What’s the purpose of `rm -P`

optionsrm

I was reading the man page for rm when I came across this option:

-P    Overwrite regular files before deleting them.  Files are overwritten 
      three times, first with the byte pattern 0xff, then 0x00, and 
      then 0xff again, before they are deleted.

I guess -P is meant for thoroughly deleting a file, but wouldn't setting all the bytes to 0xff or 0x00 be enough? Why does it have to toggle between the two three times?

Best Answer

There is a technique called residual information retrieval that can read data that was deleted based on the idea that when the drive is magnetized in order to store data other parts that are close to the data is also affected by this and it should be possible to re-read data this way ... this is though a costly technique, but use it if you are paranoid ;)

By writing data 3 times (in this case) the parts next to the track on the drive should be re-set as well in order to make it impossible to re-read this way.

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What’s the reason that “rm -rf a/b” says “a/b is not empty”

rmdir(2) will fail if the directory is not empty. If another process is creating files while rm(1) is removing them, it will not know to delete them and consequently when it comes time for rm(1) to try to delete what it believes should be an empty directory, it will fail with the error you've posted.

One way to delete the directory in the face of concurrent file creations in the directory is to rename it:

mv a a~
rm -rf a~

It's possible that this may not work if the processes creating the files in a/b are not doing so by path (open(2) vs. openat(2)).

I am assuming that the process(es) that creates files in a/b will recreate that directory if it does not exist, or will handle failure gracefully if it does not exist. Since you are already trying to delete the directory from under other processes, that seems like a safe assumption.

What’s the reason for this specific “rm” sequence in the xkcd joke

If you start at the very top, it's possible that you'll wipe out something that rm (or some other critical part of the system) needs to continue, and the evilness will be left incomplete.

These commands will make sure that at least the cwd and the user's home directory are gone before going nuclear.

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What’s the reason that “rm -rf a/b” says “a/b is not empty”

What’s the reason for this specific “rm” sequence in the xkcd joke

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