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.

Currently, this behavior is only operative in HP-UX. Solaris appears to have abandoned this in 2005. 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.

Operating System Excerpt from Man page Links
Solaris 10 If a regular file is not executable and has {stick bit} set, the file is assumed to be a swap file. In this case, the system's page cache will not be used to hold the file's data. If the {stick bit} bit is set on any other file, the results are unspecified. Solaris chmod(2)
HP-UX prevents the system from abandoning the swap-space image of the program-text portion of the file when its last user terminates. Then, when the next user of the file executes it, the text need not be read from the file system but can simply be swapped in, thus saving time. hp-ux chmod(2)
Linux No specific mention of sticky bit, so assumed to ignore Linux chmod(2)
FreeBSD The FreeBSD VM system totally ignores the sticky bit (ISVTX) for executables. FreeBSD chmod(2)
IRIX If the sticky bit, S_ISVTX, is set on a file that is a dynamic loader for an ELF executable, then when the executable is execed the old process's read only address spaces will be made available to the dynamic loader in the new process. This can improve program start up time considerably. The setting of the sticky bit on any other file has no effect. IRIX chmod(2)

[edit] Usage

The most common use of the sticky bit today is on directories, where, when set, items inside the directory can be renamed or deleted only by the item's owner, the directory's owner, or the superuser; without the sticky bit set, any user with write and execute permissions for the directory can rename or delete contained files, regardless of owner. Typically 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 today it is found in most modern Unix systems.

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.

The sticky bit is also set by the automounter to indicate that a file has not been mounted yet. This allows programs like ls to ignore unmounted remote files.

The Linux kernel ignores the sticky bit on files.

[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

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