the shadow versions if you're running under privilege or if there exists the shadow(3) functions as found in System V (this includes Solaris and Linux). Those systems that implement a proprietary shadow password facility are unlikely to be supported.The $members value returned by getgr*() is a space-separated list of the login names of the members of the group.
For the gethost*() functions, if the
variable is supported in C, it will be returned to you via$?
if the function call fails. The@addrs
value returned by a successful call is a list of raw addresses returned by the corresponding library call. In the Internet domain, each address is four bytes long; you can unpack it by saying something like:my ($w,$x,$y,$z) = unpack('W4',$addr[0]);
The Socket library makes this slightly easier:
use Socket; my $iaddr = inet_aton("127.1"); # or whatever address my $name = gethostbyaddr($iaddr, AF_INET); # or going the other way my $straddr = inet_ntoa($iaddr);
In the opposite way, to resolve a hostname to the IP address you can write this:
use Socket; my $packed_ip = gethostbyname("www.perl.org"); my $ip_address; if (defined $packed_ip) { $ip_address = inet_ntoa($packed_ip); }
Make sure
is called in SCALAR context and that its return value is checked for definedness.The
function, even though it only takes one argument, has the precedence of a list operator, so beware:getprotobynumber $number eq 'icmp' # WRONG getprotobynumber($number eq 'icmp') # actually means this getprotobynumber($number) eq 'icmp' # better this way
If you get tired of remembering which element of the return list contains which return value, by-name interfaces are provided in standard modules:
, andUser::grent
. These override the normal built-ins, supplying versions that return objects with the appropriate names for each field. For example:use File::stat; use User::pwent; my $is_theirs = (stat($filename)->uid == getpwnam($whoever)->uid);
Even though it looks as though they're the same method calls (uid), they aren't, because a
object is different from aUser::pwent
object.Many of these functions are not safe in a multi-threaded environment where more than one thread can be using them. In particular, functions like
iterate per-process and not per-thread, so if two threads are simultaneously iterating, neither will get all the records.Some systems have thread-safe versions of some of the functions, such as
instead ofgetpwnam()
. There, Perl automatically and invisibly substitutes the thread-safe version, without notice. This means that code that safely runs on some systems can fail on others that lack the thread-safe versions.Portability issues: "getpwnam" in perlport to "endservent" in perlport.
- getsockname SOCKET
Returns the packed sockaddr address of this end of the SOCKET connection, in case you don't know the address because you have several different IPs that the connection might have come in on.
use Socket; my $mysockaddr = getsockname($sock); my ($port, $myaddr) = sockaddr_in($mysockaddr); printf "Connect to %s [%s]\n", scalar gethostbyaddr($myaddr, AF_INET), inet_ntoa($myaddr);
- getsockopt SOCKET,LEVEL,OPTNAME
Queries the option named OPTNAME associated with SOCKET at a given LEVEL. Options may exist at multiple protocol levels depending on the socket type, but at least the uppermost socket level SOL_SOCKET (defined in the
module) will exist. To query options at another level the protocol number of the appropriate protocol controlling the option should be supplied. For example, to indicate that an option is to be interpreted by the TCP protocol, LEVEL should be set to the protocol number of TCP, which you can get usinggetprotobyname
.The function returns a packed string representing the requested socket option, or
on error, with the reason for the error placed in$!
. Just what is in the packed string depends on LEVEL and OPTNAME; consult getsockopt(2) for details. A common case is that the option is an integer, in which case the result is a packed integer, which you can decode usingunpack
with thei
) format.Here's an example to test whether Nagle's algorithm is enabled on a socket:
use Socket qw(:all); defined(my $tcp = getprotobyname("tcp")) or die "Could not determine the protocol number for tcp"; # my $tcp = IPPROTO_TCP; # Alternative my $packed = getsockopt($socket, $tcp, TCP_NODELAY) or die "getsockopt TCP_NODELAY: $!"; my $nodelay = unpack("I", $packed); print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
Portability issues: "getsockopt" in perlport.
- glob EXPR
- glob
In list context, returns a (possibly empty) list of filename expansions on the value of EXPR such as the Unix shell Bash would do. In scalar context, glob iterates through such filename expansions, returning
when the list is exhausted. If EXPR is omitted,$_
is used.# List context my @txt_files = glob("*.txt"); my @perl_files = glob("*.pl *.pm"); # Scalar context while (my $file = glob("*.mp3")) { # Do stuff }
Glob also supports an alternate syntax using
as delimiters. While this syntax is supported, it is recommended that you useglob
instead as it is more readable and searchable.my @txt_files = <"*.txt">;
If you need case insensitive file globbing that can be achieved using the
parameter of thebsd_glob
module.use File::Glob qw(:globally :nocase); my @txt = glob("readme*"); # README readme.txt Readme.md
Note that
splits its arguments on whitespace and treats each segment as separate pattern. As such,glob("*.c *.h")
matches all files with a .c or .h extension. The expressionglob(".* *")
matches all files in the current working directory. If you want to glob filenames that might contain whitespace, you'll have to use extra quotes around the spacey filename to protect it. For example, to glob filenames that have ane
followed by a space followed by anf
, use one of:my @spacies = <"*e f*">; my @spacies = glob('"*e f*"'); my @spacies = glob(q("*e f*"));
If you had to get a variable through, you could do this:
my @spacies = glob("'*${var}e f*'"); my @spacies = glob(qq("*${var}e f*"));
If non-empty braces are the only wildcard characters used in the
, no filenames are matched, but potentially many strings are returned. For example, this produces nine strings, one for each pairing of fruits and colors:my @many = glob("{apple,tomato,cherry}={green,yellow,red}");
This operator is implemented using the standard
extension. Seebsd_glob
for details, includingbsd_glob
, which does not treat whitespace as a pattern separator.If a
expression is used as the condition of awhile
loop, then it will be implicitly assigned to$_
. If either aglob
expression or an explicit assignment of aglob
expression to a scalar is used as awhile
condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value.Internal implemenation details:
This is the internal function implementing the
operator, but you can use it directly. The<*.c>
operator is discussed in more detail in "I/O Operators" in perlop.Portability issues: "glob" in perlport.
- gmtime EXPR
- gmtime
Works just like
, but the returned values are localized for the standard Greenwich time zone.Note: When called in list context, $isdst, the last value returned by gmtime, is always
. There is no Daylight Saving Time in GMT.Portability issues: "gmtime" in perlport.
- goto LABEL
- goto EXPR
- goto &NAME
goto LABEL
form finds the statement labeled with LABEL and resumes execution there. It can't be used to get out of a block or subroutine given tosort
. It can be used to go almost anywhere else within the dynamic scope, including out of subroutines, but it's usually better to use some other construct such aslast
. The author of Perl has never felt the need to use this form ofgoto
(in Perl, that is; C is another matter). (The difference is that C does not offer named loops combined with loop control. Perl does, and this replaces most structured uses ofgoto
in other languages.)The
goto EXPR
form expects to evaluateEXPR
to a code reference or a label name. If it evaluates to a code reference, it will be handled likegoto &NAME
, below. This is especially useful for implementing tail recursion viagoto __SUB__
.If the expression evaluates to a label name, its scope will be resolved dynamically. This allows for computed
s per FORTRAN, but isn't necessarily recommended if you're optimizing for maintainability:goto ("FOO", "BAR", "GLARCH")[$i];
As shown in this example,
goto EXPR
is exempt from the "looks like a function" rule. A pair of parentheses following it does not (necessarily) delimit its argument.goto("NE")."XT"
is equivalent togoto NEXT
. Also, unlike most named operators, this has the same precedence as assignment.Use of
goto LABEL
orgoto EXPR
to jump into a construct is deprecated and will issue a warning; it will become a fatal error in Perl 5.42. While still available, it may not be used to go into any construct that requires initialization, such as a subroutine, aforeach
loop, or agiven
block. In general, it may not be used to jump into the parameter of a binary or list operator, but it may be used to jump into the first parameter of a binary operator. (The=
assignment operator's "first" operand is its right-hand operand.) It also can't be used to go into a construct that is optimized away.The
goto &NAME
form is quite different from the other forms ofgoto
. In fact, it isn't a goto in the normal sense at all, and doesn't have the stigma associated with other gotos. Instead, it exits the current subroutine (losing any changes set bylocal
) and immediately calls in its place the named subroutine using the current value of@_
. This is used byAUTOLOAD
subroutines that wish to load another subroutine and then pretend that the other subroutine had been called in the first place (except that any modifications to@_
in the current subroutine are propagated to the other subroutine.) After thegoto
, not evencaller
will be able to tell that this routine was called first.NAME needn't be the name of a subroutine; it can be a scalar variable containing a code reference or a block that evaluates to a code reference.
- grep BLOCK LIST
- grep EXPR,LIST
This is similar in spirit to, but not the same as, grep(1) and its relatives. In particular, it is not limited to using regular expressions.
Evaluates the BLOCK or EXPR for each element of LIST (locally setting
to each element) and returns the list value consisting of those elements for which the expression evaluated to true. In scalar context, returns the number of times the expression was true.my @foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
my @foo = grep {!/^#/} @bar; # weed out comments
Note that
is an alias to the list value, so it can be used to modify the elements of the LIST. While this is useful and supported, it can cause bizarre results if the elements of LIST are not variables. Similarly, grep returns aliases into the original list, much as a for loop's index variable aliases the list elements. That is, modifying an element of a list returned by grep (for example, in aforeach
or anothergrep
) actually modifies the element in the original list. This is usually something to be avoided when writing clear code.See also
for a list composed of the results of the BLOCK or EXPR. - hex EXPR
- hex
Interprets EXPR as a hex string and returns the corresponding numeric value. If EXPR is omitted, uses
.print hex '0xAf'; # prints '175' print hex 'aF'; # same $valid_input =~ /\A(?:0?[xX])?(?:_?[0-9a-fA-F])*\z/
A hex string consists of hex digits and an optional
prefix. Each hex digit may be preceded by a single underscore, which will be ignored. Any other character triggers a warning and causes the rest of the string to be ignored (even leading whitespace, unlikeoct
). Only integers can be represented, and integer overflow triggers a warning.To convert strings that might start with any of
, or0b
, seeoct
. To present something as hex, look intoprintf
, andunpack
. - import LIST
There is no builtin
function. It is just an ordinary method (subroutine) defined (or inherited) by modules that wish to export names to another module. Theuse
function calls theimport
method for the package used. See alsouse
, perlmod, and Exporter. - index STR,SUBSTR,POSITION
- index STR,SUBSTR
The index function searches for one string within another, but without the wildcard-like behavior of a full regular-expression pattern match. It returns the position of the first occurrence of SUBSTR in STR at or after POSITION. If POSITION is omitted, starts searching from the beginning of the string. POSITION before the beginning of the string or after its end is treated as if it were the beginning or the end, respectively. POSITION and the return value are based at zero. If the substring is not found,
returns -1.Find characters or strings:
index("Perl is great", "P"); # Returns 0 index("Perl is great", "g"); # Returns 8 index("Perl is great", "great"); # Also returns 8
Attempting to find something not there:
index("Perl is great", "Z"); # Returns -1 (not found)
Using an offset to find the second occurrence:
index("Perl is great", "e", 5); # Returns 10
- int EXPR
- int
Returns the integer portion of EXPR. If EXPR is omitted, uses
. You should not use this function for rounding: one because it truncates towards0
, and two because machine representations of floating-point numbers can sometimes produce counterintuitive results. For example,int(-6.725/0.025)
produces -268 rather than the correct -269; that's because it's really more like -268.99999999999994315658 instead. Usually, thesprintf
, or thePOSIX::floor
functions will serve you better than willint
. - ioctl FILEHANDLE,FUNCTION,SCALAR
Implements the ioctl(2) function. You'll probably first have to say
require "sys/ioctl.ph"; # probably in # $Config{archlib}/sys/ioctl.ph
to get the correct function definitions. If sys/ioctl.ph doesn't exist or doesn't have the correct definitions you'll have to roll your own, based on your C header files such as <sys/ioctl.h>. (There is a Perl script called h2ph that comes with the Perl kit that may help you in this, but it's nontrivial.) SCALAR will be read and/or written depending on the FUNCTION; a C pointer to the string value of SCALAR will be passed as the third argument of the actual
call. (If SCALAR has no string value but does have a numeric value, that value will be passed rather than a pointer to the string value. To guarantee this to be true, add a0
to the scalar before using it.) Thepack
functions may be needed to manipulate the values of structures used byioctl
.The return value of
) is as follows:if OS returns: then Perl returns: -1 undefined value 0 string "0 but true" anything else that number
Thus Perl returns true on success and false on failure, yet you can still easily determine the actual value returned by the operating system:
my $retval = ioctl(...) || -1; printf "System returned %d\n", $retval;
The special string
"0 but true"
is exempt fromArgument "..." isn't numeric
warnings on improper numeric conversions.Portability issues: "ioctl" in perlport.
- join EXPR,LIST
Joins the separate strings of LIST into a single string with fields separated by the value of EXPR, and returns that new string. Example:
my $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
Beware that unlike
doesn't take a pattern as its first argument. Comparesplit
. - keys HASH
- keys ARRAY
Called in list context, returns a list consisting of all the keys of the named hash, or in Perl 5.12 or later only, the indices of an array. Perl releases prior to 5.12 will produce a syntax error if you try to use an array argument. In scalar context, returns the number of keys or indices.
Hash entries are returned in an apparently random order. The actual random order is specific to a given hash; the exact same series of operations on two hashes may result in a different order for each hash. Any insertion into the hash may change the order, as will any deletion, with the exception that the most recent key returned by
may be deleted without changing the order. So long as a given hash is unmodified you may rely onkeys
to repeatedly return the same order as each other. See "Algorithmic Complexity Attacks" in perlsec for details on why hash order is randomized. Aside from the guarantees provided here the exact details of Perl's hash algorithm and the hash traversal order are subject to change in any release of Perl. Tied hashes may behave differently to Perl's hashes with respect to changes in order on insertion and deletion of items.As a side effect, calling
resets the internal iterator of the HASH or ARRAY (seeeach
) before yielding the keys. In particular, callingkeys
in void context resets the iterator with no other overhead.Here is yet another way to print your environment:
my @keys = keys %ENV; my @values = values %ENV; while (@keys) { print pop(@keys), '=', pop(@values), "\n"; }
or how about sorted by key:
foreach my $key (sort(keys %ENV)) { print $key, '=', $ENV{$key}, "\n"; }
The returned values are copies of the original keys in the hash, so modifying them will not affect the original hash. Compare
.To sort a hash by value, you'll need to use a
function. Here's a descending numeric sort of a hash by its values:foreach my $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { printf "%4d %s\n", $hash{$key}, $key; }
Used as an lvalue,
allows you to increase the number of hash buckets allocated for the given hash. This can gain you a measure of efficiency if you know the hash is going to get big. (This is similar to pre-extending an array by assigning a larger number to $#array.) If you saykeys %hash = 200;
will have at least 200 buckets allocated for it--256 of them, in fact, since it rounds up to the next power of two. These buckets will be retained even if you do%hash = ()
, useundef %hash
if you want to free the storage while%hash
is still in scope. You can't shrink the number of buckets allocated for the hash usingkeys
in this way (but you needn't worry about doing this by accident, as trying has no effect).keys @array
in an lvalue context is a syntax error.Starting with Perl 5.14, an experimental feature allowed
to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.To avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious syntax errors, put this sort of thing at the top of your file to signal that your code will work only on Perls of a recent vintage:
use v5.12; # so keys/values/each work on arrays
- kill SIGNAL, LIST
- kill SIGNAL
Sends a signal to a list of processes. Returns the number of arguments that were successfully used to signal (which is not necessarily the same as the number of processes actually killed, e.g. where a process group is killed).
my $cnt = kill 'HUP', $child1, $child2; kill 'KILL', @goners;
SIGNAL may be either a signal name (a string) or a signal number. A signal name may start with a
prefix, thusFOO
refer to the same signal. The string form of SIGNAL is recommended for portability because the same signal may have different numbers in different operating systems.A list of signal names supported by the current platform can be found in
, which is provided by theConfig
module. See Config for more details.A negative signal name is the same as a negative signal number, killing process groups instead of processes. For example,
kill '-KILL', $pgrp
andkill -9, $pgrp
will sendSIGKILL
to the entire process group specified. That means you usually want to use positive not negative signals.If SIGNAL is either the number 0 or the string
), no signal is sent to the process, butkill
checks whether it's possible to send a signal to it (that means, to be brief, that the process is owned by the same user, or we are the super-user). This is useful to check that a child process is still alive (even if only as a zombie) and hasn't changed its UID. See perlport for notes on the portability of this construct.The behavior of kill when a PROCESS number is zero or negative depends on the operating system. For example, on POSIX-conforming systems, zero will signal the current process group, -1 will signal all processes, and any other negative PROCESS number will act as a negative signal number and kill the entire process group specified.
If both the SIGNAL and the PROCESS are negative, the results are undefined. A warning may be produced in a future version.
See "Signals" in perlipc for more details.
On some platforms such as Windows where the fork(2) system call is not available, Perl can be built to emulate
at the interpreter level. This emulation has limitations related to kill that have to be considered, for code running on Windows and in code intended to be portable.See perlfork for more details.
If there is no LIST of processes, no signal is sent, and the return value is 0. This form is sometimes used, however, because it causes tainting checks to be run, if your perl support taint checks. But see "Laundering and Detecting Tainted Data" in perlsec.
Portability issues: "kill" in perlport.
- last LABEL
- last EXPR
- last
command is like thebreak
statement in C (as used in loops); it immediately exits the loop in question. If the LABEL is omitted, the command refers to the innermost enclosing loop. Thelast EXPR
form, available starting in Perl 5.18.0, allows a label name to be computed at run time, and is otherwise identical tolast LABEL
. Thecontinue
block, if any, is not executed:LINE: while (<STDIN>) { last LINE if /^$/; # exit when done with header #... }
cannot return a value from a block that typically returns a value, such aseval {}
,sub {}
, ordo {}
. It will perform its flow control behavior, which precludes any return value. It should not be used to exit agrep
operation.Note that a block by itself is semantically identical to a loop that executes once. Thus
can be used to effect an early exit out of such a block.See also
for an illustration of howlast
, andredo
work.Unlike most named operators, this has the same precedence as assignment. It is also exempt from the looks-like-a-function rule, so
last ("foo")."bar"
will cause "bar" to be part of the argument tolast
. - lc EXPR
- lc
Returns a lowercased version of EXPR. If EXPR is omitted, uses
.my $str = lc("Perl is GREAT"); # "perl is great"
What gets returned depends on several factors:
- If
use bytes
is in effect: -
The results follow ASCII rules. Only the characters
change, toa-z
respectively. - Otherwise, if
use locale
is in effect: -
Respects current
locale for code points < 256; and uses Unicode rules for the remaining code points (this last can only happen if the UTF8 flag is also set). See perllocale.Starting in v5.20, Perl uses full Unicode rules if the locale is UTF-8. Otherwise, there is a deficiency in this scheme, which is that case changes that cross the 255/256 boundary are not well-defined. For example, the lower case of LATIN CAPITAL LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII platforms). But under
use locale
(prior to v5.20 or not a UTF-8 locale), the lower case of U+1E9E is itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the current locale, and Perl has no way of knowing if that character even exists in the locale, much less what code point it is. Perl returns a result that is above 255 (almost always the input character unchanged), for all instances (and there aren't many) where the 255/256 boundary would otherwise be crossed; and starting in v5.22, it raises a locale warning. - Otherwise, If EXPR has the UTF8 flag set:
Unicode rules are used for the case change.
- Otherwise, if
use feature 'unicode_strings'
oruse locale ':not_characters'
is in effect: -
Unicode rules are used for the case change.
- Otherwise:
ASCII rules are used for the case change. The lowercase of any character outside the ASCII range is the character itself.
Note: This is the internal function implementing the
escape in double-quoted strings.my $str = "Perl is \LGREAT\E"; # "Perl is great"
- If
- lcfirst EXPR
- lcfirst
Returns the value of EXPR with the first character lowercased. This is the internal function implementing the
escape in double-quoted strings.If EXPR is omitted, uses
.This function behaves the same way under various pragmas, such as in a locale, as
does. - length EXPR
- length
Returns the length in characters of the value of EXPR. If EXPR is omitted, returns the length of
. If EXPR is undefined, returnsundef
.This function cannot be used on an entire array or hash to find out how many elements these have. For that, use
scalar @array
andscalar keys %hash
, respectively.Like all Perl character operations,
normally deals in logical characters, not physical bytes. For how many bytes a string encoded as UTF-8 would take up, uselength(Encode::encode('UTF-8', EXPR))
(you'll have touse Encode
first). See Encode and perlunicode. - __LINE__
A special token that compiles to the current line number. It can be altered by the mechanism described at "Plain Old Comments (Not!)" in perlsyn.
- link OLDFILE,NEWFILE
Creates a new filename linked to the old filename. Returns true for success, false otherwise.
Portability issues: "link" in perlport.
- listen SOCKET,QUEUESIZE
Does the same thing that the listen(2) system call does. Returns true if it succeeded, false otherwise. See the example in "Sockets: Client/Server Communication" in perlipc.
- local EXPR
You really probably want to be using
instead, becauselocal
isn't what most people think of as "local". See "Private Variables via my()" in perlsub for details.A local modifies the listed variables to be local to the enclosing block, file, or eval. If more than one value is listed, the list must be placed in parentheses. See "Temporary Values via local()" in perlsub for details, including issues with tied arrays and hashes.
, andour
can operate on a variable anywhere it appears in an expression (aside from interpolation in strings). Unlike the other declarations, the effect oflocal
happens at runtime, and so it will apply to additional uses of the same variable executed after the declaration, even within the same statement. Note that this does not include uses within an expression assigned to the variable when it is localized, because the assigned expression is evaluated before the localization.package main; our $x = 2; { foo($x, local $x = $x + 1, $x); # foo() receives (2, 3, 3) # $main::x is 3 within the call to foo() } foo($x); # foo() receives (2) and $main::x is 2
delete local EXPR
construct can also be used to localize the deletion of array/hash elements to the current block. See "Localized deletion of elements of composite types" in perlsub. - localtime EXPR
- localtime
Converts a time as returned by the time function to a 9-element list with the time analyzed for the local time zone. Typically used as follows:
# 0 1 2 3 4 5 6 7 8 my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime(time);
All list elements are numeric and come straight out of the C 'struct tm'.
, and$hour
are the seconds, minutes, and hours of the specified time.$mday
is the day of the month and$mon
the month in the range0..11
, with 0 indicating January and 11 indicating December. This makes it easy to get a month name from a list:my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec); print "$abbr[$mon] $mday"; # $mon=9, $mday=18 gives "Oct 18"
contains the number of years since 1900. To get the full year write:$year += 1900;
To get the last two digits of the year (e.g., "01" in 2001) do:
$year = sprintf("%02d", $year % 100);
is the day of the week, with 0 indicating Sunday and 3 indicating Wednesday.$yday
is the day of the year, in the range0..364
in leap years.)$isdst
is true if the specified time occurs when Daylight Saving Time is in effect, false otherwise.If EXPR is omitted,
uses the current time (as returned bytime
).In scalar context,
returns the ctime(3) value:my $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
This scalar value is always in English, and is not locale-dependent. To get similar but locale-dependent date strings, try for example:
use POSIX qw(strftime); my $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; # or for GMT formatted appropriately for your locale: my $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
will be formatted according to the current LC_TIME locale the program or thread is running in. See perllocale for how to set up and change that locale. Note that%a
, the short forms of the day of the week and the month of the year, may not necessarily be three characters wide.The Time::gmtime and Time::localtime modules provide a convenient, by-name access mechanism to the
functions, respectively.For a comprehensive date and time representation look at the DateTime module on CPAN.
For GMT instead of local time use the
builtin.See also the
module (for converting seconds, minutes, hours, and such back to the integer value returned bytime
), and the POSIX module'smktime
function.Portability issues: "localtime" in perlport.
