sticky text and append-only
A special file mode, called the sticky bit (mode S_ISVTX), is used to indicate special treatment for files and directories. See chmod(2) or the file /usr/include/sys/stat.h for an explanation of file modes.
Historically, an executable shareable file which had the sticky bit set was not immediately discarded from swap space after execution. The kernel hoarded the text segment of the file for future reuse, thus avoiding having to reload the program. This is no longer true on modern systems; the current virtual memory system keeps track of recently used executables, making the sticky bit for files redundant. The sticky bit can still be set on files, but without any effect.
Only the superuser can set the sticky bit on a file, though the owner of the file may clear the sticky bit.
A directory with the ‘sticky bit’ set places restrictions on file deletion: a file in a sticky directory may only be removed or renamed by a user if the user has write permission for the directory and the user is the owner of the file, the owner of the directory, or the superuser. This feature is usefully applied to directories such as /tmp which must be publicly writable but should deny users the license to arbitrarily delete or rename each others' files.
Any user may create a sticky directory. See chmod(1) for details about modifying file modes.
sticky command appeared in
Version 7 AT&T UNIX/32V.
Neither open(2) nor mkdir(2) will create a file with the sticky bit set.