Notes about set-gid and sticky bits for directories

I can never remember how the set-gid and sticky bits work on directories, so I finally spent some time to re-read man (but had to resort to info) about chmod. This is my cheat sheet.


Setgid (octal permission 2000) makes new files in the directory owned by the group that owns the directory. This is very useful for teams.

How to set

chmod g+s thisdir
chmod 2770 thisdir

How to clear

chmod g-s thisdir
chmod 00770 thisdir

sticky bit, or restricted deletion

Sticky bit (octal permission 1000) on a directory prevents Bob from deleting a file owned by Alice. Even if the directory is owned by one of Bob’s groups and is writable, Bob cannot delete the Alice’s files. This is particulary helpful for the /tmp directory. Check it out:

$ ls -lad /tmp
drwxrwxrwt. 4 root root 120 Jan 23 09:40 /tmp

How to set sticky bit

chmod a+t thisdir
chmod 1770 thisdir

How to clear

chmod a-t thisdir
chmod 00770 thisdir

According to info coreutils chapter 27.4, “Directories and the Set-User-ID and Set-Group-ID Bits,” gnu chmod needs a 5-digit octal to clear this bit.
Basically, if it’s worth setting set-gid, you should throw in sticky bit.

chmod 03770 thisdir