- lock THING
This function places an advisory lock on a shared variable or referenced object contained in THING until the lock goes out of scope.
The value returned is the scalar itself, if the argument is a scalar, or a reference, if the argument is a hash, array or subroutine.
is a "weak keyword"; this means that if you've defined a function by this name (before any calls to it), that function will be called instead. If you are not underuse threads::shared
this does nothing. See threads::shared. - log EXPR
- log
Returns the natural logarithm (base e) of EXPR. If EXPR is omitted, returns the log of
. To get the log of another base, use basic algebra: The base-N log of a number is equal to the natural log of that number divided by the natural log of N. For example:sub log10 { my $n = shift; return log($n)/log(10); }
See also
for the inverse operation. - lstat FILEHANDLE
- lstat EXPR
- lstat DIRHANDLE
- lstat
Does the same thing as the
function (including setting the special_
filehandle) but stats a symbolic link instead of the file the symbolic link points to. If symbolic links are unimplemented on your system, a normalstat
is done. For much more detailed information, please see the documentation forstat
.If EXPR is omitted, stats
.Portability issues: "lstat" in perlport.
- m//
The match operator. See "Regexp Quote-Like Operators" in perlop.
- map BLOCK LIST
- map EXPR,LIST
Evaluates the BLOCK or EXPR for each element of LIST (locally setting
to each element) and composes a list of the results of each such evaluation. Each element of LIST may produce zero, one, or more elements in the generated list, so the number of elements in the generated list may differ from that in LIST. In scalar context, returns the total number of elements so generated. In list context, returns the generated list.my @chars = map(chr, @numbers);
translates a list of numbers to the corresponding characters.
my @squares = map { $_ * $_ } @numbers;
translates a list of numbers to their squared values.
my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
shows that number of returned elements can differ from the number of input elements. To omit an element, return an empty list (). This could also be achieved by writing
my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
which makes the intention more clear.
Map always returns a list, which can be assigned to a hash such that the elements become key/value pairs. See perldata for more details.
my %hash = map { get_a_key_for($_) => $_ } @array;
is just a funny way to write
my %hash; foreach (@array) { $hash{get_a_key_for($_)} = $_; }
Note that
is an alias to the list value, so it can be used to modify the elements of the LIST. While this is useful and supported, it can cause bizarre results if the elements of LIST are not variables. Using a regularforeach
loop for this purpose would be clearer in most cases. See alsogrep
for a list composed of those items of the original list for which the BLOCK or EXPR evaluates to true.{
starts both hash references and blocks, somap { ...
could be either the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look ahead for the closing}
it has to take a guess at which it's dealing with based on what it finds just after the{
. Usually it gets it right, but if it doesn't it won't realize something is wrong until it gets to the}
and encounters the missing (or unexpected) comma. The syntax error will be reported close to the}
, but you'll need to change something near the{
such as using a unary+
or semicolon to give Perl some help:my %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong my %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right my %hash = map {; "\L$_" => 1 } @array # this also works my %hash = map { ("\L$_" => 1) } @array # as does this my %hash = map { lc($_) => 1 } @array # and this. my %hash = map +( lc($_) => 1 ), @array # this is EXPR and works! my %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
or to force an anon hash constructor use
:my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs # comma at end
to get a list of anonymous hashes each with only one entry apiece.
- method NAME BLOCK
- method NAME : ATTRS BLOCK
Creates a new named method in the scope of the class that it appears within. This is only valid inside a
declaration. - mkdir FILENAME,MODE
- mkdir FILENAME
- mkdir
Creates the directory specified by FILENAME, with permissions specified by MODE (as modified by
). If it succeeds it returns true; otherwise it returns false and sets$!
(errno). MODE defaults to 0777 if omitted, and FILENAME defaults to$_
if omitted.In general, it is better to create directories with a permissive MODE and let the user modify that with their
than it is to supply a restrictive MODE and give the user no way to be more permissive. The exceptions to this rule are when the file or directory should be kept private (mail files, for instance). The documentation forumask
discusses the choice of MODE in more detail. If bits in MODE other than the permission bits are set, the result may be implementation defined, per POSIX 1003.1-2008.Note that according to the POSIX 1003.1-1996 the FILENAME may have any number of trailing slashes. Some operating and filesystems do not get this right, so Perl automatically removes all trailing slashes to keep everyone happy.
To recursively create a directory structure, look at the
function of the File::Path module. - msgctl ID,CMD,ARG
Calls the System V IPC function msgctl(2). You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is
, then ARG must be a variable that will hold the returnedmsqid_ds
structure. Returns likeioctl
: the undefined value for error,"0 but true"
for zero, or the actual return value otherwise. See also "SysV IPC" in perlipc and the documentation forIPC::SysV
.Portability issues: "msgctl" in perlport.
- msgget KEY,FLAGS
Calls the System V IPC function msgget(2). Returns the message queue id, or
on error. See also "SysV IPC" in perlipc and the documentation forIPC::SysV
.Portability issues: "msgget" in perlport.
- msgrcv ID,VAR,SIZE,TYPE,FLAGS
Calls the System V IPC function msgrcv to receive a message from message queue ID into variable VAR with a maximum message size of SIZE. Note that when a message is received, the message type as a native long integer will be the first thing in VAR, followed by the actual message. This packing may be opened with
unpack("l! a*")
. Taints the variable. Returns true if successful, false on error. See also "SysV IPC" in perlipc and the documentation forIPC::SysV
.Portability issues: "msgrcv" in perlport.
- msgsnd ID,MSG,FLAGS
Calls the System V IPC function msgsnd to send the message MSG to the message queue ID. MSG must begin with the native long integer message type, followed by the message itself. This kind of packing can be achieved with
pack("l! a*", $type, $message)
. Returns true if successful, false on error. See also "SysV IPC" in perlipc and the documentation forIPC::SysV
.Portability issues: "msgsnd" in perlport.
- my VARLIST
- my TYPE VARLIST
- my VARLIST : ATTRS
- my TYPE VARLIST : ATTRS
declares the listed variables to be local (lexically) to the enclosing block, file, oreval
. If more than one variable is listed, the list must be placed in parentheses.Note that with a parenthesised list,
can be used as a dummy placeholder, for example to skip assignment of initial values:my ( undef, $min, $hour ) = localtime;
, andour
can operate on a variable anywhere it appears in an expression (aside from interpolation in strings). The declaration will not apply to additional uses of the same variable until the next statement. This means additional uses of that variable within the same statement will act as they would have before that declaration occurred, or result in a strict 'vars' error, as appropriate.package main; our $x = 2; foo($x, my $x = $x + 1, $x); # foo() receives (2, 3, 2) foo($x, $main::x); # foo() receives (3, 2)
Redeclaring a variable in the same scope or statement will "shadow" the previous declaration, creating a new instance and preventing access to the previous one. This is usually undesired and, if warnings are enabled, will result in a warning in the
category.The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE may be a bareword, a constant declared with
use constant
, or__PACKAGE__
. It is currently bound to the use of the fields pragma, and attributes are handled using the attributes pragma, or starting from Perl 5.8.0 also via the Attribute::Handlers module. See "Private Variables via my()" in perlsub for details. - next LABEL
- next EXPR
- next
command is like thecontinue
statement in C; it starts the next iteration of the loop:LINE: while (<STDIN>) { next LINE if /^#/; # discard comments #... }
Note that if there were a
block on the above, it would get executed even on discarded lines. If LABEL is omitted, the command refers to the innermost enclosing loop. Thenext EXPR
form, available as of Perl 5.18.0, allows a label name to be computed at run time, being otherwise identical tonext LABEL
cannot return a value from a block that typically returns a value, such aseval {}
,sub {}
, ordo {}
. It will perform its flow control behavior, which precludes any return value. It should not be used to exit agrep
operation.Note that a block by itself is semantically identical to a loop that executes once. Thus
will exit such a block early.See also
for an illustration of howlast
, andredo
work.Unlike most named operators, this has the same precedence as assignment. It is also exempt from the looks-like-a-function rule, so
next ("foo")."bar"
will cause "bar" to be part of the argument tonext
. - no MODULE VERSION LIST
- no MODULE VERSION
- no MODULE LIST
- no MODULE
- no VERSION
- oct EXPR
- oct
Interprets EXPR as an octal string and returns the corresponding value. An octal string consists of octal digits and, as of Perl 5.33.5, an optional
prefix. Each octal digit may be preceded by a single underscore, which will be ignored. (If EXPR happens to start off with0x
, interprets it as a hex string. If EXPR starts off with0b
, it is interpreted as a binary string. Leading whitespace is ignored in all three cases.) The following will handle decimal, binary, octal, and hex in standard Perl notation:$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses
. To go the other way (produce a number in octal), usesprintf
:my $dec_perms = (stat("filename"))[2] & 07777; my $oct_perm_str = sprintf "%o", $perms;
function is commonly used when a string such as644
needs to be converted into a file mode, for example. Although Perl automatically converts strings into numbers as needed, this automatic conversion assumes base 10.Leading white space is ignored without warning, as too are any trailing non-digits, such as a decimal point (
only handles non-negative integers, not negative integers or floating point). - open FILEHANDLE,MODE,EXPR
- open FILEHANDLE,MODE,EXPR,LIST
- open FILEHANDLE,MODE,REFERENCE
- open FILEHANDLE,EXPR
- open FILEHANDLE
Associates an internal FILEHANDLE with the external file specified by EXPR. That filehandle will subsequently allow you to perform I/O operations on that file, such as reading from it or writing to it.
Instead of a filename, you may specify an external command (plus an optional argument list) or a scalar reference, in order to open filehandles on commands or in-memory scalars, respectively.
A thorough reference to
follows. For a gentler introduction to the basics ofopen
, see also the perlopentut manual page.- Working with files
Most often,
gets invoked with three arguments: the required FILEHANDLE (usually an empty scalar variable), followed by MODE (usually a literal describing the I/O mode the filehandle will use), and then the filename that the new filehandle will refer to.- Simple examples
Reading from a file:
open(my $fh, "<", "input.txt") or die "Can't open < input.txt: $!"; # Process every line in input.txt while (my $line = readline($fh)) { # # ... do something interesting with $line here ... # }
or writing to one:
open(my $fh, ">", "output.txt") or die "Can't open > output.txt: $!"; print $fh "This line gets printed into output.txt.\n";
For a summary of common filehandle operations such as these, see "Files and I/O" in perlintro.
- About filehandles
The first argument to
, labeled FILEHANDLE in this reference, is usually a scalar variable. (Exceptions exist, described in "Other considerations", below.) If the call toopen
succeeds, then the expression provided as FILEHANDLE will get assigned an open filehandle. That filehandle provides an internal reference to the specified external file, conveniently stored in a Perl variable, and ready for I/O operations such as reading and writing. - About modes
When calling
with three or more arguments, the second argument -- labeled MODE here -- defines the open mode. MODE is usually a literal string comprising special characters that define the intended I/O role of the filehandle being created: whether it's read-only, or read-and-write, and so on.If MODE is
, the file is opened for input (read-only). If MODE is>
, the file is opened for output, with existing files first being truncated ("clobbered") and nonexisting files newly created. If MODE is>>
, the file is opened for appending, again being created if necessary.You can put a
in front of the>
to indicate that you want both read and write access to the file; thus+<
is almost always preferred for read/write updates--the+>
mode would clobber the file first. You can't usually use either read-write mode for updating textfiles, since they have variable-length records. See the -i switch in perlrun for a better approach. The file is created with permissions of0666
modified by the process'sumask
value.These various prefixes correspond to the fopen(3) modes of
, anda+
.More examples of different modes in action:
# Open a file for concatenation open(my $log, ">>", "/usr/spool/news/twitlog") or warn "Couldn't open log file; discarding input"; # Open a file for reading and writing open(my $dbase, "+<", "dbase.mine") or die "Can't open 'dbase.mine' for update: $!";
- Checking the return value
Open returns nonzero on success, the undefined value otherwise. If the
involved a pipe, the return value happens to be the pid of the subprocess.When opening a file, it's seldom a good idea to continue if the request failed, so
is frequently used withdie
. Even if you want your code to do something other thandie
on a failed open, you should still always check the return value from opening a file.
- Specifying I/O layers in MODE
You can use the three-argument form of open to specify I/O layers (sometimes referred to as "disciplines") to apply to the new filehandle. These affect how the input and output are processed (see open and PerlIO for more details). For example:
# loads PerlIO::encoding automatically open(my $fh, "<:encoding(UTF-8)", $filename) || die "Can't open UTF-8 encoded $filename: $!";
This opens the UTF8-encoded file containing Unicode characters; see perluniintro. Note that if layers are specified in the three-argument form, then default layers stored in
(usually set by the open pragma or the switch-CioD
) are ignored. Those layers will also be ignored if you specify a colon with no name following it. In that case the default layer for the operating system (:raw on Unix, :crlf on Windows) is used.On some systems (in general, DOS- and Windows-based systems)
is necessary when you're not working with a text file. For the sake of portability it is a good idea always to use it when appropriate, and never to use it when it isn't appropriate. Also, people can set their I/O to be by default UTF8-encoded Unicode, not bytes. - Using
for temporary files -
As a special case the three-argument form with a read/write mode and the third argument being
:open(my $tmp, "+>", undef) or die ...
opens a filehandle to a newly created empty anonymous temporary file. (This happens under any mode, which makes
the only useful and sensible mode to use.) You will need toseek
to do the reading. - Opening a filehandle into an in-memory scalar
You can open filehandles directly to Perl scalars instead of a file or other resource external to the program. To do so, provide a reference to that scalar as the third argument to
, like so:open(my $memory, ">", \$var) or die "Can't open memory file: $!"; print $memory "foo!\n"; # output will appear in $var
To (re)open
as an in-memory file, close it first:close STDOUT; open(STDOUT, ">", \$variable) or die "Can't open STDOUT: $!";
The scalars for in-memory files are treated as octet strings: unless the file is being opened with truncation the scalar may not contain any code points over 0xFF.
Opening in-memory files can fail for a variety of reasons. As with any other
, check the return value for success.Technical note: This feature works only when Perl is built with PerlIO -- the default, except with older (pre-5.16) Perl installations that were configured to not include it (e.g. via
Configure -Uuseperlio
). You can see whether your Perl was built with PerlIO by runningperl -V:useperlio
. If it says'define'
, you have PerlIO; otherwise you don't.See perliol for detailed info on PerlIO.
- Opening a filehandle into a command
If MODE is
, then the filename is interpreted as a command to which output is to be piped, and if MODE is-|
, the filename is interpreted as a command that pipes output to us. In the two-argument (and one-argument) form, one should replace dash (-
) with the command. See "Using open() for IPC" in perlipc for more examples of this. (You are not allowed toopen
to a command that pipes both in and out, but see IPC::Open2, IPC::Open3, and "Bidirectional Communication with Another Process" in perlipc for alternatives.)open(my $article_fh, "-|", "caesar <$article") # decrypt # article or die "Can't start caesar: $!"; open(my $article_fh, "caesar <$article |") # ditto or die "Can't start caesar: $!"; open(my $out_fh, "|-", "sort >Tmp$$") # $$ is our process id or die "Can't start sort: $!";
In the form of pipe opens taking three or more arguments, if LIST is specified (extra arguments after the command name) then LIST becomes arguments to the command invoked if the platform supports it. The meaning of
with more than three arguments for non-pipe modes is not yet defined, but experimental "layers" may give extra LIST arguments meaning.If you open a pipe on the command
(that is, specify either|-
with the one- or two-argument forms ofopen
), an implicitfork
is done, soopen
returns twice: in the parent process it returns the pid of the child process, and in the child process it returns (a defined)0
. Usedefined($pid)
to determine whether the open was successful.For example, use either
my $child_pid = open(my $from_kid, "-|") // die "Can't fork: $!";
my $child_pid = open(my $to_kid, "|-") // die "Can't fork: $!";
followed by
if ($child_pid) { # am the parent: # either write $to_kid or else read $from_kid ... waitpid $child_pid, 0; } else { # am the child; use STDIN/STDOUT normally ... exit; }
The filehandle behaves normally for the parent, but I/O to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child process, the filehandle isn't opened--I/O happens from/to the new STDOUT/STDIN. Typically this is used like the normal piped open when you want to exercise more control over just how the pipe command gets executed, such as when running setuid and you don't want to have to scan shell commands for metacharacters.
The following blocks are more or less equivalent:
open(my $fh, "|tr '[a-z]' '[A-Z]'"); open(my $fh, "|-", "tr '[a-z]' '[A-Z]'"); open(my $fh, "|-") || exec 'tr', '[a-z]', '[A-Z]'; open(my $fh, "|-", "tr", '[a-z]', '[A-Z]'); open(my $fh, "cat -n '$file'|"); open(my $fh, "-|", "cat -n '$file'"); open(my $fh, "-|") || exec "cat", "-n", $file; open(my $fh, "-|", "cat", "-n", $file);
The last two examples in each block show the pipe as "list form", which is not yet supported on all platforms. (If your platform has a real
, such as Linux and macOS, you can use the list form; it also works on Windows with Perl 5.22 or later.) You would want to use the list form of the pipe so you can pass literal arguments to the command without risk of the shell interpreting any shell metacharacters in them. However, this also bars you from opening pipes to commands that intentionally contain shell metacharacters, such as:open(my $fh, "|cat -n | expand -4 | lpr") || die "Can't open pipeline to lpr: $!";
See "Safe Pipe Opens" in perlipc for more examples of this.
- Duping filehandles
You may also, in the Bourne shell tradition, specify an EXPR beginning with
, in which case the rest of the string is interpreted as the name of a filehandle (or file descriptor, if numeric) to be duped (as in dup(2)) and opened. You may use&
, and+<
. The mode you specify should match the mode of the original filehandle. (Duping a filehandle does not take into account any existing contents of IO buffers.) If you use the three-argument form, then you can pass either a number, the name of a filehandle, or the normal "reference to a glob".Here is a script that saves, redirects, and restores
using various methods:#!/usr/bin/perl open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!"; open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!"; open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!"; open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!"; select STDERR; $| = 1; # make unbuffered select STDOUT; $| = 1; # make unbuffered print STDOUT "stdout 1\n"; # this works for print STDERR "stderr 1\n"; # subprocesses too open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!"; open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!"; print STDOUT "stdout 2\n"; print STDERR "stderr 2\n";
If you specify
, whereX
is a file descriptor number or a filehandle, then Perl will do an equivalent of C's fdopen(3) of that file descriptor (and not call dup(2)); this is more parsimonious of file descriptors. For example:# open for input, reusing the fileno of $fd open(my $fh, "<&=", $fd)
open(my $fh, "<&=$fd")
# open for append, using the fileno of $oldfh open(my $fh, ">>&=", $oldfh)
Being parsimonious on filehandles is also useful (besides being parsimonious) for example when something is dependent on file descriptors, like for example locking using
. If you do justopen(my $A, ">>&", $B)
, the filehandle$A
will not have the same file descriptor as$B
, and thereforeflock($A)
will notflock($B)
nor vice versa. But withopen(my $A, ">>&=", $B)
, the filehandles will share the same underlying system file descriptor.Note that under Perls older than 5.8.0, Perl uses the standard C library's' fdopen(3) to implement the
functionality. On many Unix systems, fdopen(3) fails when file descriptors exceed a certain value, typically 255. For Perls 5.8.0 and later, PerlIO is (most often) the default. - Legacy usage
This section describes ways to call
outside of best practices; you may encounter these uses in older code. Perl does not consider their use deprecated, exactly, but neither is it recommended in new code, for the sake of clarity and readability.- Specifying mode and filename as a single argument
In the one- and two-argument forms of the call, the mode and filename should be concatenated (in that order), preferably separated by white space. You can--but shouldn't--omit the mode in these forms when that mode is
. It is safe to use the two-argument form ofopen
if the filename argument is a known literal.open(my $dbase, "+<dbase.mine") # ditto or die "Can't open 'dbase.mine' for update: $!";
In the two-argument (and one-argument) form, opening
opens STDIN and opening>-
opens STDOUT.New code should favor the three-argument form of
over this older form. Declaring the mode and the filename as two distinct arguments avoids any confusion between the two. - Assigning a filehandle to a bareword
An older style is to use a bareword as the filehandle, as
open(FH, "<", "input.txt") or die "Can't open < input.txt: $!";
Then you can use
as the filehandle, inclose FH
and so on. Note that it's a global variable, so this form is not recommended when dealing with filehandles other than Perl's built-in ones (e.g. STDOUT and STDIN). In fact, using a bareword for the filehandle is an error when the"bareword_filehandles"
feature has been disabled. This feature is disabled automatically when in the scope ofuse v5.36.0
or later. - Calling
with one argument via global variables -
As a shortcut, a one-argument call takes the filename from the global scalar variable of the same name as the bareword filehandle:
$ARTICLE = 100; open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
must be a global scalar variable in the same package as the filehandle - not one declared withmy
- Other considerations
- Automatic filehandle closure
The filehandle will be closed when its reference count reaches zero. If it is a lexically scoped variable declared with
, that usually means the end of the enclosing scope. However, this automatic close does not check for errors, so it is better to explicitly close filehandles, especially those used for writing:close($handle) || warn "close failed: $!";
- Automatic pipe flushing
Perl will attempt to flush all files opened for output before any operation that may do a fork, but this may not be supported on some platforms (see perlport). To be safe, you may need to set
in English) or call theautoflush
method ofIO::Handle
on any open handles.On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor as determined by the value of
. See "$^F" in perlvar.Closing any piped filehandle causes the parent process to wait for the child to finish, then returns the status value in
. - Direct versus by-reference assignment of filehandles
If FILEHANDLE -- the first argument in a call to
-- is an undefined scalar variable (or array or hash element), a new filehandle is autovivified, meaning that the variable is assigned a reference to a newly allocated anonymous filehandle. Otherwise if FILEHANDLE is an expression, its value is the real filehandle. (This is considered a symbolic reference, souse strict "refs"
should not be in effect.) - Whitespace and special characters in the filename argument
The filename passed to the one- and two-argument forms of
will have leading and trailing whitespace deleted and normal redirection characters honored. This property, known as "magic open", can often be used to good effect. A user could specify a filename of "rsh cat file |", or you could change certain filenames as needed:$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; open(my $fh, $filename) or die "Can't open $filename: $!";
Use the three-argument form to open a file with arbitrary weird characters in it,
open(my $fh, "<", $file) || die "Can't open $file: $!";
otherwise it's necessary to protect any leading and trailing whitespace:
$file =~ s#^(\s)#./$1#; open(my $fh, "< $file\0") || die "Can't open $file: $!";
(this may not work on some bizarre filesystems). One should conscientiously choose between the magic and three-argument form of
:open(my $in, $ARGV[0]) || die "Can't open $ARGV[0]: $!";
will allow the user to specify an argument of the form
"rsh cat file |"
, but will not work on a filename that happens to have a trailing space, whileopen(my $in, "<", $ARGV[0]) || die "Can't open $ARGV[0]: $!";
will have exactly the opposite restrictions. (However, some shells support the syntax
perl your_program.pl <( rsh cat file )
, which produces a filename that can be opened normally.) - Invoking C-style
If you want a "real" C open(2), then you should use the
function, which involves no such magic (but uses different filemodes than Perlopen
, which corresponds to C fopen(3)). This is another way to protect your filenames from interpretation. For example:use IO::Handle; sysopen(my $fh, $path, O_RDWR|O_CREAT|O_EXCL) or die "Can't open $path: $!"; $fh->autoflush(1); print $fh "stuff $$\n"; seek($fh, 0, 0); print "File contains: ", readline($fh);
for some details about mixing reading and writing. - Portability issues
See "open" in perlport.
