If you have Emacs (or any Info browser) and the GNU C Library Reference Manual installed, you can do this:
- Start Emacs and press Ctrl-h i (or click “Help > More Manuals > All Other Manuals (Info)”) to start its built-in Info reader.
- Press m (or click on “Info > Menu item > Other”)
libc and press Enter.
- Type i (or click on “Info > Index > Lookup a String…”)
sticky and press Enter.
You will get this text:
This is the "sticky" bit, usually 01000.
For a directory it gives permission to delete a file in that
directory only if you own that file. Ordinarily, a user can
either delete all the files in a directory or cannot delete any of
them (based on whether the user has write permission for the
directory). The same restriction applies--you must have both
write permission for the directory and own the file you want to
delete. The one exception is that the owner of the directory can
delete any file in the directory, no matter who owns it (provided
the owner has given himself write permission for the directory).
This is commonly used for the ‘/tmp’ directory, where anyone may
create files but not delete files created by other users.
Originally the sticky bit on an executable file modified the
swapping policies of the system. Normally, when a program
terminated, its pages in core were immediately freed and reused.
If the sticky bit was set on the executable file, the system kept
the pages in core for a while as if the program were still
running. This was advantageous for a program likely to be run
many times in succession. This usage is obsolete in modern
systems. When a program terminates, its pages always remain in
core as long as there is no shortage of memory in the system.
When the program is next run, its pages will still be in core if
no shortage arose since the last run.
On some modern systems where the sticky bit has no useful meaning
for an executable file, you cannot set the bit at all for a
non-directory. If you try, ‘chmod’ fails with ‘EFTYPE’; see
Some systems (particularly SunOS) have yet another use for the
sticky bit. If the sticky bit is set on a file that is not
executable, it means the opposite: never cache the pages of this
file at all. The main use of this is for the files on an NFS
server machine which are used as the swap area of diskless client
machines. The idea is that the pages of the file will be cached
in the client’s memory, so it is a waste of the server’s memory to
cache them a second time. With this usage the sticky bit also
implies that the filesystem may fail to record the file’s
modification time onto disk reliably (the idea being that no-one
cares for a swap file).
This bit is only available on BSD systems (and those derived from
them). Therefore one has to use the ‘_BSD_SOURCE feature select
macro to get the definition (see Feature Test Macros).