What is the difference between a SetUID and SetGID bit?

Question

Options

• ASetUID applies to files, while SetGID applies to directories
• BSetUID allows a file to be executed with the permissions of the file owner, while SetGID allows a
• CSetUID allows a user to change the owner of a file, while SetGID allows a user to change the
• DThere is no difference between SetUID and SetGID

• A
  8% (5)
• B
  87% (52)
• C
  3% (2)
• D
  2% (1)

Explanation

SetUID (Set User ID) and SetGID (Set Group ID) are special permission bits in Unix/Linux that change whose privileges are used during execution. Option B is correct because SetUID causes an executable to run with the file owner's permissions (not the caller's), while SetGID causes it to run with the file's group permissions - the classic example being passwd, which runs as root regardless of who invokes it.

Why the distractors are wrong:

• A is false - both bits apply to both files and directories (on directories, SetGID makes new files inherit the directory's group, which is a distinct but real use case).
• C describes chown/chgrp functionality, not SetUID/SetGID at all - these bits have nothing to do with changing ownership.
• D is obviously false; they differ in which identity is assumed (owner vs. group).

Memory tip: Think of the letters - UID = User (owner), GID = Group. Both "set" a temporary identity for execution, just at different levels of the permission hierarchy.

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