- opendir DIRHANDLE,EXPR
Opens a directory named EXPR for processing by
, andclosedir
. Returns true if successful. DIRHANDLE may be an expression whose value can be used as an indirect dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined scalar variable (or array or hash element), the variable is assigned a reference to a new anonymous dirhandle; that is, it's autovivified. Dirhandles are the same objects as filehandles; an I/O object can only be open as one of these handle types at once.See the example at
. - ord EXPR
- ord
Returns the code point of the first character of EXPR. If EXPR is an empty string, returns 0. If EXPR is omitted, uses
. (Note character, not byte.)For the reverse, see
. See perlunicode for more about Unicode. - our VARLIST
- our TYPE VARLIST
- our VARLIST : ATTRS
- our TYPE VARLIST : ATTRS
makes a lexical alias to a package (i.e. global) variable of the same name in the current package for use within the current lexical scope.our
has the same scoping rules asmy
, meaning that it is only valid within a lexical scope. Unlikemy
, which both declare new (lexical) variables,our
only creates an alias to an existing variable: a package variable of the same name.This means that when
use strict 'vars'
is in effect,our
lets you use a package variable without qualifying it with the package name, but only within the lexical scope of theour
declaration. This applies immediately--even within the same statement.package Foo; use v5.36; # which implies "use strict;" $Foo::foo = 23; { our $foo; # alias to $Foo::foo print $foo; # prints 23 } print $Foo::foo; # prints 23 print $foo; # ERROR: requires explicit package name
This works even if the package variable has not been used before, as package variables spring into existence when first used.
package Foo; use v5.36; our $foo = 23; # just like $Foo::foo = 23 print $Foo::foo; # prints 23
Because the variable becomes legal immediately under
use strict 'vars'
, so long as there is no variable with that name is already in scope, you can then reference the package variable again even within the same statement.package Foo; use v5.36; my $foo = $foo; # error, undeclared $foo on right-hand side our $foo = $foo; # no errors
If more than one variable is listed, the list must be placed in parentheses.
our($bar, $baz);
, andlocal
can operate on a variable anywhere it appears in an expression (aside from interpolation in strings). The declaration will not apply to additional uses of the same variable until the next statement. This means additional uses of that variable within the same statement will act as they would have before that declaration occurred, with the exception that it will still satisfy strict 'vars' and interpret that variable as the newly aliased package variable if it was not yet declared in that scope.package main; my $x = 2; foo($x, our $x = $x + 1, $x); # foo() receives (2, 3, 2) foo($x, our $z = 5, $z); # foo() receives (3, 5, 5)
declaration declares an alias for a package variable that will be visible across its entire lexical scope, even across package boundaries. The package in which the variable is entered is determined at the point of the declaration, not at the point of use. This means the following behavior holds:package Foo; our $bar; # declares $Foo::bar for rest of lexical scope $bar = 20; package Bar; print $bar; # prints 20, as it refers to $Foo::bar
declarations with the same name in the same lexical scope are allowed if they are in different packages. If they happen to be in the same package, Perl will emit warnings if you have asked for them, just like multiplemy
declarations. Unlike a secondmy
declaration, which will bind the name to a fresh variable, a secondour
declaration in the same package, in the same scope, is merely redundant.use warnings; package Foo; our $bar; # declares $Foo::bar for rest of lexical scope $bar = 20; package Bar; our $bar = 30; # declares $Bar::bar for rest of lexical scope print $bar; # prints 30 our $bar; # emits warning but has no other effect print $bar; # still prints 30
declaration may also have a list of attributes associated with it.The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE is currently bound to the use of the fields pragma, and attributes are handled using the attributes pragma, or, starting from Perl 5.8.0, also via the Attribute::Handlers module. See "Private Variables via my()" in perlsub for details.
Note that with a parenthesised list,
can be used as a dummy placeholder, for example to skip assignment of initial values:our ( undef, $min, $hour ) = localtime;
differs fromuse vars
, which allows use of an unqualified name only within the affected package, but across scopes. - pack TEMPLATE,LIST
Takes a LIST of values and converts it into a string using the rules given by the TEMPLATE. The resulting string is the concatenation of the converted values. Typically, each converted value looks like its machine-level representation. For example, on 32-bit machines an integer may be represented by a sequence of 4 bytes, which will in Perl be presented as a string that's 4 characters long.
See perlpacktut for an introduction to this function.
The TEMPLATE is a sequence of characters that give the order and type of values, as follows:
a A string with arbitrary binary data, will be null padded. A A text (ASCII) string, will be space padded. Z A null-terminated (ASCIZ) string, will be null padded. b A bit string (ascending bit order inside each byte, like vec()). B A bit string (descending bit order inside each byte). h A hex string (low nybble first). H A hex string (high nybble first). c A signed char (8-bit) value. C An unsigned char (octet) value. W An unsigned char value (can be greater than 255). s A signed short (16-bit) value. S An unsigned short value. l A signed long (32-bit) value. L An unsigned long value. q A signed quad (64-bit) value. Q An unsigned quad value. (Quads are available only if your system supports 64-bit integer values _and_ if Perl has been compiled to support those. Raises an exception otherwise.) i A signed integer value. I An unsigned integer value. (This 'integer' is _at_least_ 32 bits wide. Its exact size depends on what a local C compiler calls 'int'.) n An unsigned short (16-bit) in "network" (big-endian) order. N An unsigned long (32-bit) in "network" (big-endian) order. v An unsigned short (16-bit) in "VAX" (little-endian) order. V An unsigned long (32-bit) in "VAX" (little-endian) order. j A Perl internal signed integer value (IV). J A Perl internal unsigned integer value (UV). f A single-precision float in native format. d A double-precision float in native format. F A Perl internal floating-point value (NV) in native format D A float of long-double precision in native format. (Long doubles are available only if your system supports long double values. Raises an exception otherwise. Note that there are different long double formats.) p A pointer to a null-terminated string. P A pointer to a structure (fixed-length string). u A uuencoded string. U A Unicode character number. Encodes to a character in char- acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode. Also on EBCDIC platforms, the character number will be the native EBCDIC value for character numbers below 256. This allows most programs using this feature to not have to care which type of platform they are running on. w A BER compressed integer (not an ASN.1 BER, see perlpacktut for details). Its bytes represent an unsigned integer in base 128, most significant digit first, with as few digits as possible. Bit eight (the high bit) is set on each byte except the last. x A null byte (a.k.a ASCII NUL, "\000", chr(0)) X Back up a byte. @ Null-fill or truncate to absolute position, counted from the start of the innermost ()-group. . Null-fill or truncate to absolute position specified by the value. ( Start of a ()-group.
One or more modifiers below may optionally follow certain letters in the TEMPLATE (the second column lists letters for which the modifier is valid):
! sSlLiI Forces native (short, long, int) sizes instead of fixed (16-/32-bit) sizes. ! xX Make x and X act as alignment commands. ! nNvV Treat integers as signed instead of unsigned. ! @. Specify position as byte offset in the internal representation of the packed string. Efficient but dangerous. > sSiIlLqQ Force big-endian byte-order on the type. jJfFdDpP (The "big end" touches the construct.) < sSiIlLqQ Force little-endian byte-order on the type. jJfFdDpP (The "little end" touches the construct.)
modifiers can also be used on()
groups to force a particular byte-order on all components in that group, including all its subgroups.The following rules apply:
Each letter may optionally be followed by a number indicating the repeat count. A numeric repeat count may optionally be enclosed in brackets, as in
pack("C[80]", @arr)
. The repeat count gobbles that many values from the LIST when used with all format types other thana
, andP
, where it means something else, described below. Supplying a*
for the repeat count instead of a number means to use however many items are left, except for:@
, andX
, where it is equivalent to0
.<.>, where it means relative to the start of the string.
, where it is equivalent to 1 (or 45, which here is equivalent).
One can replace a numeric repeat count with a template letter enclosed in brackets to use the packed byte length of the bracketed template for the repeat count.
For example, the template
skips as many bytes as in a packed long, and the template"$t X[$t] $t"
unpacks twice whatever $t (when variable-expanded) unpacks. If the template in brackets contains alignment commands (such asx![d]
), its packed length is calculated as if the start of the template had the maximal possible alignment.When used with
, a*
as the repeat count is guaranteed to add a trailing null byte, so the resulting string is always one byte longer than the byte length of the item itself.When used with
, the repeat count represents an offset from the start of the innermost()
group.When used with
, the repeat count determines the starting position to calculate the value offset as follows:If the repeat count is
, it's relative to the current position.If the repeat count is
, the offset is relative to the start of the packed string.And if it's an integer n, the offset is relative to the start of the nth innermost
( )
group, or to the start of the string if n is bigger then the group level.
The repeat count for
is interpreted as the maximal number of bytes to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat count should not be more than 65.The
, andZ
types gobble just one value, but pack it as a string of length count, padding with nulls or spaces as needed. When unpacking,A
strips trailing whitespace and nulls,Z
strips everything after the first null, anda
returns data with no stripping at all.If the value to pack is too long, the result is truncated. If it's too long and an explicit count is provided,
packs only$count-1
bytes, followed by a null byte. ThusZ
always packs a trailing null, except when the count is 0.Likewise, the
formats pack a string that's that many bits long. Each such format generates 1 bit of the result. These are typically followed by a repeat count likeB8
.Each result bit is based on the least-significant bit of the corresponding input character, i.e., on
. In particular, characters"0"
generate bits 0 and 1, as do characters"\000"
.Starting from the beginning of the input string, each 8-tuple of characters is converted to 1 character of output. With format
, the first character of the 8-tuple determines the least-significant bit of a character; with formatB
, it determines the most-significant bit of a character.If the length of the input string is not evenly divisible by 8, the remainder is packed as if the input string were padded by null characters at the end. Similarly during unpacking, "extra" bits are ignored.
If the input string is longer than needed, remaining characters are ignored.
for the repeat count uses all characters of the input field. On unpacking, bits are converted to a string of0
s and1
formats pack a string that many nybbles (4-bit groups, representable as hexadecimal digits,"0".."9"
) long.For each such format,
generates 4 bits of result. With non-alphabetical characters, the result is based on the 4 least-significant bits of the input character, i.e., onord($char)%16
. In particular, characters"0"
generate nybbles 0 and 1, as do bytes"\000"
. For characters"a".."f"
, the result is compatible with the usual hexadecimal digits, so that"a"
both generate the nybble0xA==10
. Use only these specific hex characters with this format.Starting from the beginning of the template to
, each pair of characters is converted to 1 character of output. With formath
, the first character of the pair determines the least-significant nybble of the output character; with formatH
, it determines the most-significant nybble.If the length of the input string is not even, it behaves as if padded by a null character at the end. Similarly, "extra" nybbles are ignored during unpacking.
If the input string is longer than needed, extra characters are ignored.
for the repeat count uses all characters of the input field. Forunpack
, nybbles are converted to a string of hexadecimal digits.The
format packs a pointer to a null-terminated string. You are responsible for ensuring that the string is not a temporary value, as that could potentially get deallocated before you got around to using the packed result. TheP
format packs a pointer to a structure of the size indicated by the length. A null pointer is created if the corresponding value forp
; similarly withunpack
, where a null pointer unpacks intoundef
.If your system has a strange pointer size--meaning a pointer is neither as big as an int nor as big as a long--it may not be possible to pack or unpack pointers in big- or little-endian byte order. Attempting to do so raises an exception.
template character allows packing and unpacking of a sequence of items where the packed structure contains a packed item count followed by the packed items themselves. This is useful when the structure you're unpacking has encoded the sizes or repeat counts for some of its fields within the structure itself as separate fields.For
, you write length-item/
sequence-item, and the length-item describes how the length value is packed. Formats likely to be of most use are integer-packing ones liken
for Java strings,w
for ASN.1 or SNMP, andN
for Sun XDR.For
, sequence-item may have a repeat count, in which case the minimum of that and the number of available items is used as the argument for length-item. If it has no repeat count or uses a '*', the number of available items is used.For
, an internal stack of integer arguments unpacked so far is used. You write/
sequence-item and the repeat count is obtained by popping off the last element from the stack. The sequence-item must not have a repeat count.If sequence-item refers to a string type (
, or"Z"
), the length-item is the string length, not the number of strings. With an explicit repeat count for pack, the packed string is adjusted to that length. For example:This code: gives this result: unpack("W/a", "\004Gurusamy") ("Guru") unpack("a3/A A*", "007 Bond J ") (" Bond", "J") unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".") pack("n/a* w/a","hello,","world") "\000\006hello,\005world" pack("a/W2", ord("a") .. ord("z")) "2ab"
The length-item is not returned explicitly from
.Supplying a count to the length-item format letter is only useful with
, orZ
. Packing with a length-item ofa
may introduce"\000"
characters, which Perl does not regard as legal in numeric strings.The integer types
, andL
may be followed by a!
modifier to specify native shorts or longs. As shown in the example above, a barel
means exactly 32 bits, although the nativelong
as seen by the local C compiler may be larger. This is mainly an issue on 64-bit platforms. You can see whether using!
makes any difference this way:printf "format s is %d, s! is %d\n", length pack("s"), length pack("s!"); printf "format l is %d, l! is %d\n", length pack("l"), length pack("l!");
are also allowed, but only for completeness' sake: they are identical toi
.The actual sizes (in bytes) of native shorts, ints, longs, and long longs on the platform where Perl was built are also available from the command line:
$ perl -V:{short,int,long{,long}}size shortsize='2'; intsize='4'; longsize='4'; longlongsize='8';
or programmatically via the
module:use Config; print $Config{shortsize}, "\n"; print $Config{intsize}, "\n"; print $Config{longsize}, "\n"; print $Config{longlongsize}, "\n";
is undefined on systems without long long support.The integer formats
, andJ
are inherently non-portable between processors and operating systems because they obey native byteorder and endianness. For example, a 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively (arranged in and handled by the CPU registers) into bytes as0x12 0x34 0x56 0x78 # big-endian 0x78 0x56 0x34 0x12 # little-endian
Basically, Intel and VAX CPUs are little-endian, while everybody else, including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used) them in little-endian mode, but SGI/Cray uses them in big-endian mode.
The names big-endian and little-endian are comic references to the egg-eating habits of the little-endian Lilliputians and the big-endian Blefuscudians from the classic Jonathan Swift satire, Gulliver's Travels. This entered computer lingo via the paper "On Holy Wars and a Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
Some systems may have even weirder byte orders such as
0x56 0x78 0x12 0x34 0x34 0x12 0x78 0x56
These are called mid-endian, middle-endian, mixed-endian, or just weird.
You can determine your system endianness with this incantation:
printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
The byteorder on the platform where Perl was built is also available via Config:
use Config; print "$Config{byteorder}\n";
or from the command line:
$ perl -V:byteorder
are little-endian;"4321"
are big-endian. Systems with multiarchitecture binaries will have"ffff"
, signifying that static information doesn't work, one must use runtime probing.For portably packed integers, either use the formats
, andV
or else use the>
modifiers described immediately below. See also perlport.Also floating point numbers have endianness. Usually (but not always) this agrees with the integer endianness. Even though most platforms these days use the IEEE 754 binary format, there are differences, especially if the long doubles are involved. You can see the
): the "kind" values are enums, unlikebyteorder
.Portability-wise the best option is probably to keep to the IEEE 754 64-bit doubles, and of agreed-upon endianness. Another possibility is the
) format ofprintf
.Starting with Perl 5.10.0, integer and floating-point formats, along with the
formats and()
groups, may all be followed by the>
endianness modifiers to respectively enforce big- or little-endian byte-order. These modifiers are especially useful given hown
, andV
don't cover signed integers, 64-bit integers, or floating-point values.Here are some concerns to keep in mind when using an endianness modifier:
Exchanging signed integers between different platforms works only when all platforms store them in the same format. Most platforms store signed integers in two's-complement notation, so usually this is not an issue.
modifiers can only be used on floating-point formats on big- or little-endian machines. Otherwise, attempting to use them raises an exception.Forcing big- or little-endian byte-order on floating-point values for data exchange can work only if all platforms use the same binary representation such as IEEE floating-point. Even if all platforms are using IEEE, there may still be subtle differences. Being able to use
on floating-point values can be useful, but also dangerous if you don't know exactly what you're doing. It is not a general way to portably store floating-point values.When using
on a()
group, this affects all types inside the group that accept byte-order modifiers, including all subgroups. It is silently ignored for all other types. You are not allowed to override the byte-order within a group that already has a byte-order modifier suffix.
Real numbers (floats and doubles) are in native machine format only. Due to the multiplicity of floating-point formats and the lack of a standard "network" representation for them, no facility for interchange has been made. This means that packed floating-point data written on one machine may not be readable on another, even if both use IEEE floating-point arithmetic (because the endianness of the memory representation is not part of the IEEE spec). See also perlport.
If you know exactly what you're doing, you can use the
modifiers to force big- or little-endian byte-order on floating-point values.Because Perl uses doubles (or long doubles, if configured) internally for all numeric calculation, converting from double into float and thence to double again loses precision, so
unpack("f", pack("f", $foo)
) will not in general equal $foo.Pack and unpack can operate in two modes: character mode (
mode) where the packed string is processed per character, and UTF-8 byte mode (U0
mode) where the packed string is processed in its UTF-8-encoded Unicode form on a byte-by-byte basis. Character mode is the default unless the format string starts withU
. You can always switch mode mid-format with an explicitC0
in the format. This mode remains in effect until the next mode change, or until the end of the()
group it (directly) applies to.Using
to get Unicode characters while usingU0
to get non-Unicode bytes is not necessarily obvious. Probably only the first of these is what you want:$ perl -CS -E 'say "\x{3B1}\x{3C9}"' | perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)' 03B1.03C9 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)' CE.B1.CF.89 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)' CE.B1.CF.89 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)' C3.8E.C2.B1.C3.8F.C2.89
Those examples also illustrate that you should not try to use
as a substitute for the Encode module.You must yourself do any alignment or padding by inserting, for example, enough
es while packing. There is no way forpack
to know where characters are going to or coming from, so they handle their output and input as flat sequences of characters.A
group is a sub-TEMPLATE enclosed in parentheses. A group may take a repeat count either as postfix, or forunpack
, also via the/
template character. Within each repetition of a group, positioning with@
starts over at 0. Therefore, the result ofpack("@1A((@2A)@3A)", qw[X Y Z])
is the string
accept the!
modifier to act as alignment commands: they jump forward or back to the closest position aligned at a multiple ofcount
characters. For example, topack
a C structure likestruct { char c; /* one signed, 8-bit character */ double d; char cc[2]; }
one may need to use the template
c x![d] d c[2]
. This assumes that doubles must be aligned to the size of double.For alignment commands, a
of 0 is equivalent to acount
of 1; both are no-ops.n
accept the!
modifier to represent signed 16-/32-bit integers in big-/little-endian order. This is portable only when all platforms sharing packed data use the same binary representation for signed integers; for example, when all platforms use two's-complement representation.Comments can be embedded in a TEMPLATE using
through the end of line. White space can separate pack codes from each other, but modifiers and repeat counts must follow immediately. Breaking complex templates into individual line-by-line components, suitably annotated, can do as much to improve legibility and maintainability of pack/unpack formats as/x
can for complicated pattern matches.If TEMPLATE requires more arguments than
is given,pack
assumes additional""
arguments. If TEMPLATE requires fewer arguments than given, extra arguments are ignored.Attempting to pack the special floating point values
(infinity, also in negative, and not-a-number) into packed integer values (like"L"
) is a fatal error. The reason for this is that there simply isn't any sensible mapping for these special values into integers.
$foo = pack("WWWW",65,66,67,68); # foo eq "ABCD" $foo = pack("W4",65,66,67,68); # same thing $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9); # same thing with Unicode circled letters. $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); # same thing with Unicode circled letters. You don't get the # UTF-8 bytes because the U at the start of the format caused # a switch to U0-mode, so the UTF-8 bytes get joined into # characters $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9); # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9" # This is the UTF-8 encoding of the string in the # previous example $foo = pack("ccxxcc",65,66,67,68); # foo eq "AB\0\0CD" # NOTE: The examples above featuring "W" and "c" are true # only on ASCII and ASCII-derived systems such as ISO Latin 1 # and UTF-8. On EBCDIC systems, the first example would be # $foo = pack("WWWW",193,194,195,196); $foo = pack("s2",1,2); # "\001\000\002\000" on little-endian # "\000\001\000\002" on big-endian $foo = pack("a4","abcd","x","y","z"); # "abcd" $foo = pack("aaaa","abcd","x","y","z"); # "axyz" $foo = pack("a14","abcdefg"); # "abcdefg\0\0\0\0\0\0\0" $foo = pack("i9pl", gmtime); # a real struct tm (on my system anyway) $utmp_template = "Z8 Z8 Z16 L"; $utmp = pack($utmp_template, @utmp1); # a struct utmp (BSDish) @utmp2 = unpack($utmp_template, $utmp); # "@utmp1" eq "@utmp2" sub bintodec { unpack("N", pack("B32", substr("0" x 32 . shift, -32))); } $foo = pack('sx2l', 12, 34); # short 12, two zero bytes padding, long 34 $bar = pack('s@4l', 12, 34); # short 12, zero fill to position 4, long 34 # $foo eq $bar $baz = pack('s.l', 12, 4, 34); # short 12, zero fill to position 4, long 34 $foo = pack('nN', 42, 4711); # pack big-endian 16- and 32-bit unsigned integers $foo = pack('S>L>', 42, 4711); # exactly the same $foo = pack('s<l<', -42, 4711); # pack little-endian 16- and 32-bit signed integers $foo = pack('(sl)<', -42, 4711); # exactly the same
The same template may generally also be used in
. - package NAMESPACE
- package NAMESPACE VERSION
- package NAMESPACE BLOCK
- package NAMESPACE VERSION BLOCK
Declares the BLOCK or the rest of the compilation unit as being in the given namespace. The scope of the package declaration is either the supplied code BLOCK or, in the absence of a BLOCK, from the declaration itself through the end of current scope (the enclosing block, file, or
). That is, the forms without a BLOCK are operative through the end of the current scope, just like themy
, andour
operators. All unqualified dynamic identifiers in this scope will be in the given namespace, except where overridden by anotherpackage
declaration or when they're one of the special identifiers that qualify intomain::
, likeSTDOUT
, and the punctuation variables.A package statement affects dynamic variables only, including those you've used
on, but not lexically-scoped variables, which are created withmy
, orour
. Typically it would be the first declaration in a file included byrequire
. You can switch into a package in more than one place, since this only determines which default symbol table the compiler uses for the rest of that block. You can refer to identifiers in other packages than the current one by prefixing the identifier with the package name and a double colon, as in$SomePack::var
. If package name is omitted, themain
package is assumed. That is,$::sail
is equivalent to$main::sail
(as well as to$main'sail
, still seen in ancient code, mostly from Perl 4).If VERSION is provided,
sets the$VERSION
variable in the given namespace to a version object with the VERSION provided. VERSION must be a "strict" style version number as defined by the version module: a positive decimal number (integer or decimal-fraction) without exponentiation or else a dotted-decimal v-string with a leading 'v' character and at least three components. You should set$VERSION
only once per package.See "Packages" in perlmod for more information about packages, modules, and classes. See perlsub for other scoping issues.
A special token that returns the name of the package in which it occurs.
Invoked within a
, or similar location, such as a field initializer expression, this token returns the name of the class of the invoking instance. Essentially it is equivalent toref($self)
except that it can additionally be used in a field initializer to gain access to class methods, before the instance is fully constructed.use feature 'class'; class Example1 { field $f = __CLASS__->default_f; sub default_f { 10 } }
In a basic class, this value will be the same as
. The distinction can be seen when a subclass is constructed; it will give the class name of the instance being constructed, rather than just the package name that the actual code belongs to.class Example2 :isa(Example1) { sub default_f { 20 } } my $obj = Example2->new; # The $f field now has the value 20
- pipe READHANDLE,WRITEHANDLE
Opens a pair of connected pipes like the corresponding system call. Note that if you set up a loop of piped processes, deadlock can occur unless you are very careful. In addition, note that Perl's pipes use IO buffering, so you may need to set
to flush your WRITEHANDLE after each command, depending on the application.Returns true on success.
See IPC::Open2, IPC::Open3, and "Bidirectional Communication with Another Process" in perlipc for examples of such things.
