stickybit

The sticky bit, formally known as the sticky permission bit, is a special file mode in Unix-like operating systems. When set on a directory, it restricts deletion and renaming of the directory’s files to the file’s owner, the directory owner, or the superuser. It is most commonly used on world-writable directories such as /tmp.

In practice, if a directory has the sticky bit set, ordinary users can create or modify files there, but they cannot delete or rename files owned by others within that directory. The ability to delete or rename depends on the usual write permissions on the directory, plus the ownership of the file in question. The sticky bit therefore helps prevent users from removing or altering each other’s files in shared spaces.

The sticky bit is one of three special permission bits in the Unix file mode (the others being setuid and setgid). Its octal value is 01000. When listing a directory with ls -l, the sticky bit appears in the others’ execute position as a t (lowercase) if others have execute permission, or as a T (uppercase) if others do not.

Historically, the sticky bit on regular files had different meanings in some early Unix systems, but on modern systems its primary practical use is on directories. On such directories, it provides a simple access control mechanism for shared locations.

Examples: to enable the sticky bit on /tmp with standard permissions, use chmod 1777 /tmp or chmod +t /tmp. To remove it, use chmod 0777 /tmp or chmod -t /tmp.