Sticky bit
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The sticky bit is an access-right flag that can be assigned to files and directories on Unix systems.
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[edit] History
The sticky bit was introduced in the Fifth Edition of Unix in 1974 for use with pure executable files. When set, it instructed the operating system to retain the text segment of the program in swap space after the process exited. This speeded subsequent executions by allowing the kernel to make a single operation of moving the program from swap to real memory. Thus, frequently-used programs like editors would load notably faster. One notable problem with "stickied" programs was replacing the executable (for instance, during patching); to do so required removing the sticky bit from the executable, executing the program and exiting to flush the cache, replacing the binary executable, and then restoring the sticky bit.
The speed gain was largely lost when Unix was ported to hardware supporting direct memory access (as well as somewhat obsoleted with the replacement of swapping with demand paging). Nonetheless, it remains operative in several System V variants (notably Solaris[1] and HP-UX). The 4.4-Lite release of BSD retained the old sticky bit behavior but it has been subsequently dropped from OpenBSD (as of release 3.7) and FreeBSD (as of release 2.2.1); it remains in NetBSD. No version of Linux has ever supported the traditional behavior.
[edit] Usage
The most common use of the sticky bit today is on directories, where, when set, items inside the directory can only be renamed or deleted by the item's owner, the directory's owner, or the superuser. Frequently this is set on the /tmp directory to prevent ordinary users from deleting or moving other users' files. This feature was introduced in 4.3BSD in 1986 and is found in most modern Unixes.
In addition, Solaris (as of Solaris 2.5) defines special behavior when the sticky bit is set on non-executable files: those files, when accessed, will not be cached by the kernel. This is usually set on swap files to prevent access on the file from flushing more important data from the system cache. It is also used occasionally for benchmarking tests.
[edit] Examples
The sticky bit can be set using the chmod command and can be set using its octal mode 1000 or by its symbol t (s is already used by the setuid bit). For example, to add the bit on the directory /usr/local/tmp, one would type chmod +t /usr/local/tmp. Or, to make sure that directory has standard tmp permissions, one could also type chmod 1777 /usr/local/tmp.
In Unix symbolic file system permission notation, the sticky bit is represented by the letter t in the final character-place. For instance, on Solaris 8, the /tmp directory, which by default has the sticky-bit set, shows up as:
$ ls -ld /tmp drwxrwxrwt 4 root sys 485 Nov 10 06:01 /tmp
If the sticky-bit is set on a file or directory without the execution bit set for the others category (non-user-owner and non-group-owner), it is indicated with a capital T:
# ls -l test -rw-r--r-- 1 root other 0 Nov 10 12:57 test # chmod +t test; ls -l test -rw-r--r-T 1 root other 0 Nov 10 12:57 test
