To delete a file in Linux:  What minimal permissions do we need to set on it?  What minimal permissions do we need to set on its parent directory?
If I create a file and then change its permissions to 444 (read-only), how come rm can remove it?  If I do this:  echo test &gt; test.txt chmod 444 test.txt rm test.txt   ...rm will ask if I want to remove the write-protected file test.txt. I would have expected that rm can not remove such a file and that I would have to do a chmod +w test.txt first. If I do rm -f test.txt then rm will remove the file without even asking, even though it's read-only.   Can anyone clarify? I'm using Ubuntu 12.04/bash.
How to insert strings containing slashes with sed? Software recommendations Four balls with different colors in a box, how many times do I need to pick to see all four colors? In Gmail, what is the safest way to download, save & print all emails with a given label? Attempting to securely download all the emails with a given label in Gmail, save them to a single file, and print that file. How can I make a material only apply to a side of a plane? Infinite Series $\sum\limits_{k=1}^{\infty}\frac{k^n}{k!}$ I need a robust and simple way to remove illegal path and file characters from a simple string. I've used the below code but it doesn't seem to do anything, what am I missing?  using System; using System.IO;  namespace ConsoleApplication1 {     class Program     {         static void Main(string[] args)         {             string illegal = "\"M&lt;&gt;\"\\a/ry/ h**ad:&gt;&gt; a\\/:*?\"&lt;&gt;| li*tt|le|| la\"mb.?";              illegal = illegal.Trim(Path.GetInvalidFileNameChars());             illegal = illegal.Trim(Path.GetInvalidPathChars());              Console.WriteLine(illegal);             Console.ReadLine();         }     } } I want to search for all .conf files in /etc/. I tried grep -r *.conf /etc, but the result isn't right. What am I doing wrong? Suppose the mean noon-time temperature for September days in San Diego is 24∘ and the standard deviation is 4.9. (Temperature in this problem is measured in degrees celsius)  On September 26, 1963, the all-time record of noon-time temperature in San Diego of 44∘ was hit. Assume the temperature distribution is symmetric around the mean, what is the Chebyshev bound for the probability of breaking (or tieing) this record?  I am having a hard time understanding this. Could someone explain to me how to do this? Windows does not mount USB NTFS superfloppy I have a Transcend Storejet external USB hard disk. This is not an SSD, but a mechanical disk with rotating plates. I have formatted the whole disk, without partitioning, as NTFS. I have used mkntfs tool under Linux.  When plugged into a Linux machine, the system sees a drive with two partitions (/dev/sdc /dev/sdc1 /dev/sdc2). However, as I know it is a partitionless disk, I can mount the whole device (mount -t ntfs /dev/sdc /mnt) and it works without any problems.  When plugged into a MS Windows XP machine, the systems sees a disk with two unformatted partitions, and it does not assign a drive letter to either the whole disk, nor to any of the partitions.  Would anyone know how to get MS Windows to mount my disk as a NTFS superfloppy?  I have already tried removing old USB mounting points and residual devices using 'DriveCleanup'. It did not help.  By the way, I also have an external Kingston USB SSD, also formatted as NTFS superfloppy, without partitioning. However, this one is recognized and mounted normally by MS Windows. overset text over equal sign in in an Align environment Look at the following example  \begin{align}      &amp; x \\      \overset{\text{some long explanation}}= &amp; foo \\      &amp; = bar \end{align}   what is the best way to get the equal signs aligned? Now I have seen different solution such as adding \overset{\hphantom{\ţext{some long explanation}}}= to the beginning of every line, but I don't really think that is the best solution. Any better ideas?    UPDATE  I guess what i am actually after is   aligning the first column of the align environment right aligning the equal sign under the overset text right Command-line mv exclusion list How much memory can a 32 bit process access on a 64 bit operating system? Nautilus doesn't launch on Ubuntu Interpretation of log transformed predictor and/or response I'm wondering if it makes a difference in interpretation whether only the dependent, both the dependent and independent, or only the independent variables are log transformed.  Consider the case of  log(DV) = Intercept + B1*IV + Error    I can interpret the IV as the percent increase but how does this change when I have   log(DV) = Intercept + B1*log(IV) + Error   or when I have   DV = Intercept + B1*log(IV) + Error   ? Update help center on syntax highlighting to point to an actual list of options Reading the help center article on syntax highlighting, one of the paragraphs within it reads:     You can use either , like lang-cpp or lang-sql, or you can specify a tag, and the syntax highlighting language associated with this tag will be used:   Note the link in there, which goes to .  Problem is, a long time ago we removed the list of options from several questions/answers across meta Stack Exchange because it was a pain trying to keep all of them updated. It makes a lot more sense to just have one list that we keep updated, and the most logical place for that list was in the , which is where it now resides.  Can the link in the help center be updated to point directly to the list, rather than to a post that explains that the list has been moved? In the calculus of variations, particularly Lagrangian mechanics, people often say we vary the position and the velocity independently. But velocity is the derivative of position, so how can you treat them as independent variables? Is there a simple reason why the expected number of coin flips till getting $m$ more heads than tails or $n$ more tails than heads should be $mn$? I flip a coin until I get $m$ more heads than tails, or $n$ more tails than heads. Let the expected number of flips of the coin before stopping be $f(m,n)$.  I obtained $f(m,n)=mn$ from the recursion $f(m,n)=1+\frac{f(m-1,n+1)+f(m+1,n-1)}2$ with $f(k,0)=f(0,k)=0$ for all $k$.   Other than going through this recursion (and either solving by inspection or by writing as linear recurrence in single variable and solving brute force), is there an intuitive reason you should expect this process to take $mn$ flips? I was thinking about the more general problem with probability $p$ of getting heads and was struck by how simple the formula became when handling, what turned out to be a special case (general formula broke down) of $p=\frac12$. Just curious whether the process takes a while, and whether it scrapes through history to determine if it was once valid or not. Currently, I have 6 0-vote accepted answers and 20 1+-vote answers (), and that makes it > 5 and > 20% but not for long if voting goes up or down from here. ;-P  Update:  Special thanks to Grace Note for pointing out the  to follow. I upgraded this to a bug because it's been 48 hours during which the criteria supposedly were met. 6 0-votes, 20-23 1+ votes during the whole period. The criteria only last so long, so it may go out of scope soon. If it only checks every 3 days, I'm not sure if that's intentional or a bug. I have a current view set up that displays a feed of stories and is sorted in descending order. I now need to have the first 4 stories display as a different style then the remaining ones. I have pagination set to 10. I only want the first 4 to show differently on page 1, and then the current style will show on the remaining pages. Is there a way to do this?