Gfilui

Suppose you have permission to write to a file but not to delete it.

What rights do we talk about?

Read and write rights?

To write to an existing file you need write permissions for that file.

To delete a file you need write permission for the folder that contains that file.

Florian's answer is not quite complete on my system: I need both write and execute permission on the directory in order to delete a file in it.

Sample session:

I'm running a well-seasoned Debian with the ext4 file system. I create a directory with a file in it and then change the directory and file permissions before I try to delete the file.

File system ext4:

testuser@www:~$ df -T .
Filesystem Type 1K-blocks Used Available Use% Mounted on
/dev/sda3 ext4 152326340 44429904 100196624 31% /

Let's create a directory and a file in it:

testuser@www:~$ mkdir d
testuser@www:~$ ls -ld d
drwxr-xr-x 2 testuser testuser 4096 May 21 16:19 d
testuser@www:~$ echo kjhkjh > d/f
testuser@www:~$ ls -l d/f
-rw-r--r-- 1 testuser testuser 7 May 21 16:19 d/f

Expected: Cannot change file catalog without write permission on it:

testuser@www:~$ chmod 100 d && ls -ld d
d--x------ 2 testuser testuser 4096 May 21 16:19 d
testuser@www:~$ rm d/f
rm: cannot remove 'd/f': Permission denied

Unexpected: Cannot change file catalog without execute permission on it:

testuser@www:~$ chmod 200 d && ls -ld d
d-w------- 2 testuser testuser 4096 May 21 16:19 d
testuser@www:~$ rm d/f
rm: cannot remove 'd/f': Permission denied

I need both write and execute permission on it:

testuser@www:~$ chmod 300 d && ls -ld d
d-wx------ 2 testuser testuser 4096 May 21 16:19 d
testuser@www:~$ rm d/f

As an aside: When the file (but not the directory) is write protected rm asks for permission before deleting. After all, deleting a file can be considered an extreme form of writing to it. There is no difference in lost data between echo -n "" > d/f and rm d/f in the common case that there is no other hardlink to the file.

testuser@www:~$ !echo
echo kjhkjh > d/f
testuser@www:~$ chmod 700 d && ls -ld d
drwx------ 2 testuser testuser 4096 May 21 16:21 d
testuser@www:~$ chmod 000 d/f && ls -l d/f
---------- 1 testuser testuser 7 May 21 16:21 d/f
testuser@www:~$ rm d/f
rm: remove write-protected regular file 'd/f'? y
testuser@www:~$ ls -l d
total 0
testuser@www:~$