On systems that support a close-on-exec flag on files, that flag is set on all newly opened file descriptors whose
s are higher than the current value of$^F
(by default 2 forSTDERR
). See "$^F" in perlvar. - pop ARRAY
- pop
Removes and returns the last element of the array, shortening the array by one element.
my @arr = ('cat', 'dog', 'mouse'); my $item = pop(@arr); # 'mouse' # @arr is now ('cat', 'dog')
if the array is empty.Note:
may also returnundef
if the last element in the array isundef
.my @arr = ('one', 'two', undef); my $item = pop(@arr); # undef
If ARRAY is omitted,
operates on the@ARGV
array in the main program, but the@_
array in subroutines.pop
will operate on the@ARGV
array ineval STRING
blocks.Starting with Perl 5.14, an experimental feature allowed
to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24. - pos SCALAR
- pos
Returns the offset of where the last
search left off for the variable in question ($_
is used when the variable is not specified). This offset is in characters unless the (no-longer-recommended)use bytes
pragma is in effect, in which case the offset is in bytes. Note that 0 is a valid match offset.undef
indicates that the search position is reset (usually due to match failure, but can also be because no match has yet been run on the scalar).pos
directly accesses the location used by the regexp engine to store the offset, so assigning topos
will change that offset, and so will also influence the\G
zero-width assertion in regular expressions. Both of these effects take place for the next match, so you can't affect the position withpos
during the current match, such as in(?{pos() = 5})
ors//pos() = 5/e
also resets the matched with zero-length flag, described under "Repeated Patterns Matching a Zero-length Substring" in perlre.Because a failed
match doesn't reset the offset, the return frompos
won't change either in this case. See perlre and perlop. - print FILEHANDLE LIST
- print FILEHANDLE
- print LIST
Prints a string or a list of strings. Returns true if successful. FILEHANDLE may be a scalar variable containing the name of or a reference to the filehandle, thus introducing one level of indirection. (NOTE: If FILEHANDLE is a variable and the next token is a term, it may be misinterpreted as an operator unless you interpose a
or put parentheses around the arguments.) If FILEHANDLE is omitted, prints to the last selected (seeselect
) output handle. If LIST is omitted, prints$_
to the currently selected output handle. To use FILEHANDLE alone to print the content of$_
to it, you must use a bareword filehandle likeFH
, not an indirect one like$fh
. To set the default output handle to something other than STDOUT, use the select operation.The current value of
(if any) is printed between each LIST item. The current value of$\
(if any) is printed after the entire LIST has been printed. Because print takes a LIST, anything in the LIST is evaluated in list context, including any subroutines whose return lists you pass toprint
. Be careful not to follow the print keyword with a left parenthesis unless you want the corresponding right parenthesis to terminate the arguments to the print; put parentheses around all arguments (or interpose a+
, but that doesn't look as good).If you're storing handles in an array or hash, or in general whenever you're using any expression more complex than a bareword handle or a plain, unsubscripted scalar variable to retrieve it, you will have to use a block returning the filehandle value instead, in which case the LIST may not be omitted:
print { $files[$i] } "stuff\n"; print { $OK ? *STDOUT : *STDERR } "stuff\n";
Printing to a closed pipe or socket will generate a SIGPIPE signal. See perlipc for more on signal handling.
- printf FILEHANDLE FORMAT, LIST
- printf FILEHANDLE
- printf FORMAT, LIST
- printf
Equivalent to
print FILEHANDLE sprintf(FORMAT, LIST)
, except that$\
(the output record separator) is not appended. The FORMAT and the LIST are actually parsed as a single list. The first argument of the list will be interpreted as theprintf
format. This means thatprintf(@_)
will use$_[0]
as the format. See sprintf for an explanation of the format argument. Ifuse locale
(includinguse locale ':not_characters'
) is in effect andPOSIX::setlocale
has been called, the character used for the decimal separator in formatted floating-point numbers is affected by theLC_NUMERIC
locale setting. See perllocale and POSIX.For historical reasons, if you omit the list,
is used as the format; to use FILEHANDLE without a list, you must use a bareword filehandle likeFH
, not an indirect one like$fh
. However, this will rarely do what you want; if$_
contains formatting codes, they will be replaced with the empty string and a warning will be emitted if warnings are enabled. Just useprint
if you want to print the contents of$_
.Don't fall into the trap of using a
when a simpleprint
would do. Theprint
is more efficient and less error prone. - prototype FUNCTION
- prototype
Returns the prototype of a function as a string (or
if the function has no prototype). FUNCTION is a reference to, or the name of, the function whose prototype you want to retrieve. If FUNCTION is omitted,$_
is used.If FUNCTION is a string starting with
, the rest is taken as a name for a Perl builtin. If the builtin's arguments cannot be adequately expressed by a prototype (such assystem
, because the builtin does not really behave like a Perl function. Otherwise, the string describing the equivalent prototype is returned. - push ARRAY,LIST
Adds one or more items to the end of an array.
my @animals = ("cat"); push(@animals, "mouse"); # ("cat", "mouse") my @colors = ("red"); push(@colors, ("blue", "green")); # ("red", "blue", "green")
Returns the number of elements in the array following the completed
.my $color_count = push(@colors, ("yellow", "purple")); say "There are $color_count colors in the updated array";
Starting with Perl 5.14, an experimental feature allowed
to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24. - q/STRING/
- qq/STRING/
- qw/STRING/
- qx/STRING/
Generalized quotes. See "Quote-Like Operators" in perlop.
- qr/STRING/
Regexp-like quote. See "Regexp Quote-Like Operators" in perlop.
- quotemeta EXPR
- quotemeta
Returns the value of EXPR with all the ASCII non-"word" characters backslashed. (That is, all ASCII characters not matching
will be preceded by a backslash in the returned string, regardless of any locale settings.) This is the internal function implementing the\Q
escape in double-quoted strings. (See below for the behavior on non-ASCII code points.)If EXPR is omitted, uses
.quotemeta (and
) are useful when interpolating strings into regular expressions, because by default an interpolated variable will be considered a mini-regular expression. For example:my $sentence = 'The quick brown fox jumped over the lazy dog'; my $substring = 'quick.*?fox'; $sentence =~ s{$substring}{big bad wolf};
Will cause
to become'The big bad wolf jumped over...'
.On the other hand:
my $sentence = 'The quick brown fox jumped over the lazy dog'; my $substring = 'quick.*?fox'; $sentence =~ s{\Q$substring\E}{big bad wolf};
my $sentence = 'The quick brown fox jumped over the lazy dog'; my $substring = 'quick.*?fox'; my $quoted_substring = quotemeta($substring); $sentence =~ s{$quoted_substring}{big bad wolf};
Will both leave the sentence as is. Normally, when accepting literal string input from the user,
must be used.Beware that if you put literal backslashes (those not inside interpolated variables) between
, double-quotish backslash interpolation may lead to confusing results. If you need to use literal backslashes within\Q...\E
, consult "Gory details of parsing quoted constructs" in perlop.Because the result of
has all metacharacters quoted, there is no way to insert a literal$
inside a\Q\E
pair. If protected by\
will be quoted to become"\\\$"
; if not, it is interpreted as the start of an interpolated scalar.In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded strings, but not quoted in UTF-8 strings.
Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for quoting non-ASCII characters; the quoting of ASCII characters is unchanged.
Also unchanged is the quoting of non-UTF-8 strings when outside the scope of a
use feature 'unicode_strings'
, which is to quote all characters in the upper Latin1 range. This provides complete backwards compatibility for old programs which do not use Unicode. (Note thatunicode_strings
is automatically enabled within the scope of ause v5.12
or greater.)Within the scope of
use locale
, all non-ASCII Latin1 code points are quoted whether the string is encoded as UTF-8 or not. As mentioned above, locale does not affect the quoting of ASCII-range characters. This protects against those locales where characters such as"|"
are considered to be word characters.Otherwise, Perl quotes non-ASCII characters using an adaptation from Unicode (see https://www.unicode.org/reports/tr31/). The only code points that are quoted are those that have any of the Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space, Default_Ignorable_Code_Point, or General_Category=Control.
Of these properties, the two important ones are Pattern_Syntax and Pattern_White_Space. They have been set up by Unicode for exactly this purpose of deciding which characters in a regular expression pattern should be quoted. No character that can be in an identifier has these properties.
Perl promises, that if we ever add regular expression pattern metacharacters to the dozen already defined (
\ | ( ) [ { ^ $ * + ? .
), that we will only use ones that have the Pattern_Syntax property. Perl also promises, that if we ever add characters that are considered to be white space in regular expressions (currently mostly affected by/x
), they will all have the Pattern_White_Space property.Unicode promises that the set of code points that have these two properties will never change, so something that is not quoted in v5.16 will never need to be quoted in any future Perl release. (Not all the code points that match Pattern_Syntax have actually had characters assigned to them; so there is room to grow, but they are quoted whether assigned or not. Perl, of course, would never use an unassigned code point as an actual metacharacter.)
Quoting characters that have the other 3 properties is done to enhance the readability of the regular expression and not because they actually need to be quoted for regular expression purposes (characters with the White_Space property are likely to be indistinguishable on the page or screen from those with the Pattern_White_Space property; and the other two properties contain non-printing characters).
- rand EXPR
- rand
Returns a random fractional number greater than or equal to
and less than the value of EXPR. (EXPR should be positive.) If EXPR is omitted, the value1
is used. Currently EXPR with the value0
is also special-cased as1
(this was undocumented before Perl 5.8.0 and is subject to change in future versions of Perl). Automatically callssrand
has already been called. See alsosrand
to the value returned byrand
if you want random integers instead of random fractional numbers. For example,int(rand(10))
returns a random integer between
, inclusive.(Note: If your rand function consistently returns numbers that are too large or too small, then your version of Perl was probably compiled with the wrong number of RANDBITS.)
is not cryptographically secure. You should not rely on it in security-sensitive situations. As of this writing, a number of third-party CPAN modules offer random number generators intended by their authors to be cryptographically secure, including: Data::Entropy, Crypt::Random, Math::Random::Secure, and Math::TrulyRandom. - read FILEHANDLE,SCALAR,LENGTH,OFFSET
- read FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH characters of data into variable SCALAR from the specified FILEHANDLE. Returns the number of characters actually read,
at end of file, or undef if there was an error (in the latter case$!
is also set). SCALAR will be grown or shrunk so that the last character actually read is the last character of the scalar after the read.An OFFSET may be specified to place the read data at some place in the string other than the beginning. A negative OFFSET specifies placement at that many characters counting backwards from the end of the string. A positive OFFSET greater than the length of SCALAR results in the string being padded to the required size with
bytes before the result of the read is appended.The call is implemented in terms of either Perl's or your system's native fread(3) library function, via the PerlIO layers applied to the handle. To get a true read(2) system call, see sysread.
Note the characters: depending on the status of the filehandle, either (8-bit) bytes or characters are read. By default, all filehandles operate on bytes, but for example if the filehandle has been opened with the
I/O layer (seeopen
, and the open pragma), the I/O will operate on UTF8-encoded Unicode characters, not bytes. Similarly for the:encoding
layer: in that case pretty much any characters can be read. - readdir DIRHANDLE
Returns the next directory entry for a directory opened by
. If used in list context, returns all the rest of the entries in the directory. If there are no more entries, returns the undefined value in scalar context and the empty list in list context.If you're planning to filetest the return values out of a
, you'd better prepend the directory in question. Otherwise, because we didn'tchdir
there, it would have been testing the wrong file.opendir(my $dh, $some_dir) || die "Can't opendir $some_dir: $!"; my @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh); closedir $dh;
As of Perl 5.12 you can use a bare
in awhile
loop, which will set$_
on every iteration. If either areaddir
expression or an explicit assignment of areaddir
expression to a scalar is used as awhile
condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value.opendir(my $dh, $some_dir) || die "Can't open $some_dir: $!"; while (readdir $dh) { print "$some_dir/$_\n"; } closedir $dh;
To avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious failures, put this sort of thing at the top of your file to signal that your code will work only on Perls of a recent vintage:
use v5.12; # so readdir assigns to $_ in a lone while test
- readline EXPR
- readline
Reads from the filehandle whose typeglob is contained in EXPR (or from
if EXPR is not provided). In scalar context, each call reads and returns the next line until end-of-file is reached, whereupon the subsequent call returnsundef
. In list context, reads until end-of-file is reached and returns a list of lines. Note that the notion of "line" used here is whatever you may have defined with$/
in English). See "$/" in perlvar.When
is set toundef
, whenreadline
is in scalar context (i.e., file slurp mode), and when an empty file is read, it returns''
the first time, followed byundef
subsequently.This is the internal function implementing the
operator, but you can use it directly. The<EXPR>
operator is discussed in more detail in "I/O Operators" in perlop.my $line = <STDIN>; my $line = readline(STDIN); # same thing
encounters an operating system error,$!
will be set with the corresponding error message. It can be helpful to check$!
when you are reading from filehandles you don't trust, such as a tty or a socket. The following example uses the operator form ofreadline
and dies if the result is not defined.while ( ! eof($fh) ) { defined( $_ = readline $fh ) or die "readline failed: $!"; ... }
Note that you can't handle
errors that way with theARGV
filehandle. In that case, you have to open each element of@ARGV
yourself sinceeof
differently.foreach my $arg (@ARGV) { open(my $fh, $arg) or warn "Can't open $arg: $!"; while ( ! eof($fh) ) { defined( $_ = readline $fh ) or die "readline failed for $arg: $!"; ... } }
Like the
operator, if areadline
expression is used as the condition of awhile
loop, then it will be implicitly assigned to$_
. If either areadline
expression or an explicit assignment of areadline
expression to a scalar is used as awhile
condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value. - readlink EXPR
- readlink
Returns the value of a symbolic link, if symbolic links are implemented. If not, raises an exception. If there is a system error, returns the undefined value and sets
(errno). If EXPR is omitted, uses$_
.Portability issues: "readlink" in perlport.
- readpipe EXPR
- readpipe
EXPR is executed as a system command. The collected standard output of the command is returned. In scalar context, it comes back as a single (potentially multi-line) string. In list context, returns a list of lines (however you've defined lines with
in English)). This is the internal function implementing theqx/EXPR/
operator, but you can use it directly. Theqx/EXPR/
operator is discussed in more detail in "qx/STRING/
" in perlop. If EXPR is omitted, uses$_
. - recv SOCKET,SCALAR,LENGTH,FLAGS
Receives a message on a socket. Attempts to receive LENGTH characters of data into variable SCALAR from the specified SOCKET filehandle. SCALAR will be grown or shrunk to the length actually read. Takes the same flags as the system call of the same name. Returns the address of the sender if SOCKET's protocol supports this; returns an empty string otherwise. If there's an error, returns the undefined value. This call is actually implemented in terms of the recvfrom(2) system call. See "UDP: Message Passing" in perlipc for examples.
Note that if the socket has been marked as
will throw an exception. The:encoding(...)
layer implicitly introduces the:utf8
layer. Seebinmode
. - redo LABEL
- redo EXPR
- redo
command restarts the loop block without evaluating the conditional again. Thecontinue
block, if any, is not executed. If the LABEL is omitted, the command refers to the innermost enclosing loop. Theredo EXPR
form, available starting in Perl 5.18.0, allows a label name to be computed at run time, and is otherwise identical toredo LABEL
. Programs that want to lie to themselves about what was just input normally use this command:# a simpleminded Pascal comment stripper # (warning: assumes no { or } in strings) LINE: while (<STDIN>) { while (s|({.*}.*){.*}|$1 |) {} s|{.*}| |; if (s|{.*| |) { my $front = $_; while (<STDIN>) { if (/}/) { # end of comment? s|^|$front\{|; redo LINE; } } } print; }
cannot return a value from a block that typically returns a value, such aseval {}
,sub {}
, ordo {}
. It will perform its flow control behavior, which precludes any return value. It should not be used to exit agrep
operation.Note that a block by itself is semantically identical to a loop that executes once. Thus
inside such a block will effectively turn it into a looping construct.See also
for an illustration of howlast
, andredo
work.Unlike most named operators, this has the same precedence as assignment. It is also exempt from the looks-like-a-function rule, so
redo ("foo")."bar"
will cause "bar" to be part of the argument toredo
. - ref EXPR
- ref
Examines the value of EXPR, expecting it to be a reference, and returns a string giving information about the reference and the type of referent. If EXPR is not specified,
will be used.If the operand is not a reference, then the empty string will be returned. An empty string will only be returned in this situation.
is often useful to just test whether a value is a reference, which can be done by comparing the result to the empty string. It is a common mistake to use the result ofref
directly as a truth value: this goes wrong because0
(which is false) can be returned for a reference.If the operand is a reference to a blessed object, then the name of the class into which the referent is blessed will be returned.
doesn't care what the physical type of the referent is; blessing takes precedence over such concerns. Beware that exact comparison ofref
results against a class name doesn't perform a class membership test: a class's members also include objects blessed into subclasses, for whichref
will return the name of the subclass. Also beware that class names can clash with the built-in type names (described below).If the operand is a reference to an unblessed object, then the return value indicates the type of object. If the unblessed referent is not a scalar, then the return value will be one of the strings
, orIO
, indicating only which kind of object it is. If the unblessed referent is a scalar, then the return value will be one of the stringsSCALAR
, orREGEXP
, depending on the kind of value the scalar currently has. But note thatqr//
scalars are created already blessed, soref qr/.../
will likely returnRegexp
. Beware that these built-in type names can also be used as class names, soref
returning one of these names doesn't unambiguously indicate that the referent is of the kind to which the name refers.The ambiguity between built-in type names and class names significantly limits the utility of
. For unambiguous information, useScalar::Util::blessed()
for information about blessing, andScalar::Util::reftype()
for information about physical types. Use theisa
method for class membership tests, though one must be sure of blessedness before attempting a method call. Alternatively, theisa
operator can test class membership without checking blessedness first. - rename OLDNAME,NEWNAME
Changes the name of a file; an existing file NEWNAME will be clobbered. Returns true for success; on failure returns false and sets
.Behavior of this function varies wildly depending on your system implementation. For example, it will usually not work across file system boundaries, even though the system mv command sometimes compensates for this. Other restrictions include whether it works on directories, open files, or pre-existing files. Check perlport and either the rename(2) manpage or equivalent system documentation for details.
For a platform independent
function look at the File::Copy module.Portability issues: "rename" in perlport.
- require VERSION
- require EXPR
- require
Demands a version of Perl specified by VERSION, or demands some semantics specified by EXPR or by
if EXPR is not supplied.VERSION may be either a literal such as v5.24.1, which will be compared to
in English), or a numeric argument of the form 5.024001, which will be compared to$]
. An exception is raised if VERSION is greater than the version of the current Perl interpreter. Compare withuse
, which can do a similar check at compile time.Specifying VERSION as a numeric argument of the form 5.024001 should generally be avoided as older less readable syntax compared to v5.24.1. Before perl 5.8.0 (released in 2002), the more verbose numeric form was the only supported syntax, which is why you might see it in older code.
require v5.24.1; # run time version check require 5.24.1; # ditto require 5.024_001; # ditto; older syntax compatible with perl 5.6
demands that a library file be included if it hasn't already been included. The file is included via the do-FILE mechanism, which is essentially just a variety ofeval
with the caveat that lexical variables in the invoking script will be invisible to the included code. If it were implemented in pure Perl, it would have semantics similar to the following:use Carp 'croak'; use version; sub require { my ($filename) = @_; if ( my $version = eval { version->parse($filename) } ) { if ( $version > $^V ) { my $vn = $version->normal; croak "Perl $vn required--this is only $^V, stopped"; } return 1; } if (exists $INC{$filename}) { return 1 if $INC{$filename}; croak "Compilation failed in require"; } local $INC; # this type of loop lets a hook overwrite $INC if they wish for($INC = 0; $INC < @INC; $INC++) { my $prefix = $INC[$INC]; if (!defined $prefix) { next; } if (ref $prefix) { #... do other stuff - see text below .... } # (see text below about possible appending of .pmc # suffix to $filename) my $realfilename = "$prefix/$filename"; next if ! -e $realfilename || -d _ || -b _; $INC{$filename} = $realfilename; my $result = do($realfilename); # but run in caller's namespace if (!defined $result) { $INC{$filename} = undef; croak $@ ? "$@Compilation failed in require" : "Can't locate $filename: $!\n"; } if (!$result) { delete $INC{$filename}; croak "$filename did not return true value"; } $! = 0; return $result; } croak "Can't locate $filename in \@INC ..."; }
Note that the file will not be included twice under the same specified name.
Historically the file must return true as the last statement to indicate successful execution of any initialization code, so it's customary to end such a file with
unless you're sure it'll return true otherwise. But it's better just to put the1;
, in case you add more statements. As of 5.37.6 this requirement may be avoided by enabling the 'module_true' feature, which is enabled by default in modern version bundles. Thus code withuse v5.37;
no longer needs to concern itself with this issue. See feature for more details. Note that this affects the compilation unit within which the feature is used, and using it before requiring a module will not change the behavior of existing modules that do not themselves also use it.If EXPR is a bareword,
assumes a .pm extension and replaces::
in the filename for you, to make it easy to load standard modules. This form of loading of modules does not risk altering your namespace, however it will autovivify the stash for the required module.In other words, if you try this:
require Foo::Bar; # a splendid bareword
The require function will actually look for the Foo/Bar.pm file in the directories specified in the
array, and it will autovivify theFoo::Bar::
stash at compile time.But if you try this:
my $class = 'Foo::Bar'; require $class; # $class is not a bareword #or require "Foo::Bar"; # not a bareword because of the ""
The require function will look for the Foo::Bar file in the
array and will complain about not finding Foo::Bar there. In this case you can do:eval "require $class";
or you could do
require "Foo/Bar.pm";
Neither of these forms will autovivify any stashes at compile time and only have run time effects.
Now that you understand how
looks for files with a bareword argument, there is a little extra functionality going on behind the scenes. Beforerequire
looks for a .pm extension, it will first look for a similar filename with a .pmc extension. If this file is found, it will be loaded in place of any file ending in a .pm extension. This applies to both the explicitrequire "Foo/Bar.pm";
form and therequire Foo::Bar;
form.You can also insert hooks into the import facility by putting Perl coderefs or objects directly into the
array. There are two types of hooks, INC filters, and INCDIR hooks, and there are three forms of representing a hook: subroutine references, array references, and blessed objects.Subroutine references are the simplest case. When the inclusion system walks through
and encounters a subroutine, unless this subroutine is blessed and supports an INCDIR hook this subroutine will be assumed to be an INC hook will be called with two parameters, the first a reference to itself, and the second the name of the file to be included (e.g., Foo/Bar.pm). The subroutine should return either nothing or else a list of up to four values in the following order:A reference to a scalar, containing any initial source code to prepend to the file or generator output.
A filehandle, from which the file will be read.
