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It's straightforward to realise that the permissions of a file are not relevant to the ability to delete that file. The ability to modify the directory listing is controlled by the directory's permissions.
However, for years I have believed that the purpose of the write permission was to allow modification of the directory, and the execute permission is for 'search' - listing files, or changing into the directory.
Today I discovered that one cannot
rm a file in a directory unless both the write and execute bits are set. In fact, without execute set, write appears almost useless.
$ tree foo/ foo/ └── file_to_delete 0 directories, 1 file $ chmod -x foo $ ls -ld foo drw-rw-r-- 2 ire_and_curses users 4096 Sep 18 22:08 foo/ $ rm foo/file_to_delete rm: cannot remove ‘foo/file_to_delete’: Permission denied $ chmod +x foo/ $ rm foo/file_to_delete $ tree foo/ foo/ 0 directories, 0 files $
I find this behaviour pretty surprising. For directories, what is the reason that execute is required to make write useful in practice?