sticky text and append-only directories
A special file mode, called the sticky bit
S_ISVTX), is used to indicate special treatment for files and directories. See
or the file
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
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
which must be publicly writable but
should deny users the license to arbitrarily delete or rename each others'
Any user may create a sticky directory. See
for details about
modifying file modes.
command appeared in
Version 32V AT&T UNIX
will create a file with
the sticky bit set.