A reference to a subroutine. If there is no filehandle (previous item), then this subroutine is expected to generate one line of source code per call, writing the line into
and returning 1, then finally at end of file returning 0. If there is a filehandle, then the subroutine will be called to act as a simple source filter, with the line as read in$_
. Again, return 1 for each valid line, and 0 after all lines have been returned. For historical reasons the subroutine will receive a meaningless argument (in fact always the numeric value zero) as$_[0]
.Optional state for the subroutine. The state is passed in as
cannot be used to resolve theINCDIR
is checked first, andAUTOLOAD
would resolve that.If an empty list,
, or nothing that matches the first 3 values above is returned, thenrequire
looks at the remaining elements of@INC
. Note that this filehandle must be a real filehandle (strictly a typeglob or reference to a typeglob, whether blessed or unblessed); tied filehandles will be ignored and processing will stop there.If the hook is an object, it should provide an
method that will be called as above, the first parameter being the object itself. If it does not provide either method, and the object is not CODE ref then an exception will be thrown, otherwise it will simply be executed like an unblessed CODE ref would. Note that you must fully qualify the method name when you declare anINC
sub (unlike theINCDIR
sub), as the unqualified symbolINC
is always forced into packagemain
. Here is a typical code layout for anINC
hook:# In Foo.pm package Foo; sub new { ... } sub Foo::INC { my ($self, $filename) = @_; ... } # In the main program push @INC, Foo->new(...);
If the hook is an array reference, its first element must be a subroutine reference or an object as described above. When the first element is an object that supports an
method then the method will be called with the object as the first argument, the filename requested as the second, and the hook array reference as the the third. When the first element is a subroutine then it will be called with the array as the first argument, and the filename as the second, no third parameter will be passed in. In both forms you can modify the contents of the array to provide state between calls, or whatever you like.In other words, you can write:
push @INC, \&my_sub; sub my_sub { my ($coderef, $filename) = @_; # $coderef is \&my_sub ... }
push @INC, [ \&my_sub, $x, $y, ... ]; sub my_sub { my ($arrayref, $filename) = @_; # Retrieve $x, $y, ... my (undef, @parameters) = @$arrayref; ... }
push @INC, [ HookObj->new(), $x, $y, ... ]; sub HookObj::INC { my ($self, $filename, $arrayref)= @_; my (undef, @parameters) = @$arrayref; ... }
These hooks are also permitted to set the
entry corresponding to the files they have loaded. See "%INC" in perlvar. Should anINC
hook not do this then perl will set the%INC
entry to be the hook reference itself.A hook may also be used to rewrite the
array. While this might sound strange, there are situations where it can be very useful to do this. Such hooks usually just return undef and do not mix filtering and@INC
modifications. While in older versions of perl having a hook modify@INC
was fraught with issues and could even result in segfaults or assert failures, as of 5.37.7 the logic has been made much more robust and the hook now has control over the loop iteration if it wishes to do so.There is a now a facility to control the iterator for the
array traversal that is performed during require. The$INC
variable will be initialized with the index of the currently executing hook. Once the hook returns the next slot in@INC
that will be checked will be the integer successor of value in$INC
(or -1 if it is undef). For example the following codepush @INC, sub { splice @INC, $INC, 1; # remove this hook from @INC unshift @INC, sub { warn "A" }; undef $INC; # reset the $INC iterator so we # execute the newly installed sub # immediately. };
would install a sub into
that when executed as a hook (by for instance a require of a file that does not exist), the hook will splice itself out of@INC
, and add a new sub to the front that will warn whenever someone does a require operation that requires an@INC
search, and then immediately execute that hook.Prior to 5.37.7, there was no way to cause perl to use the newly installed hook immediately, or to inspect any changed items in
to the left of the iterator, and so the warning would only be generated on the second call to require. In more recent perl the presence of the last statement which undefines$INC
will cause perl to restart the traversal of the@INC
array at the beginning and execute the newly installed sub immediately.Whatever value
held, if any, will be restored at the end of the require. Any changes made to$INC
during the lifetime of the hook will be unrolled after the hook exits, and its value only has meaning immediately after execution of the hook, thus setting$INC
to some value prior to executing arequire
will have no effect on how the require executes at all.As of 5.37.7
values of undef will be silently ignored.The function
is difficult to wrap properly. Many modules consult the stack to find information about their caller, and injecting a new stack frame by wrappingrequire()
often breaks things. Nevertheless it can be very helpful to have the ability to perform actions before and after arequire
, for instance for trace utilities likeDevel::TraceUse
or to measure time to load and the memory consumption of the require graph. Because of the difficulties in safely creating arequire()
wrapper in 5.37.10 we introduced a new mechanism.As of 5.37.10, prior to any other actions it performs,
will check if${^HOOK}{require__before}
contains a coderef, and if it does it will be called with the filename form of the item being loaded. The hook may modify$_[0]
to load a different filename, or it may throw a fatal exception to cause the require to fail, which will be treated as though the required code itself had thrown an exception.The
hook may return a code reference, in which case the code reference will be executed (in an eval with the filname as a parameter) after the require completes. It will be executed regardless of how the compilation completed, and even if the require throws a fatal exception. The function may consult%INC
to determine if the require failed or not. For instance the following code will print some diagnostics before and after everyrequire
statement. The example also includes logic to chain the signal, so that multiple signals can cooperate. Well behaved${^HOOK}{require__before}
handlers should always take this into account.{ use Scalar::Util qw(reftype); my $old_hook = ${^HOOK}{require__before}; local ${^HOOK}{require__before} = sub { my ($name) = @_; my $old_hook_ret; $old_hook_ret = $old_hook->($name) if $old_hook; warn "Requiring: $name\n"; return sub { $old_hook_ret->() if ref($old_hook_ret) && reftype($old_hook_ret) eq "CODE"; warn sprintf "Finished requiring %s: %s\n", $name, $INC{$name} ? "loaded" :"failed"; }; }; require Whatever; }
This hook executes for ALL
statements, unlikeINC
hooks, which are only executed for relative file names, and it executes first before any other special behaviour inside of require. Note that the initial hook in${^HOOK}{require__before}
is *not* executed inside of an eval, and throwing an exception will stop further processing, but the after hook it may return is executed inside of an eval, and any exceptions it throws will be silently ignored. This is because it executes inside of the scope cleanup logic that is triggered after the require completes, and an exception at this time would not stop the module from being loaded, etc.There is a similar hook that fires after require completes,
, which will be called after each require statement completes, either via an exception or successfully. It will be called with the filename of the most recently executed require statement. It is executed in an eval, and will not in any way affect execution.For a yet-more-powerful import facility built around
, seeuse
and perlmod. - reset EXPR
- reset
Generally used in a
block at the end of a loop to clear variables and resetm?pattern?
searches so that they work again. The expression is interpreted as a list of single characters (hyphens allowed for ranges). All variables (scalars, arrays, and hashes) in the current package beginning with one of those letters are reset to their pristine state. If the expression is omitted, one-match searches (m?pattern?
) are reset to match again. Only resets variables or searches in the current package. Always returns 1. Examples:reset 'X'; # reset all X variables reset 'a-z'; # reset lower case variables reset; # just reset m?one-time? searches
is not recommended because you'll wipe out your@ARGV
arrays and your%ENV
hash.Resets only package variables; lexical variables are unaffected, but they clean themselves up on scope exit anyway, so you'll probably want to use them instead. See
. - return EXPR
- return
Returns from a subroutine,
,do FILE
block or regex eval block (but not agrep
, ordo BLOCK
block) with the value given in EXPR. Evaluation of EXPR may be in list, scalar, or void context, depending on how the return value will be used, and the context may vary from one execution to the next (seewantarray
). If no EXPR is given, returns an empty list in list context, the undefined value in scalar context, and (of course) nothing at all in void context.(In the absence of an explicit
, a subroutine,eval
, ordo FILE
automatically returns the value of the last expression evaluated.)Unlike most named operators, this is also exempt from the looks-like-a-function rule, so
return ("foo")."bar"
will cause"bar"
to be part of the argument toreturn
. - reverse LIST
In list context, returns a list value consisting of the elements of LIST in the opposite order. In scalar context, concatenates the elements of LIST and returns a string value with all characters in the opposite order.
print join(", ", reverse "world", "Hello"); # Hello, world print scalar reverse "dlrow ,", "olleH"; # Hello, world
Used without arguments in scalar context,
.$_ = "dlrow ,olleH"; print reverse; # No output, list context print scalar reverse; # Hello, world
Note that reversing an array to itself (as in
@a = reverse @a
) will preserve non-existent elements whenever possible; i.e., for non-magical arrays or for tied arrays withEXISTS
methods.This operator is also handy for inverting a hash, although there are some caveats. If a value is duplicated in the original hash, only one of those can be represented as a key in the inverted hash. Also, this has to unwind one hash and build a whole new one, which may take some time on a large hash, such as from a DBM file.
my %by_name = reverse %by_address; # Invert the hash
- rewinddir DIRHANDLE
Sets the current position to the beginning of the directory for the
routine on DIRHANDLE.Portability issues: "rewinddir" in perlport.
- rindex STR,SUBSTR,POSITION
- rindex STR,SUBSTR
Works just like
except that it returns the position of the last occurrence of SUBSTR in STR. If POSITION is specified, returns the last occurrence beginning at or before that position. - rmdir FILENAME
- rmdir
Deletes the directory specified by FILENAME if that directory is empty. If it succeeds it returns true; otherwise it returns false and sets
(errno). If FILENAME is omitted, uses$_
.To remove a directory tree recursively (
rm -rf
on Unix) look at thermtree
function of the File::Path module. - s///
The substitution operator. See "Regexp Quote-Like Operators" in perlop.
- say FILEHANDLE LIST
- say FILEHANDLE
- say LIST
- say
Just like
, but implicitly appends a newline at the end of the LIST instead of any value$\
might have. To use FILEHANDLE without a LIST to print the contents of$_
to it, you must use a bareword filehandle likeFH
, not an indirect one like$fh
is available only if the"say"
feature is enabled or if it is prefixed withCORE::
. The"say"
feature is enabled automatically with ause v5.10
(or higher) declaration in the current scope. - scalar EXPR
Forces EXPR to be interpreted in scalar context and returns the value of EXPR.
my @counts = ( scalar @a, scalar @b, scalar @c );
There is no equivalent operator to force an expression to be interpolated in list context because in practice, this is never needed. If you really wanted to do so, however, you could use the construction
@{[ (some expression) ]}
, but usually a simple(some expression)
is a unary operator, if you accidentally use a parenthesized list for the EXPR, this behaves as a scalar comma expression, evaluating all but the last element in void context and returning the final element evaluated in scalar context. This is seldom what you want.The following single statement:
print uc(scalar(foo(), $bar)), $baz;
is the moral equivalent of these two:
foo(); print(uc($bar), $baz);
See perlop for more details on unary operators and the comma operator, and perldata for details on evaluating a hash in scalar context.
- seek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's position, just like the fseek(3) call of C
. FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are0
to set the new position in bytes to POSITION;1
to set it to the current position plus POSITION; and2
to set it to EOF plus POSITION, typically negative. For WHENCE you may use the constantsSEEK_SET
, andSEEK_END
(start of the file, current position, end of the file) from the Fcntl module. Returns1
on success, false otherwise.Note the emphasis on bytes: even if the filehandle has been set to operate on characters (for example using the
I/O layer), theseek
, andsysseek
family of functions use byte offsets, not character offsets, because seeking to a character offset would be very slow in a UTF-8 file.If you want to position the file for
, don't useseek
, because buffering makes its effect on the file's read-write position unpredictable and non-portable. Usesysseek
instead.Due to the rules and rigors of ANSI C, on some systems you have to do a seek whenever you switch between reading and writing. Amongst other things, this may have the effect of calling stdio's clearerr(3). A WHENCE of
) is useful for not moving the file position:seek($fh, 0, 1);
This is also useful for applications emulating
tail -f
. Once you hit EOF on your read and then sleep for a while, you (probably) have to stick in a dummyseek
to reset things. Theseek
doesn't change the position, but it does clear the end-of-file condition on the handle, so that the nextreadline FILE
makes Perl try again to read something. (We hope.)If that doesn't work (some I/O implementations are particularly cantankerous), you might need something like this:
for (;;) { for ($curpos = tell($fh); $_ = readline($fh); $curpos = tell($fh)) { # search for some stuff and put it into files } sleep($for_a_while); seek($fh, $curpos, 0); }
- seekdir DIRHANDLE,POS
Sets the current position for the
routine on DIRHANDLE. POS must be a value returned bytelldir
also has the same caveats about possible directory compaction as the corresponding system library routine. - select FILEHANDLE
- select
Returns the currently selected filehandle. If FILEHANDLE is supplied, sets the new current default filehandle for output. This has two effects: first, a
, orsay
without a filehandle will default to this FILEHANDLE. Second, references to variables related to output will refer to this output channel.For example, to set the top-of-form format for more than one output channel, you might do the following:
select(REPORT1); $^ = 'report1_top'; select(REPORT2); $^ = 'report2_top';
FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus:
my $oldfh = select(STDERR); $| = 1; select($oldfh);
Some programmers may prefer to think of filehandles as objects with methods, preferring to write the last example as:
(Prior to Perl version 5.14, you have to
use IO::Handle;
explicitly first.)Whilst you can use
to temporarily "capture" the output ofprint
like this:{ my $old_handle = select $new_handle; # This goes to $new_handle: print "ok 1\n"; ... select $old_handle; }
you might find it easier to localize the typeglob instead:
{ local *STDOUT = $new_handle; print "ok 1\n"; ... }
The two are not exactly equivalent, but the latter might be clearer and will restore STDOUT if the wrapped code dies. The difference is that in the former, the original STDOUT can still be accessed by explicitly using it in a
statement (asprint STDOUT ...
), whereas in the latter the meaning of the STDOUT handle itself has temporarily been changed.Portability issues: "select" in perlport.
- select RBITS,WBITS,EBITS,TIMEOUT
This calls the select(2) syscall with the bit masks specified, which can be constructed using
, along these lines:my $rin = my $win = my $ein = ''; vec($rin, fileno(STDIN), 1) = 1; vec($win, fileno(STDOUT), 1) = 1; $ein = $rin | $win;
If you want to select on many filehandles, you may wish to write a subroutine like this:
sub fhbits { my @fhlist = @_; my $bits = ""; for my $fh (@fhlist) { vec($bits, fileno($fh), 1) = 1; } return $bits; } my $rin = fhbits(\*STDIN, $tty, $mysock);
The usual idiom is:
my ($nfound, $timeleft) = select(my $rout = $rin, my $wout = $win, my $eout = $ein, $timeout);
or to block until something becomes ready just do this
my $nfound = select(my $rout = $rin, my $wout = $win, my $eout = $ein, undef);
Most systems do not bother to return anything useful in
, so callingselect
in scalar context just returns$nfound
.Any of the bit masks can also be
. The timeout, if specified, is in seconds, which may be fractional. Note: not all implementations are capable of returning the$timeleft
. If not, they always return$timeleft
equal to the supplied$timeout
.You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
Note that whether
gets restarted after signals (say, SIGALRM) is implementation-dependent. See also perlport for notes on the portability ofselect
.On error,
behaves just like select(2): it returns-1
and sets$!
.On some Unixes, select(2) may report a socket file descriptor as "ready for reading" even when no data is available, and thus any subsequent
would block. This can be avoided if you always useO_NONBLOCK
on the socket. See select(2) and fcntl(2) for further details.The standard
module provides a user-friendlier interface toselect
, mostly because it does all the bit-mask work for you.WARNING: One should not attempt to mix buffered I/O (like
) withselect
, except as permitted by POSIX, and even then only on POSIX systems. You have to usesysread
instead.Portability issues: "select" in perlport.
- semctl ID,SEMNUM,CMD,ARG
Calls the System V IPC function semctl(2). You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is IPC_STAT or GETALL, then ARG must be a variable that will hold the returned semid_ds structure or semaphore value array. Returns like
: the undefined value for error, "0 but true
" for zero, or the actual return value otherwise. The ARG must consist of a vector of native short integers, which may be created withpack("s!",(0)x$nsem)
. See also "SysV IPC" in perlipc and the documentation forIPC::SysV
.Portability issues: "semctl" in perlport.
- semget KEY,NSEMS,FLAGS
Calls the System V IPC function semget(2). Returns the semaphore id, or the undefined value on error. See also "SysV IPC" in perlipc and the documentation for
.Portability issues: "semget" in perlport.
- semop KEY,OPSTRING
Calls the System V IPC function semop(2) for semaphore operations such as signalling and waiting. OPSTRING must be a packed array of semop structures. Each semop structure can be generated with
pack("s!3", $semnum, $semop, $semflag)
. The length of OPSTRING implies the number of semaphore operations. Returns true if successful, false on error. As an example, the following code waits on semaphore $semnum of semaphore id $semid:my $semop = pack("s!3", $semnum, -1, 0); die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace
. See also "SysV IPC" in perlipc and the documentation forIPC::SysV
.Portability issues: "semop" in perlport.
- send SOCKET,MSG,FLAGS,TO
- send SOCKET,MSG,FLAGS
Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET filehandle. Takes the same flags as the system call of the same name. On unconnected sockets, you must specify a destination to send to, in which case it does a sendto(2) syscall. Returns the number of characters sent, or the undefined value on error. The sendmsg(2) syscall is currently unimplemented. See "UDP: Message Passing" in perlipc for examples.
Note that if the socket has been marked as
will throw an exception. The:encoding(...)
layer implicitly introduces the:utf8
layer. Seebinmode
. - setpgrp PID,PGRP
Sets the current process group for the specified PID,
for the current process. Raises an exception when used on a machine that doesn't implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, it defaults to0,0
. Note that the BSD 4.2 version ofsetpgrp
does not accept any arguments, so onlysetpgrp(0,0)
is portable. See alsoPOSIX::setsid()
.Portability issues: "setpgrp" in perlport.
- setpriority WHICH,WHO,PRIORITY
Sets the current priority for a process, a process group, or a user. (See setpriority(2).) Raises an exception when used on a machine that doesn't implement setpriority(2).
can be any ofPRIO_PROCESS
imported from "RESOURCE CONSTANTS" in POSIX.Portability issues: "setpriority" in perlport.
- setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
Sets the socket option requested. Returns
on error. Use integer constants provided by theSocket
module for LEVEL and OPNAME. Values for LEVEL can also be obtained from getprotobyname. OPTVAL might either be a packed string or an integer. An integer OPTVAL is shorthand for pack("i", OPTVAL).An example disabling Nagle's algorithm on a socket:
use Socket qw(IPPROTO_TCP TCP_NODELAY); setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
Portability issues: "setsockopt" in perlport.
- shift ARRAY
- shift
Removes and returns the first element of an array. This shortens the array by one and moves everything down.
my @arr = ('cat', 'dog'); my $item = shift(@arr); # 'cat' # @arr is now ('dog');
if the array is empty.Note:
may also returnundef
if the first element in the array isundef
.my @arr = (undef, 'two', 'three'); my $item = shift(@arr); # undef
If ARRAY is omitted,
operates on the@ARGV
array in the main program, and the@_
array in subroutines.shift
will operate on the@ARGV
array ineval STRING
blocks.Starting with Perl 5.14, an experimental feature allowed
to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.See also
, andpop
do the same thing to the left end of an array thatpop
do to the right end. - shmctl ID,CMD,ARG
Calls the System V IPC function shmctl. You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is
, then ARG must be a variable that will hold the returnedshmid_ds
structure. Returns like ioctl:undef
for error; "0
but true" for zero; and the actual return value otherwise. See also "SysV IPC" in perlipc and the documentation forIPC::SysV
.Portability issues: "shmctl" in perlport.
- shmget KEY,SIZE,FLAGS
Calls the System V IPC function shmget. Returns the shared memory segment id, or
on error. See also "SysV IPC" in perlipc and the documentation forIPC::SysV
.Portability issues: "shmget" in perlport.
- shmread ID,VAR,POS,SIZE
- shmwrite ID,STRING,POS,SIZE
Reads or writes the System V shared memory segment ID starting at position POS for size SIZE by attaching to it, copying in/out, and detaching from it. When reading, VAR must be a variable that will hold the data read. When writing, if STRING is too long, only SIZE bytes are used; if STRING is too short, nulls are written to fill out SIZE bytes. Return true if successful, false on error.
taints the variable. See also "SysV IPC" in perlipc and the documentation forIPC::SysV
and theIPC::Shareable
module from CPAN.Portability issues: "shmread" in perlport and "shmwrite" in perlport.
- shutdown SOCKET,HOW
Shuts down a socket connection in the manner indicated by HOW, which has the same interpretation as in the syscall of the same name.
shutdown($socket, 0); # I/we have stopped reading data shutdown($socket, 1); # I/we have stopped writing data shutdown($socket, 2); # I/we have stopped using this socket
This is useful with sockets when you want to tell the other side you're done writing but not done reading, or vice versa. It's also a more insistent form of close because it also disables the file descriptor in any forked copies in other processes.
for success; on error, returnsundef
if the first argument is not a valid filehandle, or returns0
and sets$!
for any other failure. - sin EXPR
- sin
Returns the sine of EXPR (expressed in radians). If EXPR is omitted, returns sine of
.For the inverse sine operation, you may use the
function, or use this relation:sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
- sleep EXPR
- sleep
Causes the script to sleep for (integer) EXPR seconds, or forever if no argument is given. Returns the integer number of seconds actually slept.
EXPR should be a positive integer. If called with a negative integer,
does not sleep but instead emits a warning, sets$!
), and returns zero.If called with a non-integer, the fractional part is ignored.
sleep 0
is permitted, but a function call to the underlying platform implementation still occurs, with any side effects that may have.sleep 0
is therefore not exactly identical to not sleeping at all.May be interrupted if the process receives a signal such as
.eval { local $SIG{ALRM} = sub { die "Alarm!\n" }; sleep; }; die $@ unless $@ eq "Alarm!\n";
You probably cannot mix
calls, becausesleep
is often implemented usingalarm
.On some older systems, it may sleep up to a full second less than what you requested, depending on how it counts seconds. Most modern systems always sleep the full amount. They may appear to sleep longer than that, however, because your process might not be scheduled right away in a busy multitasking system.
For delays of finer granularity than one second, the Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard distribution) provides
. You may also use Perl's four-argument version ofselect
leaving the first three arguments undefined, or you might be able to use thesyscall
interface to access setitimer(2) if your system supports it. See perlfaq8 for details. - socket SOCKET,DOMAIN,TYPE,PROTOCOL
Opens a socket of the specified kind and attaches it to filehandle SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the syscall of the same name. You should
use Socket
first to get the proper definitions imported. See the examples in "Sockets: Client/Server Communication" in perlipc.On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of
. See "$^F" in perlvar. - socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
Creates an unnamed pair of sockets in the specified domain, of the specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as for the syscall of the same name. If unimplemented, raises an exception. Returns true if successful.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptors, as determined by the value of
. See "$^F" in perlvar.Some systems define
in terms ofsocketpair
, in which a call topipe($rdr, $wtr)
is essentially:use Socket; socketpair(my $rdr, my $wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); shutdown($rdr, 1); # no more writing for reader shutdown($wtr, 0); # no more reading for writer
See perlipc for an example of socketpair use. Perl 5.8 and later will emulate socketpair using IP sockets to localhost if your system implements sockets but not socketpair.
Portability issues: "socketpair" in perlport.
- sort SUBNAME LIST
- sort BLOCK LIST
- sort LIST
In list context, this sorts the LIST and returns the sorted list value. In scalar context, the behaviour of
is undefined.If SUBNAME or BLOCK is omitted,
s in standard string comparison order. If SUBNAME is specified, it gives the name of a subroutine that returns a numeric value less than, equal to, or greater than0
, depending on how the elements of the list are to be ordered. (The<=>
operators are extremely useful in such routines.) SUBNAME may be a scalar variable name (unsubscripted), in which case the value provides the name of (or a reference to) the actual subroutine to use. In place of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort subroutine.If the subroutine's prototype is
, the elements to be compared are passed by reference in@_
, as for a normal subroutine. This is slower than unprototyped subroutines, where the elements to be compared are passed into the subroutine as the package global variables$a
(see example below).If the subroutine is an XSUB, the elements to be compared are pushed on to the stack, the way arguments are usually passed to XSUBs.
are not set.The values to be compared are always passed by reference and should not be modified.
You also cannot exit out of the sort block or subroutine using any of the loop control operators described in perlsyn or with
use locale
(but notuse locale ':not_characters'
) is in effect,sort LIST
sorts LIST according to the current collation locale. See perllocale.sort
returns aliases into the original list, much as a for loop's index variable aliases the list elements. That is, modifying an element of a list returned bysort
(for example, in aforeach
) actually modifies the element in the original list. This is usually something to be avoided when writing clear code.Historically Perl has varied in whether sorting is stable by default. If stability matters, it can be controlled explicitly by using the sort pragma.
