File Access Permissions (POSIX)

Additional Bits

Three additional bits with special meanings are available:

• SUID (4): Change the UID of the process to that of the owner of the file when the program is executed (only if file is executable)
• SGID (2): Like SUID, but change the GID of the process to that of the group of the file
• Sticky (1): For directory: restrict deletions so that one user cannot delete files that are owned by another user

SUID is used often. It gives non-root users a temporary privilege as superuser to perform a task (e.g. for ping which is owned by root).

The sticky bit is usually set on /tmp and /var/tmp.