`ls -l /usr/bin/passwd` shows `-rwsr-xr-x 1 root root`. What does the s in the owner's execute position mean?
LPIC-1 Exam 101-500, objective 104. Devices, Linux filesystems, filesystem hierarchy standard medium
Draft — not checked yet. This question was written from the published exam objectives and cites the clause it comes from, but nobody has yet read it against that clause and signed for it.
It is kept out of search results and out of mock exams until they have. Read the source below before you take the answer as settled.
The options
Not correct The file is shared, so the kernel keeps a single copy of its text segment in memory.
Wrong. That was the original meaning of the sticky bit on executables in old Unix, and Linux ignores it on regular files. It has nothing to do with s.
Not correct The program runs with the effective group ID of the file's group.
Wrong. That is SGID, and it appears as an s in the group execute position (-rwxr-sr-x), not the owner's.
Not correct Only root may delete or rename the file.
Wrong. That is roughly what the sticky bit does, and only for files inside a directory that carries it; it appears as t in the other-execute position.
Correct The program runs with the effective user ID of the file's owner, root, whoever starts it.
Correct. That is the SUID bit. passwd needs it because it must write /etc/shadow, which ordinary users cannot open at all.
Why
Special bits in ls output: s in the owner-execute slot is SUID (octal 4000), s in the group-execute slot is SGID (2000), t in the other-execute slot is the sticky bit (1000). A capital S or T means the bit is set while the underlying execute bit is not. Set them with chmod u+s, g+s, +t, or with a fourth leading octal digit such as 4755.
Where this comes from
- Cited
- LPI exam objective 104.5
- What it says
- Understand and apply the suid, sgid and sticky bits.
Practise this
Reading one question is not practice. The trainer will draw a set from objective 104 and space the ones you get wrong.