# sort lexically my @articles = sort @files; # same thing, but with explicit sort routine my @articles = sort {$a cmp $b} @files; # now case-insensitively my @articles = sort {fc($a) cmp fc($b)} @files; # same thing in reversed order my @articles = sort {$b cmp $a} @files; # sort numerically ascending my @articles = sort {$a <=> $b} @files; # sort numerically descending my @articles = sort {$b <=> $a} @files; # this sorts the %age hash by value instead of key # using an in-line function my @eldest = sort { $age{$b} <=> $age{$a} } keys %age; # sort using explicit subroutine name sub byage { $age{$a} <=> $age{$b}; # presuming numeric } my @sortedclass = sort byage @class; sub backwards { $b cmp $a } my @harry = qw(dog cat x Cain Abel); my @george = qw(gone chased yz Punished Axed); print sort @harry; # prints AbelCaincatdogx print sort backwards @harry; # prints xdogcatCainAbel print sort @george, 'to', @harry; # prints AbelAxedCainPunishedcatchaseddoggonetoxyz # inefficiently sort by descending numeric compare using # the first integer after the first = sign, or the # whole record case-insensitively otherwise my @new = sort { ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] || fc($a) cmp fc($b) } @old; # same thing, but much more efficiently; # we'll build auxiliary indices instead # for speed my (@nums, @caps); for (@old) { push @nums, ( /=(\d+)/ ? $1 : undef ); push @caps, fc($_); } my @new = @old[ sort { $nums[$b] <=> $nums[$a] || $caps[$a] cmp $caps[$b] } 0..$#old ]; # same thing, but without any temps my @new = map { $_->[0] } sort { $b->[1] <=> $a->[1] || $a->[2] cmp $b->[2] } map { [$_, /=(\d+)/, fc($_)] } @old; # using a prototype allows you to use any comparison subroutine # as a sort subroutine (including other package's subroutines) package Other; sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are # not set here package main; my @new = sort Other::backwards @old; ## using a prototype with function signature use feature 'signatures'; sub function_with_signature :prototype($$) ($one, $two) { return $one <=> $two } my @new = sort function_with_signature @old; # guarantee stability use sort 'stable'; my @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
Warning: syntactical care is required when sorting the list returned from a function. If you want to sort the list returned by the function call
, you can use:my @contact = sort { $a cmp $b } find_records @key; my @contact = sort +find_records(@key); my @contact = sort &find_records(@key); my @contact = sort(find_records(@key));
If instead you want to sort the array
with the comparison routinefind_records()
then you can use:my @contact = sort { find_records() } @key; my @contact = sort find_records(@key); my @contact = sort(find_records @key); my @contact = sort(find_records (@key));
are set as package globals in the package the sort() is called from. That means$main::a
) in themain
in theFooPack
package, etc. If the sort block is in scope of amy
declaration of$a
, you must spell out the full name of the variables in the sort block :package main; my $a = "C"; # DANGER, Will Robinson, DANGER !!! print sort { $a cmp $b } qw(A C E G B D F H); # WRONG sub badlexi { $a cmp $b } print sort badlexi qw(A C E G B D F H); # WRONG # the above prints BACFEDGH or some other incorrect ordering print sort { $::a cmp $::b } qw(A C E G B D F H); # OK print sort { our $a cmp our $b } qw(A C E G B D F H); # also OK print sort { our ($a, $b); $a cmp $b } qw(A C E G B D F H); # also OK sub lexi { our $a cmp our $b } print sort lexi qw(A C E G B D F H); # also OK # the above print ABCDEFGH
With proper care you may mix package and my (or state)
:my $a = { tiny => -2, small => -1, normal => 0, big => 1, huge => 2 }; say sort { $a->{our $a} <=> $a->{our $b} } qw{ huge normal tiny small big}; # prints tinysmallnormalbighuge
are implicitly local to the sort() execution and regain their former values upon completing the sort.Sort subroutines written using
are bound to their calling package. It is possible, but of limited interest, to define them in a different package, since the subroutine must still refer to the calling package's$a
:package Foo; sub lexi { $Bar::a cmp $Bar::b } package Bar; ... sort Foo::lexi ...
Use the prototyped versions (see above) for a more generic alternative.
The comparison function is required to behave. If it returns inconsistent results (sometimes saying
is less than$x[2]
and sometimes saying the opposite, for example) the results are not well-defined.Because
when either operand isNaN
(not-a-number), be careful when sorting with a comparison function like$a <=> $b
any lists that might contain aNaN
. The following example takes advantage thatNaN != NaN
to eliminate anyNaN
s from the input list.my @result = sort { $a <=> $b } grep { $_ == $_ } @input;
In this version of perl, the
function is implemented via the mergesort algorithm. - splice ARRAY,OFFSET,LENGTH,LIST
- splice ARRAY,OFFSET,LENGTH
- splice ARRAY,OFFSET
- splice ARRAY
Removes the elements designated by OFFSET and LENGTH from an array, and replaces them with the elements of LIST, if any. In list context, returns the elements removed from the array. In scalar context, returns the last element removed, or
if no elements are removed. The array grows or shrinks as necessary. If OFFSET is negative then it starts that far from the end of the array. If LENGTH is omitted, removes everything from OFFSET onward. If LENGTH is negative, removes the elements from OFFSET onward except for -LENGTH elements at the end of the array. If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is past the end of the array and a LENGTH was provided, Perl issues a warning, and splices at the end of the array.The following equivalences hold (assuming
$#a >= $i
)push(@a,$x,$y) splice(@a,@a,0,$x,$y) pop(@a) splice(@a,-1) shift(@a) splice(@a,0,1) unshift(@a,$x,$y) splice(@a,0,0,$x,$y) $a[$i] = $y splice(@a,$i,1,$y)
can be used, for example, to implement n-ary queue processing:sub nary_print { my $n = shift; while (my @next_n = splice @_, 0, $n) { say join q{ -- }, @next_n; } } nary_print(3, qw(a b c d e f g h)); # prints: # a -- b -- c # d -- e -- f # g -- h
Starting with Perl 5.14, an experimental feature allowed
to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24. - split /PATTERN/,EXPR,LIMIT
- split /PATTERN/,EXPR
- split /PATTERN/
- split
Splits the string EXPR into a list of strings and returns the list in list context, or the size of the list in scalar context. (Prior to Perl 5.11, it also overwrote
with the list in void and scalar context. If you target old perls, beware.)If only PATTERN is given, EXPR defaults to
.Anything in EXPR that matches PATTERN is taken to be a separator that separates the EXPR into substrings (called "fields") that do not include the separator. Note that a separator may be longer than one character or even have no characters at all (the empty string, which is a zero-width match).
The PATTERN need not be constant; an expression may be used to specify a pattern that varies at runtime.
If PATTERN matches the empty string, the EXPR is split at the match position (between characters). As an example, the following:
my @x = split(/b/, "abc"); # ("a", "c")
uses the
as a separator to produce the list ("a", "c"). However, this:my @x = split(//, "abc"); # ("a", "b", "c")
uses empty string matches as separators; thus, the empty string may be used to split EXPR into a list of its component characters.
As a special case for
, the empty pattern given in match operator syntax (//
) specifically matches the empty string, which is contrary to its usual interpretation as the last successful match.If PATTERN is
, then it is treated as if it used the multiline modifier (/^/m
), since it isn't much use otherwise./m
and any of the other pattern modifiers valid forqr
(summarized in "qr/STRING/msixpodualn" in perlop) may be specified explicitly.As another special case,
emulates the default behavior of the command line tool awk when the PATTERN is either omitted or a string composed of a single space character (such as' '
, but not e.g./ /
). In this case, any leading whitespace in EXPR is removed before splitting occurs, and the PATTERN is instead treated as if it were/\s+/
; in particular, this means that any contiguous whitespace (not just a single space character) is used as a separator.my @x = split(" ", " Quick brown fox\n"); # ("Quick", "brown", "fox") my @x = split(" ", "RED\tGREEN\tBLUE"); # ("RED", "GREEN", "BLUE")
Using split in this fashion is very similar to how
works.However, this special treatment can be avoided by specifying the pattern
/ /
instead of the string" "
, thereby allowing only a single space character to be a separator. In earlier Perls this special case was restricted to the use of a plain" "
as the pattern argument to split; in Perl 5.18.0 and later this special case is triggered by any expression which evaluates to the simple string" "
.As of Perl 5.28, this special-cased whitespace splitting works as expected in the scope of
"use feature 'unicode_strings'"
. In previous versions, and outside the scope of that feature, it exhibits "The "Unicode Bug"" in perlunicode: characters that are whitespace according to Unicode rules but not according to ASCII rules can be treated as part of fields rather than as field separators, depending on the string's internal encoding.As of Perl 5.39.9 the
default modifier does NOT affectsplit STRING
but does affectsplit PATTERN
, this means thatsplit " "
will produces the expected awk emulation regardless as to whether it is used in the scope of ause re "/x"
statement. If you want to split by spaces underuse re "/x"
you must do something likesplit /(?-x: )/
orsplit /\x{20}/
instead ofsplit / /
.If omitted, PATTERN defaults to a single space,
" "
, triggering the previously described awk emulation.If LIMIT is specified and positive, it represents the maximum number of fields into which the EXPR may be split; in other words, LIMIT is one greater than the maximum number of times EXPR may be split. Thus, the LIMIT value
means that EXPR may be split a maximum of zero times, producing a maximum of one field (namely, the entire value of EXPR). For instance:my @x = split(/,/, "a,b,c", 1); # ("a,b,c") my @x = split(/,/, "a,b,c", 2); # ("a", "b,c") my @x = split(/,/, "a,b,c", 3); # ("a", "b", "c") my @x = split(/,/, "a,b,c", 4); # ("a", "b", "c")
If LIMIT is negative, it is treated as if it were instead arbitrarily large; as many fields as possible are produced.
If LIMIT is omitted (or, equivalently, zero), then it is usually treated as if it were instead negative but with the exception that trailing empty fields are stripped (empty leading fields are always preserved); if all fields are empty, then all fields are considered to be trailing (and are thus stripped in this case). Thus, the following:
my @x = split(/,/, "a,b,c,,,"); # ("a", "b", "c")
produces only a three element list.
my @x = split(/,/, "a,b,c,,,", -1); # ("a", "b", "c", "", "", "")
produces a six element list.
In time-critical applications, it is worthwhile to avoid splitting into more fields than necessary. Thus, when assigning to a list, if LIMIT is omitted (or zero), then LIMIT is treated as though it were one larger than the number of variables in the list; for the following, LIMIT is implicitly 3:
my ($login, $passwd) = split(/:/);
Note that splitting an EXPR that evaluates to the empty string always produces zero fields, regardless of the LIMIT specified.
An empty leading field is produced when there is a positive-width match at the beginning of EXPR. For instance:
my @x = split(/ /, " abc"); # ("", "abc")
splits into two elements. However, a zero-width match at the beginning of EXPR never produces an empty field, so that:
my @x = split(//, " abc"); # (" ", "a", "b", "c")
splits into four elements instead of five.
An empty trailing field, on the other hand, is produced when there is a match at the end of EXPR, regardless of the length of the match (of course, unless a non-zero LIMIT is given explicitly, such fields are removed, as in the last example). Thus:
my @x = split(//, " abc", -1); # (" ", "a", "b", "c", "")
If the PATTERN contains capturing groups, then for each separator, an additional field is produced for each substring captured by a group (in the order in which the groups are specified, as per backreferences); if any group does not match, then it captures the
value instead of a substring. Also, note that any such additional field is produced whenever there is a separator (that is, whenever a split occurs), and such an additional field does not count towards the LIMIT. Consider the following expressions evaluated in list context (each returned list is provided in the associated comment):my @x = split(/-|,/ , "1-10,20", 3); # ("1", "10", "20") my @x = split(/(-|,)/ , "1-10,20", 3); # ("1", "-", "10", ",", "20") my @x = split(/-|(,)/ , "1-10,20", 3); # ("1", undef, "10", ",", "20") my @x = split(/(-)|,/ , "1-10,20", 3); # ("1", "-", "10", undef, "20") my @x = split(/(-)|(,)/, "1-10,20", 3); # ("1", "-", undef, "10", undef, ",", "20")
- sprintf FORMAT, LIST
Returns a string formatted by the usual
conventions of the C library functionsprintf
. See below for more details and see sprintf(3) or printf(3) on your system for an explanation of the general principles.For example:
# Format number with up to 8 leading zeroes my $result = sprintf("%08d", $number); # Round number to 3 digits after decimal point my $rounded = sprintf("%.3f", $number);
Perl does its own
formatting: it emulates the C function sprintf(3), but doesn't use it except for floating-point numbers, and even then only standard modifiers are allowed. Non-standard extensions in your local sprintf(3) are therefore unavailable from Perl.Unlike
does not do what you probably mean when you pass it an array as your first argument. The array is given scalar context, and instead of using the 0th element of the array as the format, Perl will use the count of elements in the array as the format, which is almost never useful.Perl's
permits the following universally-known conversions:%% a percent sign %c a character with the given number %s a string %d a signed integer, in decimal %u an unsigned integer, in decimal %o an unsigned integer, in octal %x an unsigned integer, in hexadecimal %e a floating-point number, in scientific notation %f a floating-point number, in fixed decimal notation %g a floating-point number, in %e or %f notation
In addition, Perl permits the following widely-supported conversions:
%X like %x, but using upper-case letters %E like %e, but using an upper-case "E" %G like %g, but with an upper-case "E" (if applicable) %b an unsigned integer, in binary %B like %b, but using an upper-case "B" with the # flag %p a pointer (outputs the Perl value's address in hexadecimal) %n special: *stores* the number of characters output so far into the next argument in the parameter list %a hexadecimal floating point %A like %a, but using upper-case letters %i a synonym for %d %D a synonym for %ld %U a synonym for %lu %O a synonym for %lo %F a synonym for %f
Note that the number of exponent digits in the scientific notation produced by
for numbers with the modulus of the exponent less than 100 is system-dependent: it may be three or less (zero-padded as necessary). In other words, 1.23 times ten to the 99th may be either "1.23e99" or "1.23e099". Similarly for%a
: the exponent or the hexadecimal digits may float: especially the "long doubles" Perl configuration option may cause surprises.Between the
and the format letter, you may specify several additional attributes controlling the interpretation of the format. In order, these are:- format parameter index
An explicit format parameter index, such as
. By default sprintf will format the next unused argument in the list, but this allows you to take the arguments out of order:printf '%2$d %1$d', 12, 34; # prints "34 12" printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
- flags
one or more of:
space prefix non-negative number with a space + prefix non-negative number with a plus sign - left-justify within the field 0 use zeros, not spaces, to right-justify # ensure the leading "0" for any octal, prefix non-zero hexadecimal with "0x" or "0X", prefix non-zero binary with "0b" or "0B"
For example:
printf '<% d>', 12; # prints "< 12>" printf '<% d>', 0; # prints "< 0>" printf '<% d>', -12; # prints "<-12>" printf '<%+d>', 12; # prints "<+12>" printf '<%+d>', 0; # prints "<+0>" printf '<%+d>', -12; # prints "<-12>" printf '<%6s>', 12; # prints "< 12>" printf '<%-6s>', 12; # prints "<12 >" printf '<%06s>', 12; # prints "<000012>" printf '<%#o>', 12; # prints "<014>" printf '<%#x>', 12; # prints "<0xc>" printf '<%#X>', 12; # prints "<0XC>" printf '<%#b>', 12; # prints "<0b1100>" printf '<%#B>', 12; # prints "<0B1100>"
When a space and a plus sign are given as the flags at once, the space is ignored.
printf '<%+ d>', 12; # prints "<+12>" printf '<% +d>', 12; # prints "<+12>"
When the # flag and a precision are given in the %o conversion, the precision is incremented if it's necessary for the leading "0".
printf '<%#.5o>', 012; # prints "<00012>" printf '<%#.5o>', 012345; # prints "<012345>" printf '<%#.0o>', 0; # prints "<0>"
- vector flag
This flag tells Perl to interpret the supplied string as a vector of integers, one for each character in the string. Perl applies the format to each integer in turn, then joins the resulting strings with a separator (a dot
by default). This can be useful for displaying ordinal values of characters in arbitrary strings:printf "%vd", "AB\x{100}"; # prints "65.66.256" printf "version is v%vd\n", $^V; # Perl's version
Put an asterisk
before thev
to override the string to use to separate the numbers:printf "address is %*vX\n", ":", $addr; # IPv6 address printf "bits are %0*v8b\n", " ", $bits; # random bitstring
You can also explicitly specify the argument number to use for the join string using something like
; for example:printf '%*4$vX %*4$vX %*4$vX', # 3 IPv6 addresses @addr[1..3], ":";
- (minimum) width
Arguments are usually formatted to be only as wide as required to display the given value. You can override the width by putting a number here, or get the width from the next argument (with
) or from a specified argument (e.g., with*2$
):printf "<%s>", "a"; # prints "<a>" printf "<%6s>", "a"; # prints "< a>" printf "<%*s>", 6, "a"; # prints "< a>" printf '<%*2$s>', "a", 6; # prints "< a>" printf "<%2s>", "long"; # prints "<long>" (does not truncate)
If a field width obtained through
is negative, it has the same effect as the-
flag: left-justification. - precision, or maximum width
You can specify a precision (for numeric conversions) or a maximum width (for string conversions) by specifying a
followed by a number. For floating-point formats exceptg
, this specifies how many places right of the decimal point to show (the default being 6). For example:# these examples are subject to system-specific variation printf '<%f>', 1; # prints "<1.000000>" printf '<%.1f>', 1; # prints "<1.0>" printf '<%.0f>', 1; # prints "<1>" printf '<%07.2f>', 1.3; # prints "<0001.30>" printf '<%e>', 10; # prints "<1.000000e+01>" printf '<%.1e>', 10; # prints "<1.0e+01>"
For "g" and "G", this specifies the maximum number of significant digits to show; for example:
# These examples are subject to system-specific variation. printf '<%g>', 1; # prints "<1>" printf '<%.10g>', 1; # prints "<1>" printf '<%g>', 100; # prints "<100>" printf '<%.1g>', 100; # prints "<1e+02>" printf '<%.2g>', 100.01; # prints "<1e+02>" printf '<%.5g>', 100.01; # prints "<100.01>" printf '<%.4g>', 100.01; # prints "<100>" printf '<%.1g>', 0.0111; # prints "<0.01>" printf '<%.2g>', 0.0111; # prints "<0.011>" printf '<%.3g>', 0.0111; # prints "<0.0111>"
For integer conversions, specifying a precision implies that the output of the number itself should be zero-padded to this width, where the 0 flag is ignored:
printf '<%.6d>', 1; # prints "<000001>" printf '<%+.6d>', 1; # prints "<+000001>" printf '<%-10.6d>', 1; # prints "<000001 >" printf '<%10.6d>', 1; # prints "< 000001>" printf '<%010.6d>', 1; # prints "< 000001>" printf '<%+10.6d>', 1; # prints "< +000001>" printf '<%.6x>', 1; # prints "<000001>" printf '<%#.6x>', 1; # prints "<0x000001>" printf '<%-10.6x>', 1; # prints "<000001 >" printf '<%10.6x>', 1; # prints "< 000001>" printf '<%010.6x>', 1; # prints "< 000001>" printf '<%#10.6x>', 1; # prints "< 0x000001>"
For string conversions, specifying a precision truncates the string to fit the specified width:
printf '<%.5s>', "truncated"; # prints "<trunc>" printf '<%10.5s>', "truncated"; # prints "< trunc>"
You can also get the precision from the next argument using
, or from a specified argument (e.g., with.*2$
):printf '<%.6x>', 1; # prints "<000001>" printf '<%.*x>', 6, 1; # prints "<000001>" printf '<%.*2$x>', 1, 6; # prints "<000001>" printf '<%6.*2$x>', 1, 4; # prints "< 0001>"
If a precision obtained through
is negative, it counts as having no precision at all.printf '<%.*s>', 7, "string"; # prints "<string>" printf '<%.*s>', 3, "string"; # prints "<str>" printf '<%.*s>', 0, "string"; # prints "<>" printf '<%.*s>', -1, "string"; # prints "<string>" printf '<%.*d>', 1, 0; # prints "<0>" printf '<%.*d>', 0, 0; # prints "<>" printf '<%.*d>', -1, 0; # prints "<0>"
- size
For numeric conversions, you can specify the size to interpret the number as using
, orll
. For integer conversions (d u o x X b i D U O
), numbers are usually assumed to be whatever the default integer size is on your platform (usually 32 or 64 bits), but you can override this to use instead one of the standard C types, as supported by the compiler used to build Perl:hh interpret integer as C type "char" or "unsigned char" on Perl 5.14 or later h interpret integer as C type "short" or "unsigned short" j interpret integer as C type "intmax_t" on Perl 5.14 or later. Prior to Perl 5.30 this worked only with a C99 compiler, hence was unportable before that release. l interpret integer as C type "long" or "unsigned long" q, L, or ll interpret integer as C type "long long", "unsigned long long", or "quad" (typically 64-bit integers) t interpret integer as C type "ptrdiff_t" on Perl 5.14 or later z interpret integer as C types "size_t" or "ssize_t" on Perl 5.14 or later
Note that, in general, using the
modifier (for example, when writing"%ld"
instead of"%d"
) is unnecessary when used from Perl code. Moreover, it may be harmful, for example on Windows 64-bit where a long is 32-bits.As of 5.14, none of these raises an exception if they are not supported on your platform. However, if warnings are enabled, a warning of the
warning class is issued on an unsupported conversion flag. Should you instead prefer an exception, do this:use warnings FATAL => "printf";
If you would like to know about a version dependency before you start running the program, put something like this at its top:
use v5.14; # for hh/j/t/z/ printf modifiers
You can find out whether your Perl supports quads via Config:
use Config; if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) { print "Nice quads!\n"; }
For floating-point conversions (
e f g E F G
), numbers are usually assumed to be the default floating-point size on your platform (double or long double), but you can force "long double" withq
, orll
if your platform supports them. You can find out whether your Perl supports long doubles via Config:use Config; print "long doubles\n" if $Config{d_longdbl} eq "define";
You can find out whether Perl considers "long double" to be the default floating-point size to use on your platform via Config:
use Config; if ($Config{uselongdouble} eq "define") { print "long doubles by default\n"; }
It can also be that long doubles and doubles are the same thing:
use Config; ($Config{doublesize} == $Config{longdblsize}) && print "doubles are long doubles\n";
The size specifier
has no effect for Perl code, but is supported for compatibility with XS code. It means "use the standard size for a Perl integer or floating-point number", which is the default. - order of arguments
takes the next unused argument as the value to format for each format specification. If the format specification uses*
to require additional arguments, these are consumed from the argument list in the order they appear in the format specification before the value to format. Where an argument is specified by an explicit index, this does not affect the normal order for the arguments, even when the explicitly specified index would have been the next argument.So:
printf "<%*.*s>", $x, $y, $z;
for the width,$y
for the precision, and$z
as the value to format; while:printf '<%*1$.*s>', $x, $y;
would use
for the width and precision, and$y
as the value to format.Here are some more examples; be aware that when using an explicit index, the
may need escaping:printf "%2\$d %d\n", 12, 34; # will print "34 12\n" printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n" printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n" printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n" printf "%*1\$.*f\n", 4, 5, 10; # will print "5.0000\n"
use locale
(includinguse locale ':not_characters'
) is in effect andPOSIX::setlocale
has been called, the character used for the decimal separator in formatted floating-point numbers is affected by theLC_NUMERIC
locale. See perllocale and POSIX. - sqrt EXPR
- sqrt
Return the positive square root of EXPR. If EXPR is omitted, uses
. Works only for non-negative operands unless you've loaded theMath::Complex
module.use Math::Complex; print sqrt(-4); # prints 2i
- srand EXPR
- srand
Sets and returns the random number seed for the
operator.The point of the function is to "seed" the
function so thatrand
can produce a different sequence each time you run your program. When called with a parameter,srand
uses that for the seed; otherwise it (semi-)randomly chooses a seed (see below). In either case, starting with Perl 5.14, it returns the seed. To signal that your code will work only on Perls of a recent vintage:use v5.14; # so srand returns the seed
is not called explicitly, it is called implicitly without a parameter at the first use of therand
operator. However, there are a few situations where programs are likely to want to callsrand
. One is for generating predictable results, generally for testing or debugging. There, you usesrand($seed)
, with the same$seed
each time. Another case is that you may want to callsrand
after afork
to avoid child processes sharing the same seed value as the parent (and consequently each other).Do not call
(i.e., without an argument) more than once per process. The internal state of the random number generator should contain more entropy than can be provided by any seed, so callingsrand
again actually loses randomness.Most implementations of
take an integer and will silently truncate decimal numbers. This meanssrand(42)
will usually produce the same results assrand(42.1)
. To be safe, always passsrand
an integer.A typical use of the returned seed is for a test program which has too many combinations to test comprehensively in the time available to it each run. It can test a random subset each time, and should there be a failure, log the seed used for that run so that it can later be used to reproduce the same results.
