I am building a social networking website. After user signs up an account, I add his/her information to the database and creates 3 different tables for his/her other information, they are called like this ($username . "_posts"),($username . "_friends") and ($username . "_notifications"). In the creating statements I can not use placeholders for the table name to prevent SQL injection.  So, I was thinking of a way to prevent the user from manipulating the statement I use to create the tables by his/her input.  So, what if I prevented the user from inputting characters like (back quote) , (single quote) or (double quote) ....etc to the input field (username) ,  will this work? Is there a better way? If I choose to use way mentioned above what other characters should I prevent the user from inputting? thanks in advance.  Here is one of the create statements I use:  $create_table_for_user_posts = $conn-&gt;prepare("CREATE TABLE " . $username . "_posts (     id INT(255) UNSIGNED AUTO_INCREMENT PRIMARY KEY,     posts VARCHAR(35000) NOT NULL,     poster_username VARCHAR(2000) NOT NULL,     poster_first_name VARCHAR(2000) NOT NULL,     poster_last_name VARCHAR(2000) NOT NULL,     reg_date TIMESTAMP )");
If user input is inserted without modification into an SQL query, then the application becomes vulnerable to , like in the following example:  $unsafe_variable = $_POST['user_input'];   mysql_query("INSERT INTO `table` (`column`) VALUES ('$unsafe_variable')");   That's because the user can input something like value'); DROP TABLE table;--, and the query becomes:  INSERT INTO `table` (`column`) VALUES('value'); DROP TABLE table;--')   What can be done to prevent this from happening?
Separator between command list and } The Bash Manual says   { list; }        Placing a list of commands between curly braces causes the list to be executed in the current shell context. No subshell is   created. The semicolon (or newline) following list is required.      The braces are reserved words, so they must be separated from the list   by blanks or other shell metacharacters.       The parentheses are operators, and are recognized as separate tokens   by the shell even if they are not separated from the list by   whitespace.   If I remove the semicolon, like this:  $ { date } &gt;    Why does it expect stdin input? A metacharacter is a character that separates words.  The semicolon and whitespace are both "shell metacharacters". Why can't whitespace separate words date and }? Why do we need a semicolon instead of a whitespace for separating the words? 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On my home machine with a newer version of apt-get I can  apt-get download bash   and it does exactly what I want it to.  How can I perform the same on the host machine? What is the best way to filter a Java Collection? Dark Mode for Stack Exchange sites Each Stack Exchange site should provide users the option of using a dark mode design/theme.  Why is this feature missing?  The major IDEs are already a letdown regarding dark color schemes. Is there a way to select the children of a parent object? How do I wrap a macro definition in an environment? I'm creating a style file, and want to store the body text of the abstract environment so that I can put it on my custom titlepage. I could have required that my users enter the abstract as  \customabstract{Abstract text goes here}`   which I would have implemented as something like  \newcommand{\customabstract}[1]{\gdef\abstracttext{#1}}   However, I would prefer that the user is able to use environment notation,  \begin{customabstract}Abstract text goes here\end{customabstract}   To do this, I thought I could define something like  \newenvironment{customabstract}{\gdef\abstracttext\bgroup}{\egroup}   But this definition makes \abstracttext expand to \bgroup, which is not what I wanted. I tried to solve the problem by \expandafter,  \newenvironment{customabstract}{\expandafter\gdef\expandafter\abstracttext\bgroup}{\egroup}   but this results in a runaway error.  Is there any way to acheive what I want? Answers that helps me understand the inner workings of TeX (instead of just referring to a custom package) are preferred. Prove $\sup(f+g) \le \sup f + \sup g$ Suppose $D$ is a nonempty bounded subset of reals. Let $f:D \to \mathbb R$ and $g:D \to \mathbb R$. Define $(f+g)(x)=f(x)+g(x)$. Prove $\sup(f+g)(D) \le \sup f(D) + \sup g(D)$ (also prove that $\sup (f+g)$ exists).   