/tmp is shown by ls -ld as `drwxrwxrwt`. What does the trailing t achieve on this world-writable directory?
LPIC-1 Exam 101-500, objective 104. Devices, Linux filesystems, filesystem hierarchy standard medium
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The options
Correct A file inside may only be deleted or renamed by the file's owner, the directory's owner, or root.
Correct. Without the sticky bit, write permission on the directory would let any user unlink any other user's file in /tmp, regardless of that file's own permissions.
Not correct Files created inside inherit the directory's group ownership.
Wrong. That is the SGID bit on a directory, shown as s in the group-execute position.
Not correct Programs started from inside the directory run as the directory's owner.
Wrong. SUID applies to executable files, never to directories; Linux ignores the SUID bit on a directory entirely.
Not correct The directory's contents are erased automatically at every reboot.
Wrong. Clearing /tmp at boot is a distribution policy implemented by an init script or systemd-tmpfiles, not a permission bit.
Why
On Linux the sticky bit is meaningful on directories only. It is what makes a shared, world-writable directory like /tmp or /var/tmp safe enough to use: everyone can create files, nobody can remove someone else's. Set it with `chmod +t dir` or `chmod 1777 dir`.
Where this comes from
- Cited
- LPI exam objective 104.5
- What it says
- The sticky bit restricts deletion in shared writable directories.
Practise this
Reading one question is not practice. The trainer will draw a set from objective 104 and space the ones you get wrong.