If the
environment variable is set to a non-negative integer during process startup then calls tosrand()
with no arguments will initialize the perl random number generator with a consistent seed each time it is called, whether called explicitly with no arguments or implicitly via use ofrand()
. The exact seeding that a givenPERL_RAND_SEED
will produce is deliberately unspecified, but using different values forPERL_RAND_SEED
should produce different results. This is intended for debugging and performance analysis and is only guaranteed to produce consistent results between invocations of the same perl executable running the same code when all other factors are equal. The environment variable is read only once during process startup, and changing it during the program flow will not affect the currently running process. See perlrun for more details.rand
is not cryptographically secure. You should not rely on it in security-sensitive situations. As of this writing, a number of third-party CPAN modules offer random number generators intended by their authors to be cryptographically secure, including: Data::Entropy, Crypt::Random, Math::Random::Secure, and Math::TrulyRandom. - stat FILEHANDLE
- stat EXPR
- stat DIRHANDLE
- stat
Returns a 13-element list giving the status info for a file, either the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is omitted, it stats
!). Returns the empty list ifstat
fails. Typically used as follows:my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, $atime,$mtime,$ctime,$blksize,$blocks) = stat($filename);
Not all fields are supported on all filesystem types. Here are the meanings of the fields:
0 dev device number of filesystem 1 ino inode number 2 mode file mode (type and permissions) 3 nlink number of (hard) links to the file 4 uid numeric user ID of file's owner 5 gid numeric group ID of file's owner 6 rdev the device identifier (special files only) 7 size total size of file, in bytes 8 atime last access time in seconds since the epoch 9 mtime last modify time in seconds since the epoch 10 ctime inode change time in seconds since the epoch (*) 11 blksize preferred I/O size in bytes for interacting with the file (may vary from file to file) 12 blocks actual number of system-specific blocks allocated on disk (often, but not always, 512 bytes each)
(The epoch was at 00:00 January 1, 1970 GMT.)
(*) Not all fields are supported on all filesystem types. Notably, the ctime field is non-portable. In particular, you cannot expect it to be a "creation time"; see "Files and Filesystems" in perlport for details.
is passed the special filehandle consisting of an underline, no stat is done, but the current contents of the stat structure from the laststat
, or filetest are returned. Example:if (-x $file && (($d) = stat(_)) && $d < 0) { print "$file is executable NFS file\n"; }
(This works on machines only for which the device number is negative under NFS.)
On some platforms inode numbers are of a type larger than perl knows how to handle as integer numerical values. If necessary, an inode number will be returned as a decimal string in order to preserve the entire value. If used in a numeric context, this will be converted to a floating-point numerical value, with rounding, a fate that is best avoided. Therefore, you should prefer to compare inode numbers using
rather than==
will work fine on inode numbers that are represented numerically, as well as those represented as strings.Because the mode contains both the file type and its permissions, you should mask off the file type portion and (s)printf using a
if you want to see the real permissions.my $mode = (stat($filename))[2]; printf "Permissions are %04o\n", $mode & 07777;
In scalar context,
returns a boolean value indicating success or failure, and, if successful, sets the information associated with the special filehandle_
.The File::stat module provides a convenient, by-name access mechanism:
use File::stat; my $sb = stat($filename); printf "File is %s, size is %s, perm %04o, mtime %s\n", $filename, $sb->size, $sb->mode & 07777, scalar localtime $sb->mtime;
You can import symbolic mode constants and functions (
) from the Fcntl module:use Fcntl ':mode'; my $mode = (stat($filename))[2]; my $user_rwx = ($mode & S_IRWXU) >> 6; my $group_read = ($mode & S_IRGRP) >> 3; my $other_execute = $mode & S_IXOTH; printf "Permissions are %04o\n", S_IMODE($mode), "\n"; my $is_setuid = $mode & S_ISUID; my $is_directory = S_ISDIR($mode);
You could write the last two using the
operators. Commonly availableS_I*
constants are:# Permissions: read, write, execute, for user, group, others. S_IRWXU S_IRUSR S_IWUSR S_IXUSR S_IRWXG S_IRGRP S_IWGRP S_IXGRP S_IRWXO S_IROTH S_IWOTH S_IXOTH # Setuid/Setgid/Stickiness/SaveText/EnforcedLocks. # Note that the exact meaning of these is system-dependent. S_ISUID S_ISGID S_ISVTX S_ISTXT S_ENFMT # File types. Not all are necessarily available on # your system. S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT # The following are compatibility aliases for S_IRUSR, # S_IWUSR, and S_IXUSR. S_IREAD S_IWRITE S_IEXEC
and the
functions areS_IMODE($mode) the part of $mode containing the permission bits and the setuid/setgid/sticky bits S_IFMT($mode) the part of $mode containing the file type, which will match one of the S_IF* constants (e.g. S_IFMT($mode) == S_IFDIR for directories), but see the following helper functions # The operators -f, -d, -l, -b, -c, -p, and -S. S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode) S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode) # No direct -X operator counterpart, but for the first one # the -g operator is often equivalent. The ENFMT stands for # record flocking enforcement, a platform-dependent feature. S_ISENFMT($mode) S_ISWHT($mode)
See your native chmod(2) and stat(2) documentation for more details about the
constants. To get status info for a symbolic link instead of the target file behind the link, use thelstat
function.Portability issues: "stat" in perlport.
- state VARLIST
- state TYPE VARLIST
- state VARLIST : ATTRS
- state TYPE VARLIST : ATTRS
declares a lexically scoped variable, just likemy
. However, those variables will never be reinitialized, contrary to lexical variables that are reinitialized each time their enclosing block is entered. See "Persistent Private Variables" in perlsub for details.If more than one variable is listed, the list must be placed in parentheses. With a parenthesised list,
can be used as a dummy placeholder. However, since initialization of state variables in such lists is currently not possible this would serve no purpose.Like
, andour
can operate on a variable anywhere it appears in an expression (aside from interpolation in strings). The declaration will not apply to additional uses of the same variable until the next statement. This means additional uses of that variable within the same statement will act as they would have before that declaration occurred, or result in a strict 'vars' error, as appropriate.package main; use feature 'state'; our $x = 2; foo($x, state $x = $x + 1, $x); # foo() receives (2, 3, 2) foo($x, $main::x); # foo() receives (3, 2)
Redeclaring a variable in the same scope or statement will "shadow" the previous declaration, creating a new instance and preventing access to the previous one. This is usually undesired and, if warnings are enabled, will result in a warning in the
is available only if the"state"
feature is enabled or if it is prefixed withCORE::
. The"state"
feature is enabled automatically with ause v5.10
(or higher) declaration in the current scope. - study SCALAR
- study
At this time,
does nothing. This may change in the future.Prior to Perl version 5.16, it would create an inverted index of all characters that occurred in the given SCALAR (or
if unspecified). When matching a pattern, the rarest character from the pattern would be looked up in this index. Rarity was based on some static frequency tables constructed from some C programs and English text. - sub NAME BLOCK
- sub NAME (PROTO) BLOCK
- sub NAME : ATTRS BLOCK
- sub NAME (PROTO) : ATTRS BLOCK
This is subroutine definition, not a real function per se. Without a BLOCK it's just a forward declaration. Without a NAME, it's an anonymous function declaration, so does return a value: the CODE ref of the closure just created.
See perlsub and perlref for details about subroutines and references; see attributes and Attribute::Handlers for more information about attributes.
A special token that returns a reference to the current subroutine, or
outside of a subroutine.The behaviour of
within a regex code block (such as/(?{...})/
) is subject to change.This token is only available under
use v5.16
or the"current_sub"
feature. See feature. - substr EXPR,OFFSET,LENGTH,REPLACEMENT
- substr EXPR,OFFSET,LENGTH
- substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it. First character is at offset zero. If OFFSET is negative, starts that far back from the end of the string. If LENGTH is omitted, returns everything through the end of the string. If LENGTH is negative, leaves that many characters off the end of the string.
my $s = "The black cat climbed the green tree"; my $color = substr $s, 4, 5; # black my $middle = substr $s, 4, -11; # black cat climbed the my $end = substr $s, 14; # climbed the green tree my $tail = substr $s, -4; # tree my $z = substr $s, -4, 2; # tr
You can use the
function as an lvalue, in which case EXPR must itself be an lvalue. If you assign something shorter than LENGTH, the string will shrink, and if you assign something longer than LENGTH, the string will grow to accommodate it. To keep the string the same length, you may need to pad or chop your value usingsprintf
.If OFFSET and LENGTH specify a substring that is partly outside the string, only the part within the string is returned. If the substring is beyond either end of the string,
returns the undefined value and produces a warning. When used as an lvalue, specifying a substring that is entirely outside the string raises an exception. Here's an example showing the behavior for boundary cases:my $name = 'fred'; substr($name, 4) = 'dy'; # $name is now 'freddy' my $null = substr $name, 6, 2; # returns "" (no warning) my $oops = substr $name, 7; # returns undef, with warning substr($name, 7) = 'gap'; # raises an exception
An alternative to using
as an lvalue is to specify the REPLACEMENT string as the 4th argument. This allows you to replace parts of the EXPR and return what was there before in one operation, just as you can withsplice
.my $s = "The black cat climbed the green tree"; my $z = substr $s, 14, 7, "jumped from"; # climbed # $s is now "The black cat jumped from the green tree"
Note that the lvalue returned by the three-argument version of
acts as a 'magic bullet'; each time it is assigned to, it remembers which part of the original string is being modified; for example:my $x = '1234'; for (substr($x,1,2)) { $_ = 'a'; print $x,"\n"; # prints 1a4 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4 $x = '56789'; $_ = 'pq'; print $x,"\n"; # prints 5pq9 }
With negative offsets, it remembers its position from the end of the string when the target string is modified:
my $x = '1234'; for (substr($x, -3, 2)) { $_ = 'a'; print $x,"\n"; # prints 1a4, as above $x = 'abcdefg'; print $_,"\n"; # prints f }
Prior to Perl version 5.10, the result of using an lvalue multiple times was unspecified. Prior to 5.16, the result with negative offsets was unspecified.
- symlink OLDFILE,NEWFILE
Creates a new filename symbolically linked to the old filename. Returns
for success,0
otherwise. On systems that don't support symbolic links, raises an exception. To check for that, use eval:my $symlink_exists = eval { symlink("",""); 1 };
Portability issues: "symlink" in perlport.
- syscall NUMBER, LIST
Calls the system call specified as the first element of the list, passing the remaining elements as arguments to the system call. If unimplemented, raises an exception. The arguments are interpreted as follows: if a given argument is numeric, the argument is passed as an int. If not, the pointer to the string value is passed. You are responsible to make sure a string is pre-extended long enough to receive any result that might be written into a string. You can't use a string literal (or other read-only string) as an argument to
because Perl has to assume that any string pointer might be written through. If your integer arguments are not literals and have never been interpreted in a numeric context, you may need to add0
to them to force them to look like numbers. This emulates thesyswrite
function (or vice versa):require 'syscall.ph'; # may need to run h2ph my $s = "hi there\n"; syscall(SYS_write(), fileno(STDOUT), $s, length $s);
Note that Perl supports passing of up to only 14 arguments to your syscall, which in practice should (usually) suffice.
Syscall returns whatever value returned by the system call it calls. If the system call fails,
and sets$!
(errno). Note that some system calls can legitimately return-1
. The proper way to handle such calls is to assign$! = 0
before the call, then check the value of$!
.There's a problem with
: it returns the file number of the read end of the pipe it creates, but there is no way to retrieve the file number of the other end. You can avoid this problem by usingpipe
instead.Portability issues: "syscall" in perlport.
- sysopen FILEHANDLE,FILENAME,MODE
- sysopen FILEHANDLE,FILENAME,MODE,PERMS
Opens the file whose filename is given by FILENAME, and associates it with FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real filehandle wanted; an undefined scalar will be suitably autovivified. This function calls the underlying operating system's open(2) function with the parameters FILENAME, MODE, and PERMS.
Returns true on success and
otherwise.PerlIO layers will be applied to the handle the same way they would in an
call that does not specify layers. That is, the current value of${^OPEN}
as set by the open pragma in a lexical scope, or the-C
command-line option orPERL_UNICODE
environment variable in the main program scope, falling back to the platform defaults as described in "Defaults and how to override them" in PerlIO. If you want to remove any layers that may transform the byte stream, usebinmode
after opening it.The possible values and flag bits of the MODE parameter are system-dependent; they are available via the standard module
. See the documentation of your operating system's open(2) syscall to see which values and flag bits are available. You may combine several flags using the|
-operator.Some of the most common values are
for opening the file in read-only mode,O_WRONLY
for opening the file in write-only mode, andO_RDWR
for opening the file in read-write mode.For historical reasons, some values work on almost every system supported by Perl: 0 means read-only, 1 means write-only, and 2 means read/write. We know that these values do not work under OS/390; you probably don't want to use them in new code.
If the file named by FILENAME does not exist and the
call creates it (typically because MODE includes theO_CREAT
flag), then the value of PERMS specifies the permissions of the newly created file. If you omit the PERMS argument tosysopen
, Perl uses the octal value0666
. These permission values need to be in octal, and are modified by your process's currentumask
.In many systems the
flag is available for opening files in exclusive mode. This is not locking: exclusiveness means here that if the file already exists,sysopen
may not work on network filesystems, and has no effect unless theO_CREAT
flag is set as well. SettingO_CREAT|O_EXCL
prevents the file from being opened if it is a symbolic link. It does not protect against symbolic links in the file's path.Sometimes you may want to truncate an already-existing file. This can be done using the
flag. The behavior ofO_TRUNC
is undefined.You should seldom if ever use
as argument tosysopen
, because that takes away the user's option to have a more permissive umask. Better to omit it. Seeumask
for more on this.This function has no direct relation to the usage of
, orsysseek
. A handle opened with this function can be used with buffered IO just as one opened withopen
can be used with unbuffered IO.Note that under Perls older than 5.8.0,
depends on the fdopen(3) C library function. On many Unix systems, fdopen(3) is known to fail when file descriptors exceed a certain value, typically 255. If you need more file descriptors than that, consider using thePOSIX::open
function. For Perls 5.8.0 and later, PerlIO is (most often) the default.See perlopentut for a kinder, gentler explanation of opening files.
Portability issues: "sysopen" in perlport.
- sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
- sysread FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE, using read(2). It bypasses any PerlIO layers including buffered IO (but is affected by the presence of the
layer as described later), so mixing this with other kinds of reads,print
, oreof
can cause confusion because the:perlio
layers usually buffer data. Returns the number of bytes actually read,0
at end of file, or undef if there was an error (in the latter case$!
is also set). SCALAR will be grown or shrunk so that the last byte actually read is the last byte of the scalar after the read.An OFFSET may be specified to place the read data at some place in the string other than the beginning. A negative OFFSET specifies placement at that many characters counting backwards from the end of the string. A positive OFFSET greater than the length of SCALAR results in the string being padded to the required size with
bytes before the result of the read is appended.There is no syseof() function, which is ok, since
doesn't work well on device files (like ttys) anyway. Usesysread
and check for a return value of 0 to decide whether you're done.Note that if the filehandle has been marked as
will throw an exception. The:encoding(...)
layer implicitly introduces the:utf8
layer. Seebinmode
, and the open pragma. - sysseek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are
to set the new position to POSITION;1
to set it to the current position plus POSITION; and2
to set it to EOF plus POSITION, typically negative.Note the emphasis on bytes: even if the filehandle has been set to operate on characters (for example using the
I/O layer), theseek
, andsysseek
family of functions use byte offsets, not character offsets, because seeking to a character offset would be very slow in a UTF-8 file.sysseek
bypasses normal buffered IO, so mixing it with reads other thansysread
(for examplereadline
, oreof
may cause confusion.For WHENCE, you may also use the constants
, andSEEK_END
(start of the file, current position, end of the file) from the Fcntl module. Use of the constants is also more portable than relying on 0, 1, and 2. For example to define a "systell" function:use Fcntl 'SEEK_CUR'; sub systell { sysseek($_[0], 0, SEEK_CUR) }
Returns the new position, or the undefined value on failure. A position of zero is returned as the string
"0 but true"
; thussysseek
returns true on success and false on failure, yet you can still easily determine the new position. - system LIST
- system PROGRAM LIST
Does exactly the same thing as
, except that a fork is done first and the parent process waits for the child process to exit. Note that argument processing varies depending on the number of arguments. If there is more than one argument in LIST, or if LIST is an array with more than one value, starts the program given by the first element of the list with arguments given by the rest of the list. If there is only one scalar argument, the argument is checked for shell metacharacters, and if there are any, the entire argument is passed to the system's command shell for parsing (this is/bin/sh -c
on Unix platforms, but varies on other platforms). If there are no shell metacharacters in the argument, it is split into words and passed directly toexecvp
, which is more efficient. On Windows, only thesystem PROGRAM LIST
syntax will reliably avoid using the shell;system LIST
, even with more than one element, will fall back to the shell if the first spawn fails.Perl will attempt to flush all files opened for output before any operation that may do a fork, but this may not be supported on some platforms (see perlport). To be safe, you may need to set
in English) or call theautoflush
method ofIO::Handle
on any open handles.The return value is the exit status of the program as returned by the
call. To get the actual exit value, shift right by eight (see below). See alsoexec
. This is not what you want to use to capture the output from a command; for that you should use merely backticks orqx//
, as described in "`STRING`" in perlop. Return value of -1 indicates a failure to start the program or an error of the wait(2) system call (inspect$!
for the reason).If you'd like to make
(and many other bits of Perl) die on error, have a look at the autodie pragma.Like
allows you to lie to a program about its name if you use thesystem PROGRAM LIST
syntax. Again, seeexec
are ignored during the execution ofsystem
, if you expect your program to terminate on receipt of these signals you will need to arrange to do so yourself based on the return value.my @args = ("command", "arg1", "arg2"); system(@args) == 0 or die "system @args failed: $?";
If you'd like to manually inspect
's failure, you can check all possible failure modes by inspecting$?
like this:if ($? == -1) { print "failed to execute: $!\n"; } elsif ($? & 127) { printf "child died with signal %d, %s coredump\n", ($? & 127), ($? & 128) ? 'with' : 'without'; } else { printf "child exited with value %d\n", $? >> 8; }
Alternatively, you may inspect the value of
with theW*()
calls from the POSIX module.When
's arguments are executed indirectly by the shell, results and return codes are subject to its quirks. See "`STRING`" in perlop andexec
for details.Since
does afork
it may affect aSIGCHLD
handler. See perlipc for details.Portability issues: "system" in perlport.
- syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
- syswrite FILEHANDLE,SCALAR,LENGTH
- syswrite FILEHANDLE,SCALAR
Attempts to write LENGTH bytes of data from variable SCALAR to the specified FILEHANDLE, using write(2). If LENGTH is not specified, writes whole SCALAR. It bypasses any PerlIO layers including buffered IO (but is affected by the presence of the
layer as described later), so mixing this with reads (other thansysread)
, oreof
may cause confusion because the:perlio
layers usually buffer data. Returns the number of bytes actually written, orundef
if there was an error (in this case the errno variable$!
is also set). If the LENGTH is greater than the data available in the SCALAR after the OFFSET, only as much data as is available will be written.An OFFSET may be specified to write the data from some part of the string other than the beginning. A negative OFFSET specifies writing that many characters counting backwards from the end of the string. If SCALAR is of length zero, you can only use an OFFSET of 0.
WARNING: If the filehandle is marked
will raise an exception. The:encoding(...)
layer implicitly introduces the:utf8
layer. Alternately, if the handle is not marked with an encoding but you attempt to write characters with code points over 255, raises an exception. Seebinmode
, and the open pragma. - tell FILEHANDLE
- tell
Returns the current position in bytes for FILEHANDLE, or -1 on error. FILEHANDLE may be an expression whose value gives the name of the actual filehandle. If FILEHANDLE is omitted, assumes the file last read.
Note the emphasis on bytes: even if the filehandle has been set to operate on characters (for example using the
I/O layer), theseek
, andsysseek
family of functions use byte offsets, not character offsets, because seeking to a character offset would be very slow in a UTF-8 file.The return value of
for the standard streams like the STDIN depends on the operating system: it may return -1 or something else.tell
on pipes, fifos, and sockets usually returns -1.There is no
function. Usesysseek($fh, 0, 1)
for that.Do not use
(or other buffered I/O operations) on a filehandle that has been manipulated bysysread
, orsysseek
. Those functions ignore the buffering, whiletell
does not. - telldir DIRHANDLE
Returns the current position of the
routines on DIRHANDLE. Value may be given toseekdir
to access a particular location in a directory.telldir
has the same caveats about possible directory compaction as the corresponding system library routine. - tie VARIABLE,CLASSNAME,LIST
This function binds a variable to a package class that will provide the implementation for the variable. VARIABLE is the name of the variable to be enchanted. CLASSNAME is the name of a class implementing objects of correct type. Any additional arguments are passed to the appropriate constructor method of the class (meaning
, orTIEHASH
). Typically these are arguments such as might be passed to the dbm_open(3) function of C. The object returned by the constructor is also returned by thetie
function, which would be useful if you want to access other methods in CLASSNAME.Note that functions such as
may return huge lists when used on large objects, like DBM files. You may prefer to use theeach
function to iterate over such. Example:# print out history file offsets use NDBM_File; tie(my %HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); while (my ($key,$val) = each %HIST) { print $key, ' = ', unpack('L', $val), "\n"; }
A class implementing a hash should have the following methods:
TIEHASH classname, LIST FETCH this, key STORE this, key, value DELETE this, key CLEAR this EXISTS this, key FIRSTKEY this NEXTKEY this, lastkey SCALAR this DESTROY this UNTIE this
A class implementing an ordinary array should have the following methods:
TIEARRAY classname, LIST FETCH this, key STORE this, key, value FETCHSIZE this STORESIZE this, count CLEAR this PUSH this, LIST POP this SHIFT this UNSHIFT this, LIST SPLICE this, offset, length, LIST EXTEND this, count DELETE this, key EXISTS this, key DESTROY this UNTIE this
A class implementing a filehandle should have the following methods:
TIEHANDLE classname, LIST READ this, scalar, length, offset READLINE this GETC this WRITE this, scalar, length, offset PRINT this, LIST PRINTF this, format, LIST BINMODE this EOF this FILENO this SEEK this, position, whence TELL this OPEN this, mode, LIST CLOSE this DESTROY this UNTIE this
A class implementing a scalar should have the following methods:
TIESCALAR classname, LIST FETCH this, STORE this, value DESTROY this UNTIE this
Not all methods indicated above need be implemented. See perltie, Tie::Hash, Tie::Array, Tie::Scalar, and Tie::Handle.
, thetie
function will notuse
a module for you; you need to do that explicitly yourself. See DB_File or the Config module for interestingtie
implementations. - tied VARIABLE
Returns a reference to the object underlying VARIABLE (the same value that was originally returned by the
call that bound the variable to a package.) Returns the undefined value if VARIABLE isn't tied to a package. - time
Returns the number of non-leap seconds since whatever time the system considers to be the epoch, suitable for feeding to
. On most systems the epoch is 00:00:00 UTC, January 1, 1970; a prominent exception being Mac OS Classic which uses 00:00:00, January 1, 1904 in the current local time zone for its epoch.For measuring time in better granularity than one second, use the Time::HiRes module from Perl 5.8 onwards (or from CPAN before then), or, if you have gettimeofday(2), you may be able to use the
interface of Perl. See perlfaq8 for details.For date and time processing look at the many related modules on CPAN. For a comprehensive date and time representation look at the DateTime module.
