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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 > 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.

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Clarification: I'm running these commands as my regular user, not as root. –  Magnus Sep 19 '12 at 8:42

2 Answers 2

up vote 44 down vote accepted

All rm needs is write permission on the parent directory. The permissions of the file itself are irrelevant.

Here's a reference which explains the permissions model more clearly than I ever could:

Any attempt to access a file's data requires read permission. Any attempt to modify a file's data requires write permission. Any attempt to execute a file (a program or a script) requires execute permission...

Because directories are not used in the same way as regular files, the permissions work slightly (but only slightly) differently. An attempt to list the files in a directory requires read permission for the directory, but not on the files within. An attempt to add a file to a directory, delete a file from a directory, or to rename a file, all require write permission for the directory, but (perhaps surprisingly) not for the files within. Execute permission doesn't apply to directories (a directory can't also be a program). But that permission bit is reused for directories for other purposes.

Execute permission is needed on a directory to be able to cd into it (that is, to make some directory your current working directory).

Execute is needed on a directory to access the "inode" information of the files within. You need this to search a directory to read the inodes of the files within. For this reason the execute permission on a directory is often called search permission instead.

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2  
So if I wanted to create a directory where some files could not be deleted/changed without doing chmod first, but others could be freely writable, that would be impossible? I would have to chmod the directory 555, which would mean no files in the directory could be created or modified. –  Magnus Sep 19 '12 at 8:50
1  
@Magnus - Of course, there's nothing stopping you making a writable child directory inside the read-only directory, and storing your writable files inside that. The child directory itself cannot be deleted, but its contents can. –  ire_and_curses Sep 19 '12 at 8:57
6  
Can't you make a directory sticky with +t so that people can no longer modify or remove files in that directory that they don't own even if they have write access to the directory? –  Shadur Sep 19 '12 at 12:10
3  
@Magnus If you have root access (including sudo), you can use chattr to add the immutable flag to files. If not, then ire_and_curses is quite correct. –  James O'Gorman Sep 19 '12 at 12:49
4  
Not using rm -f only works as long as I'm sober... plus, I have no idea what the retarded bash scripts I write may or may not do –  Magnus Sep 19 '12 at 16:27

Ok, according to your comment to ire_and_curses, what you really want to do is make some files immutable. You can do that with the chattr command. For example:

e.g.

$ cd /tmp
$ touch immutable-file
$ sudo chattr +i immutable-file

$ rm -f immutable-file
rm: remove write-protected regular empty file `immutable-file'? y
rm: cannot remove `immutable-file': Operation not permitted

$ mv immutable-file someothername
mv: cannot move `immutable-file' to `someothername': Operation not permitted

$ echo foo > immutable-file 
-bash: immutable-file: Permission denied

You can't do anything to an immutable file - you can't delete it, edit it, overwrite it, rename it, chmod or chown it, or anything else. The only thing you can do with it is read it (if unix permissions allow) and (as root) chattr -i to remove the immutable bit.

Not all filesystems support all attributes. AFAIK, immutable is supported by all common linux filesystems (incl ext2/3/4 and xfs. zfsonlinux doesn't support attributes at all at the moment)

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Wow, thank you! That is amazing, I didn't know the immutable attribute existed! –  Magnus Sep 19 '12 at 9:05
1  
it's occasionally useful. btw, not even root can modify or delete an immutable file (not without removing the immutable attribute first). also btw, use lsattr to list attributes. –  cas Sep 19 '12 at 9:13
1  
+1 - I'd forgotten about attributes, and was so busy answering the literal question about rm that this never even occurred to me... –  ire_and_curses Sep 19 '12 at 9:16
    
A big thanks to the both of you, you've made my day so much easier –  Magnus Sep 19 '12 at 9:25
1  
@Magnus: possible problems include backup (not all backup utilities will backup attributes - in fact, most won't) and restore (if you restore to a directory that already contains an immutable file, some programs will treat the inability to overwrite that file as a fatal error and abort). Also you can cause yourself confusion if you forget that you made a file immutable and cant figure out why you can't delete it....the 'Operation not permitted' error message is the same error message you see with some kinds of filesystem corruption, which can lead to potentially dangerous over-reaction. –  cas Sep 19 '12 at 21:53

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