Select the THREE true statements about the special permission bits on Linux.
LPIC-1 Exam 101-500, objective 104. Devices, Linux filesystems, filesystem hierarchy standard hard
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The options
Choose 3.
Not correct SUID on a shell script causes the Linux kernel to run the script with the owner's privileges.
False, and an important trap. Linux ignores the SUID bit on interpreted scripts because of unavoidable race conditions between the exec and the interpreter opening the file. Privileged helpers must be compiled binaries, or invoked through sudo.
Correct SUID on an executable file makes it run with the effective user ID of the file's owner.
True. This is how /usr/bin/passwd, owned by root, can write /etc/shadow when an ordinary user runs it.
Correct SGID on a directory makes files created inside it inherit the directory's group.
True, and it is the standard way to build a shared project directory so that everyone in the group can read each other's new files.
Correct The sticky bit on a world-writable directory stops users from deleting files they do not own.
True. Only the file's owner, the directory's owner and root may unlink or rename; this is why /tmp is mode 1777.
Not correct The sticky bit on a regular file keeps that file's contents cached in memory for faster execution.
False on Linux. That was the original 1970s Unix meaning of the bit on executables; Linux ignores it on regular files entirely. Today it is meaningful only on directories.
Why
Octal values: SUID 4000, SGID 2000, sticky 1000, written as a fourth leading digit (chmod 4755, 2775, 1777) or symbolically (u+s, g+s, +t). SGID is meaningful on both executables (run with the file's group) and directories (group inheritance); the sticky bit is meaningful only on directories; SUID is meaningful only on binary executables.
Where this comes from
- Cited
- LPI exam objective 104.5
- What it says
- Use the suid, sgid and sticky bits and know where each applies.
Practise this
Reading one question is not practice. The trainer will draw a set from objective 104 and space the ones you get wrong.