- times
Returns a four-element list giving the user and system times in seconds for this process and any exited children of this process.
my ($user,$system,$cuser,$csystem) = times;
In scalar context,
.Children's times are only included for terminated children.
Portability issues: "times" in perlport.
- tr///
The transliteration operator. Same as
. See "Quote-Like Operators" in perlop. - truncate FILEHANDLE,LENGTH
- truncate EXPR,LENGTH
Truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length. Raises an exception if truncate isn't implemented on your system. Returns true if successful,
on error.The behavior is undefined if LENGTH is greater than the length of the file.
The position in the file of FILEHANDLE is left unchanged. You may want to call seek before writing to the file.
Portability issues: "truncate" in perlport.
- uc EXPR
- uc
Returns an uppercased version of EXPR. If EXPR is omitted, uses
.my $str = uc("Perl is GREAT"); # "PERL IS GREAT"
This function behaves the same way under various pragmas, such as in a locale, as
does.If you want titlecase mapping on initial letters see
instead.Note: This is the internal function implementing the
escape in double-quoted strings.my $str = "Perl is \Ugreat\E"; # "Perl is GREAT"
- ucfirst EXPR
- ucfirst
Returns the value of EXPR with the first character in uppercase (Unicode calls this titlecase). If EXPR is omitted,
.my $str = ucfirst("hello world!"); # "Hello world!"
This function behaves the same way under various pragmas, such as in a locale, as
does.Note: This is the internal function implementing the
escape in double-quoted strings.my $str = "\uperl\E is great"; # "Perl is great"
- umask EXPR
- umask
Sets the umask for the process to EXPR and returns the previous value. If EXPR is omitted, merely returns the current umask.
The Unix permission
is represented as three sets of three bits, or three octal digits:0750
(the leading 0 indicates octal and isn't one of the digits). Theumask
value is such a number representing disabled permissions bits. The permission (or "mode") values you passmkdir
are modified by your umask, so even if you tellsysopen
to create a file with permissions0777
, if your umask is0022
, then the file will actually be created with permissions0755
. If yourumask
(group can't write; others can't read, write, or execute), then passingsysopen
would create a file with mode0640
(because0666 &~ 027
).Here's some advice: supply a creation mode of
for regular files (insysopen
) and one of0777
for directories (inmkdir
) and executable files. This gives users the freedom of choice: if they want protected files, they might choose process umasks of022
, or even the particularly antisocial mask of077
. Programs should rarely if ever make policy decisions better left to the user. The exception to this is when writing files that should be kept private: mail files, web browser cookies, .rhosts files, and so on.If umask(2) is not implemented on your system and you are trying to restrict access for yourself (i.e.,
), raises an exception. If umask(2) is not implemented and you are not trying to restrict access for yourself, returnsundef
.Remember that a umask is a number, usually given in octal; it is not a string of octal digits. See also
, if all you have is a string.Portability issues: "umask" in perlport.
- undef EXPR
- undef
Undefines the value of EXPR, which must be an lvalue. Use only on a scalar value, an array (using
), a hash (using%
), a subroutine (using&
), or a typeglob (using*
). Sayingundef $hash{$key}
will probably not do what you expect on most predefined variables or DBM list values, so don't do that; seedelete
. Always returns the undefined value. You can omit the EXPR, in which case nothing is undefined, but you still get an undefined value that you could, for instance, return from a subroutine, assign to a variable, or pass as a parameter. Examples:undef $foo; undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; undef @ary; undef %hash; undef &mysub; undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; select undef, undef, undef, 0.25; my ($x, $y, undef, $z) = foo(); # Ignore third value returned
Note that this is a unary operator, not a list operator.
- unlink LIST
- unlink
Deletes a list of files. On success, it returns the number of files it successfully deleted. On failure, it returns false and sets
(errno):my $unlinked = unlink 'a', 'b', 'c'; unlink @goners; unlink glob "*.bak";
On error,
will not tell you which files it could not remove. If you want to know which files you could not remove, try them one at a time:foreach my $file ( @goners ) { unlink $file or warn "Could not unlink $file: $!"; }
will not attempt to delete directories unless you are superuser and the -U flag is supplied to Perl. Even if these conditions are met, be warned that unlinking a directory can inflict damage on your filesystem. Finally, usingunlink
on directories is not supported on many operating systems. Usermdir
instead. - unpack TEMPLATE,EXPR
- unpack TEMPLATE
does the reverse ofpack
: it takes a string and expands it out into a list of values. (In scalar context, it returns merely the first value produced.)If EXPR is omitted, unpacks the
string. See perlpacktut for an introduction to this function.The string is broken into chunks described by the TEMPLATE. Each chunk is converted separately to a value. Typically, either the string is a result of
, or the characters of the string represent a C structure of some kind.The TEMPLATE has the same format as in the
function. Here's a subroutine that does substring:sub substr { my ($what, $where, $howmuch) = @_; unpack("x$where a$howmuch", $what); }
and then there's
sub ordinal { unpack("W",$_[0]); } # same as ord()
In addition to fields allowed in
, you may prefix a field with a %<number> to indicate that you want a <number>-bit checksum of the items instead of the items themselves. Default is a 16-bit checksum. The checksum is calculated by summing numeric values of expanded values (for string fields the sum oford($char)
is taken; for bit fields the sum of zeroes and ones).For example, the following computes the same number as the System V sum program:
my $checksum = do { local $/; # slurp! unpack("%32W*", readline) % 65535; };
The following efficiently counts the number of set bits in a bit vector:
my $setbits = unpack("%32b*", $selectmask);
formats should be used with care. Since Perl has no way of checking whether the value passed tounpack
corresponds to a valid memory location, passing a pointer value that's not known to be valid is likely to have disastrous consequences.If there are more pack codes or if the repeat count of a field or a group is larger than what the remainder of the input string allows, the result is not well defined: the repeat count may be decreased, or
may produce empty strings or zeros, or it may raise an exception. If the input string is longer than one described by the TEMPLATE, the remainder of that input string is ignored.See
for more examples and notes. - unshift ARRAY,LIST
Add one or more elements to the beginning of an array. This is the opposite of a
.my @animals = ("cat"); unshift(@animals, "mouse"); # ("mouse", "cat") my @colors = ("red"); unshift(@colors, ("blue", "green")); # ("blue", "green", "red")
Returns the new number of elements in the updated array.
# Return value is the number of items in the updated array my $color_count = unshift(@colors, ("yellow", "purple")); say "There are $color_count colors in the updated array";
Note the LIST is prepended whole, not one element at a time, so the prepended elements stay in the same order. Use
to do the reverse.Starting with Perl 5.14, an experimental feature allowed
to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24. - untie VARIABLE
Breaks the binding between a variable and a package. (See tie.) Has no effect if the variable is not tied.
- use Module VERSION LIST
- use Module VERSION
- use Module LIST
- use Module
Imports some semantics into the current package from the named module, generally by aliasing certain subroutine or variable names into your package. It is exactly equivalent to
BEGIN { require Module; Module->import( LIST ); }
except that Module must be a bareword. The importation can be made conditional by using the if module.
forces therequire
to happen at compile time. Therequire
makes sure the module is loaded into memory if it hasn't been yet. Theimport
is not a builtin; it's just an ordinary static method call into theModule
package to tell the module to import the list of features back into the current package. The module can implement itsimport
method any way it likes, though most modules just choose to derive theirimport
method via inheritance from theExporter
class that is defined in theExporter
module. See Exporter. If noimport
method can be found, then the call is skipped, even if there is an AUTOLOAD method.If you do not want to call the package's
method (for instance, to stop your namespace from being altered), explicitly supply the empty list:use Module ();
That is exactly equivalent to
BEGIN { require Module }
If the VERSION argument is present between Module and LIST, then the
will call theVERSION
method in class Module with the given version as an argument:use Module 12.34;
is equivalent to:
BEGIN { require Module; Module->VERSION(12.34) }
The default
method, inherited from theUNIVERSAL
class, croaks if the given version is larger than the value of the variable$Module::VERSION
.The VERSION argument cannot be an arbitrary expression. It only counts as a VERSION argument if it is a version number literal, starting with either a digit or
followed by a digit. Anything that doesn't look like a version literal will be parsed as the start of the LIST. Nevertheless, many attempts to use an arbitrary expression as a VERSION argument will appear to work, because Exporter'simport
method handles numeric arguments specially, performing version checks rather than treating them as things to export.Again, there is a distinction between omitting LIST (
called with no arguments) and an explicit empty LIST()
not called). Note that there is no comma after VERSION!Because this is a wide-open interface, pragmas (compiler directives) are also implemented this way. Some of the currently implemented pragmas are:
use constant; use diagnostics; use integer; use sigtrap qw(SEGV BUS); use strict qw(subs vars refs); use subs qw(afunc blurfl); use warnings qw(all); use sort qw(stable);
Some of these pseudo-modules import semantics into the current block scope (like
, unlike ordinary modules, which import symbols into the current package (which are effective through the end of the file).Because
takes effect at compile time, it doesn't respect the ordinary flow control of the code being compiled. In particular, putting ause
inside the false branch of a conditional doesn't prevent it from being processed. If a module or pragma only needs to be loaded conditionally, this can be done using the if pragma:use if $] < 5.008, "utf8"; use if WANT_WARNINGS, warnings => qw(all);
There's a corresponding
declaration that unimports meanings imported byuse
, i.e., it callsModule->unimport(LIST)
instead ofimport
. It behaves just asimport
does with VERSION, an omitted or empty LIST, or no unimport method being found.no integer; no strict 'refs'; no warnings;
See perlmodlib for a list of standard modules and pragmas. See perlrun for the
command-line options to Perl that giveuse
functionality from the command-line. - use VERSION
Lexically enables all features available in the requested version as defined by the feature pragma, disabling any features not in the requested version's feature bundle. See feature.
VERSION may be either a v-string such as v5.24.1, which will be compared to
(aka $PERL_VERSION), or a numeric argument of the form 5.024001, which will be compared to$]
. An exception is raised if VERSION is greater than the version of the current Perl interpreter; Perl will not attempt to parse the rest of the file. Compare withrequire
, which can do a similar check at run time.If the specified Perl version is 5.12 or higher, strictures are enabled lexically as with
use strict
.If the specified Perl version is 5.35.0 or higher, warnings are enabled.
If the specified Perl version is 5.39.0 or higher, builtin functions are imported lexically as with
use builtin
with a corresponding version bundle.Use of
use VERSION
while another is in effect is not allowed with ause v5.39;
or greater version. For lower versions,use VERSION
will override most behavior of a previoususe VERSION
, possibly removingwarnings
effects added by it. This behavior is deprecated, and a future release of perl will disallow changing the version once one has been declared. Additionally, ause VERSION
with a version less than 5.11 is not allowed after ause VERSION
with a version greater than 5.11.use VERSION
does not load the feature.pm, strict.pm, warnings.pm or builtin.pm files, but instead implements the equivalent functionality directly.In the current implementation, any explicit use of
no strict
overridesuse VERSION
, even if it comes before it. However, this may be subject to change in a future release of Perl, so new code should not rely on this fact. It is recommended that ause VERSION
declaration be the first significant statement within a file (possibly after apackage
statement or any amount of whitespace or comment), so that its effects happen first, and other pragmata are applied after it.Specifying VERSION as a numeric argument of the form 5.024001 should generally be avoided as older less readable syntax compared to v5.24.1. Before perl 5.8.0 released in 2002 the more verbose numeric form was the only supported syntax, which is why you might see it in older code.
use v5.24.1; # compile time version check use 5.24.1; # ditto use 5.024_001; # ditto; older syntax compatible with perl 5.6
This is often useful if you need to check the current Perl version before
ing library modules that won't work with older versions of Perl. (We try not to do this more than we have to.)Symmetrically,
no VERSION
allows you to specify that you want a version of Perl older than the specified one. Historically this was added during early designs of the Raku language (formerly "Perl 6"), so that a Perl 5 program could beginno 6;
to declare that it is not a Perl 6 program. As the two languages have different implementations, file naming conventions, and other infrastructure, this feature is now little used in practice and should be avoided in newly-written code.
Care should be taken when using the
no VERSION
form, as it is only meant to be used to assert that the running Perl is of a earlier version than its argument and not to undo the feature-enabling side effects ofuse VERSION
. - utime LIST
Changes the access and modification times on each file of a list of files. The first two elements of the list must be the NUMERIC access and modification times, in that order. Returns the number of files successfully changed. The inode change time of each file is set to the current time. For example, this code has the same effect as the Unix touch(1) command when the files already exist and belong to the user running the program:
#!/usr/bin/perl my $atime = my $mtime = time; utime $atime, $mtime, @ARGV;
Since Perl 5.8.0, if the first two elements of the list are
, the utime(2) syscall from your C library is called with a null second argument. On most systems, this will set the file's access and modification times to the current time (i.e., equivalent to the example above) and will work even on files you don't own provided you have write permission:for my $file (@ARGV) { utime(undef, undef, $file) || warn "Couldn't touch $file: $!"; }
Under NFS this will use the time of the NFS server, not the time of the local machine. If there is a time synchronization problem, the NFS server and local machine will have different times. The Unix touch(1) command will in fact normally use this form instead of the one shown in the first example.
Passing only one of the first two elements as
is equivalent to passing a 0 and will not have the effect described when both areundef
. This also triggers an uninitialized warning.On systems that support futimes(2), you may pass filehandles among the files. On systems that don't support futimes(2), passing filehandles raises an exception. Filehandles must be passed as globs or glob references to be recognized; barewords are considered filenames.
Portability issues: "utime" in perlport.
- values HASH
- values ARRAY
In list context, returns a list consisting of all the values of the named hash. In Perl 5.12 or later only, will also return a list of the values of an array; prior to that release, attempting to use an array argument will produce a syntax error. In scalar context, returns the number of values.
Hash entries are returned in an apparently random order. The actual random order is specific to a given hash; the exact same series of operations on two hashes may result in a different order for each hash. Any insertion into the hash may change the order, as will any deletion, with the exception that the most recent key returned by
may be deleted without changing the order. So long as a given hash is unmodified you may rely onkeys
to repeatedly return the same order as each other. See "Algorithmic Complexity Attacks" in perlsec for details on why hash order is randomized. Aside from the guarantees provided here the exact details of Perl's hash algorithm and the hash traversal order are subject to change in any release of Perl. Tied hashes may behave differently to Perl's hashes with respect to changes in order on insertion and deletion of items.As a side effect, calling
resets the HASH or ARRAY's internal iterator (seeeach
) before yielding the values. In particular, callingvalues
in void context resets the iterator with no other overhead.Apart from resetting the iterator,
values @array
in list context is the same as plain@array
. (We recommend that you use void contextkeys @array
for this, but reasoned that takingvalues @array
out would require more documentation than leaving it in.)Note that the values are not copied, which means modifying them will modify the contents of the hash:
for (values %hash) { s/foo/bar/g } # modifies %hash values for (@hash{keys %hash}) { s/foo/bar/g } # same
Starting with Perl 5.14, an experimental feature allowed
to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.To avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious syntax errors, put this sort of thing at the top of your file to signal that your code will work only on Perls of a recent vintage:
use v5.12; # so keys/values/each work on arrays
- vec EXPR,OFFSET,BITS
Treats the string in EXPR as a bit vector made up of elements of width BITS and returns the value of the element specified by OFFSET as an unsigned integer. BITS therefore specifies the number of bits that are reserved for each element in the bit vector. This must be a power of two from 1 to 32 (or 64, if your platform supports that).
If BITS is 8, "elements" coincide with bytes of the input string.
If BITS is 16 or more, bytes of the input string are grouped into chunks of size BITS/8, and each group is converted to a number as with
with big-endian formatsn
(and analogously for BITS==64). Seepack
for details.If bits is 4 or less, the string is broken into bytes, then the bits of each byte are broken into 8/BITS groups. Bits of a byte are numbered in a little-endian-ish way, as in
. For example, breaking the single input bytechr(0x36)
into two groups gives a list(0x6, 0x3)
; breaking it into 4 groups gives(0x2, 0x1, 0x3, 0x0)
may also be assigned to, in which case parentheses are needed to give the expression the correct precedence as invec($image, $max_x * $x + $y, 8) = 3;
If the selected element is outside the string, the value 0 is returned. If an element off the end of the string is written to, Perl will first extend the string with sufficiently many zero bytes. It is an error to try to write off the beginning of the string (i.e., negative OFFSET).
If the string happens to be encoded as UTF-8 internally (and thus has the UTF8 flag set),
tries to convert it to use a one-byte-per-character internal representation. However, if the string contains characters with values of 256 or higher, a fatal error will occur.Strings created with
can also be manipulated with the logical operators|
, and~
. These operators will assume a bit vector operation is desired when both operands are strings. See "Bitwise String Operators" in perlop.The following code will build up an ASCII string saying
. The comments show the string after each step. Note that this code works in the same way on big-endian or little-endian machines.my $foo = ''; vec($foo, 0, 32) = 0x5065726C; # 'Perl' # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') vec($foo, 2, 16) = 0x5065; # 'PerlPe' vec($foo, 3, 16) = 0x726C; # 'PerlPerl' vec($foo, 8, 8) = 0x50; # 'PerlPerlP' vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" vec($foo, 21, 4) = 7; # 'PerlPerlPer' # 'r' is "\x72" vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" vec($foo, 94, 1) = 1; # 'PerlPerlPerl' # 'l' is "\x6c"
To transform a bit vector into a string or list of 0's and 1's, use these:
my $bits = unpack("b*", $vector); my @bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used in place of the
.Here is an example to illustrate how the bits actually fall in place:
#!/usr/bin/perl -wl print <<'EOT'; 0 1 2 3 unpack("V",$_) 01234567890123456789012345678901 ------------------------------------------------------------------ EOT for $w (0..3) { $width = 2**$w; for ($shift=0; $shift < $width; ++$shift) { for ($off=0; $off < 32/$width; ++$off) { $str = pack("B*", "0"x32); $bits = (1<<$shift); vec($str, $off, $width) = $bits; $res = unpack("b*",$str); $val = unpack("V", $str); write; } } } format STDOUT = vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> $off, $width, $bits, $val, $res . __END__
Regardless of the machine architecture on which it runs, the example above should print the following table:
0 1 2 3 unpack("V",$_) 01234567890123456789012345678901 ------------------------------------------------------------------ vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
- wait
Behaves like wait(2) on your system: it waits for a child process to terminate and returns the pid of the deceased process, or
if there are no child processes. The status is returned in$?
. Note that a return value of-1
could mean that child processes are being automatically reaped, as described in perlipc.If you use
in your handler for$SIG{CHLD}
, it may accidentally wait for the child created byqx
. See perlipc for details.Equivalent to
waitpid(-1, 0)
.Portability issues: "wait" in perlport.
- waitpid PID,FLAGS
Waits for a particular child process to terminate and returns the pid of the deceased process, or
if there is no such child process. A non-blocking wait (with WNOHANG in FLAGS) can return 0 if there are child processes matching PID but none have terminated yet. The status is returned in$?
.A PID of
indicates to wait for any child process whose process group ID is equal to that of the current process. A PID of less than-1
indicates to wait for any child process whose process group ID is equal to -PID. A PID of-1
indicates to wait for any child process.If you say
use POSIX ":sys_wait_h"; my $kid; do { $kid = waitpid(-1, WNOHANG); } while $kid > 0;
1 while waitpid(-1, WNOHANG) > 0;
then you can do a non-blocking wait for all pending zombie processes (see "WAIT" in POSIX). Non-blocking wait is available on machines supporting either the waitpid(2) or wait4(2) syscalls. However, waiting for a particular pid with FLAGS of
is implemented everywhere. (Perl emulates the system call by remembering the status values of processes that have exited but have not been harvested by the Perl script yet.)Note that on some systems, a return value of
could mean that child processes are being automatically reaped. See perlipc for details, and for other examples.Portability issues: "waitpid" in perlport.
- wantarray
Returns true if the context of the currently executing subroutine or
is looking for a list value. Returns false if the context is looking for a scalar. Returns the undefined value if the context is looking for no value (void context).return unless defined wantarray; # don't bother doing more my @a = complex_calculation(); return wantarray ? @a : "@a";
's result is unspecified in the top level of a file, in aBEGIN
block, or in aDESTROY
method.This function should have been named wantlist() instead.
- warn LIST
Emits a warning, usually by printing it to
interprets its operand LIST in the same way asdie
, but is slightly different in what it defaults to when LIST is empty or makes an empty string. If it is empty and$@
already contains an exception value then that value is used after appending"\t...caught"
. If it is empty and$@
is also empty then the string"Warning: Something's wrong"
is used.By default, the exception derived from the operand LIST is stringified and printed to
. This behaviour can be altered by installing a$SIG{__WARN__}
handler. If there is such a handler then no message is automatically printed; it is the handler's responsibility to deal with the exception as it sees fit (like, for instance, converting it into adie
). Most handlers must therefore arrange to actually display the warnings that they are not prepared to deal with, by callingwarn
again in the handler. Note that this is quite safe and will not produce an endless loop, since__WARN__
hooks are not called from inside one.You will find this behavior is slightly different from that of
handlers (which don't suppress the error text, but can instead calldie
again to change it).Using a
handler provides a powerful way to silence all warnings (even the so-called mandatory ones). An example:# wipe out *all* compile-time warnings BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } my $foo = 10; my $foo = 20; # no warning about duplicate my $foo, # but hey, you asked for it! # no compile-time or run-time warnings before here $DOWARN = 1; # run-time warnings enabled after here warn "\$foo is alive and $foo!"; # does show up
See perlvar for details on setting
entries and for more examples. See the Carp module for other kinds of warnings using itscarp
functions. - write FILEHANDLE
- write EXPR
- write
Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, using the format associated with that file. By default the format for a file is the one having the same name as the filehandle, but the format for the current output channel (see the
function) may be set explicitly by assigning the name of the format to the$~
variable.Top of form processing is handled automatically: if there is insufficient room on the current page for the formatted record, the page is advanced by writing a form feed and a special top-of-page format is used to format the new page header before the record is written. By default, the top-of-page format is the name of the filehandle with
appended, ortop
in the current package if the former does not exist. This would be a problem with autovivified filehandles, but it may be dynamically set to the format of your choice by assigning the name to the$^
variable while that filehandle is selected. The number of lines remaining on the current page is in variable$-
, which can be set to0
to force a new page.If FILEHANDLE is unspecified, output goes to the current default output channel, which starts out as STDOUT but may be changed by the
operator. If the FILEHANDLE is an EXPR, then the expression is evaluated and the resulting string is used to look up the name of the FILEHANDLE at run time. For more on formats, see perlform.Note that write is not the opposite of
. Unfortunately. - y///
The transliteration operator. Same as
. See "Quote-Like Operators" in perlop.
Non-function Keywords by Cross-reference
These keywords are documented in "Special Literals" in perldata.
These compile phase keywords are documented in "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod.
This method keyword is documented in "Destructors" in perlobj.
- and
- cmp
- eq
- ge
- gt
- isa
- le
- lt
- ne
- not
- or
- x
- xor
These operators are documented in perlop.
This keyword is documented in "Autoloading" in perlsub.
- else
- elsif
- for
- foreach
- if
- unless
- until
- while
These flow-control keywords are documented in "Compound Statements" in perlsyn.
- elseif
The "else if" keyword is spelled
in Perl. There's noelif
orelse if
either. It does parseelseif
, but only to warn you about not using it.See the documentation for flow-control keywords in "Compound Statements" in perlsyn.
- default
- given
- when
These flow-control keywords related to the experimental switch feature are documented in "Switch Statements" in perlsyn.
- try
- catch
- finally
These flow-control keywords related to the experimental
feature are documented in "Try Catch Exception Handling" in perlsyn.
- defer
This flow-control keyword related to the experimental
feature is documented in "defer blocks" in perlsyn.
This class-related phaser block is documented in perlclass.