Special Permissions: setuid, setgid and the Sticky Bit

Beyond the nine rwx bits there are three special ones. You will encounter them when you inspect binaries and shared directories.

The special bits at a glance

Spotting them with ls -l

$ ls -l /usr/bin/passwd
-rwsr-xr-x 1 root root 68208 ... /usr/bin/passwd

The s where an owner x would be is setuid. passwd runs with root's powers so it can write to the password database, even when you run it as a normal user.

The sticky bit looks like a t:

$ ls -ld /tmp
drwxrwxrwt 20 root root ... /tmp

/tmp is writable by everyone, but the t means only the file owner can delete files there — preventing users from wiping each other's temp files.

Setting the special bits

chmod u+s program     # setuid
chmod g+s shared-dir  # setgid on a directory
chmod +t shared-dir   # sticky bit

In octal, the special bits are a leading digit: setuid 4, setgid 2, sticky 1.

chmod 4755 program    # setuid + rwxr-xr-x
chmod 1777 shared-dir # sticky + rwxrwxrwx (like /tmp)

The takeaway

If ls -l shows an s or a t, the file or directory is using a special permission bit. Now you know exactly what they mean and how to set — and avoid — them.

Try it

1. ls -l /usr/bin/passwd — spot the rws in the owner triad
2. ls -ld /tmp — spot the trailing t
3. mkdir ~/shared && chmod +t ~/shared && ls -ld ~/shared