specialbits

Specialbits is a term used in Unix-like file systems to refer to the set of special permission bits in a file's mode. These bits—set-user-ID (setuid), set-group-ID (setgid), and the sticky bit—modify the normal read, write, and execute permissions by enabling elevated privileges or restricting deletion rights in directories. They are stored in the file mode field and can be displayed with standard permission strings; in symbolic form they appear as s, S, t, or T, and in numeric form as octal values 4000 (setuid), 2000 (setgid), and 1000 (sticky).

On regular files, setuid and setgid cause executed programs to run with the owner’s user ID or the owning group’s ID, respectively. On directories, setgid causes newly created files to inherit the directory’s group, while setuid is typically ignored. The sticky bit, when set on directories, restricts file deletion so that only the file owner, the directory owner, or the root user can delete or rename entries.

Historically, these bits were introduced to support privilege separation and group collaboration in multi-user systems. Modern security practices emphasize minimizing the use of setuid/setgid programs and auditing their behavior; some systems employ mechanisms such as capabilities, sandboxing, or mandatory access controls to limit risk.

See also Unix file permissions, file mode, setuid, setgid, sticky bit.