I understand why this is the case, just not how to prove it. Left side is pretty much $\sup  (f(x)+g(x))$ and right side is $\sup (f(x) + g(y))$ for $x,\,y \in D$. Basically $f+g$ has to use the same variable and $f(D)+g(D)$ use different ones. But I don't know how to go about proving this.     The second part of the question is to find a specific example where strict inequality holds.    Let $D=\{a,b\}$ and $f: a \to 1,\, b\to 0, \,g: a \to 0,\, b\to 1$. $\sup f(D) = 1,\, \sup g(D) = 1,\, \sup f(D) + \sup g(D) = 2.$ $\sup (f+g)(D) = 1$ (if we choose a, $f+g = 1+0,\, b,\, f+g=0+1$). Possible to bake texture to new UV map? I believe I have solved a famous open problem. How do I convince people in the field that I am not a crank? I am interested in the situation where you have a very interesting result. For instance, you have solved a very important open problem. However, you are not known in the field and do not have any remarkable publications. Your supervisor thinks the work is good and you submit the work to a high profile journal, but you get rejected.  The thing is that the contribution is very strong. It breaks what most people believe or what they have already proven: e.g., you solve the P vs. NP problem or any other well known open problem.   The reviewers strongly reject your work with no justification and they do not state why the result is wrong. Examples of reviewer comments include:   "The proof must be wrong."  "You cannot achieve such a result."  "You do not understand well the notion of ..."   My question is what to do in this situation? Where to go? If your advisor accepts the work, but the reviewers from the top journal reject the work without even explaining the mistakes,  what should you do? What are Unity's keyboard and mouse shortcuts? In Unity, are there any pre-defined shortcuts, e.g. to open the dash or unhide the launcher? Which document class is being used? Is there a way to programmatically determine which document class is in effect?  I'd like to be able to write conditional macros that behave differently depending on which document class is currently being used. What kind of ship was flying away from Rey in her flashback? The ship flying away from Rey in her flashback of being left on Jakku looks like one of the ships in the group that takes her away when she is captured.  If so, and it is a First Order ship that dropped her off, this is an interesting Easter egg. mysql-server-5.7 package post-installation script subprocess returned error exit status 127 I have a website server on Ubuntu server 18.04 with apache and mysql. The database stopped for no apparent reason. I just did a restart of mysql server to solve the issue but after a simple update I got some errors on mysql-server package. The update was for other packages and not mysql-server package. Mysql still stops at least once a week and each time I have to restarts the mysql-server.  Can some one help me to understand the error please ?  Here the result of: dpkg --configure -a  Configure mysql-server-5.7 (5.7.23-0ubuntu0.18.04.1) ... /var/lib/dpkg/info/mysql-server-5.7.postinst: ligne 191: /usr/share/mysql-common/configure-symlinks: no file or folder  dpkg: erreur de traitement du paquet mysql-server-5.7 (--configure) : installed mysql-server-5.7 package post-installation script subprocess returned error exit status 127 I was wondering why the software center doesn't download all the requested apps and install one by one as they finish to download. Or instead why the software center doesn't download an app while the other is installing If $m^4+4^n$ is prime, then $m=n=1$ or $m$ is odd and $n$ even I have been stuck on this one for months, really simple to state, really giving me trouble.     Show that if $m^4 + 4^n$ is prime, $m&gt;0$, $n&gt;0$, then $m$ is odd and $n$ is even, except when $m=n=1$.   The case $m=n=1$ gives $5$, so that is why it is excluded. Clearly $m$ is odd, for if it was even, the number would be divisible by at least $4$, but I can't seem to get rid of the case where $n$ must be odd.   The only thing I've managed to try is to write $m$ as an odd number, say $m = 2k+1$, and $n = 2\ell + 1$, then get $$ m^4 + 4^n = (2k+1)^4 + 4^{2\ell+1} = 16k^4 + 32k^3 + 24k^2 + 8k + 1 + 4^{2 \ell + 1} $$ and this is congruent to $0 \mod 5$ unless $k \equiv 2 \mod 5$, and now it gives you an even more ugly polynomial if I write $k = 5j+2$, and it's not working. I've tried other inconclusive approaches, like trying to see if two numbers of some form generate numbers of the same form.. (for instance primes of the form 8k+1 and 8k+7 always generate primes of the same form when multiplied together, stuff like that).  Any ideas? 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