A special file mode, called the sticky bit (mode S_ISTXT), is used to indicate special treatment for directories. It is ignored for regular files. See chmod(2) or the file #include <sys/stat.h>
for an explanation of file modes.

A directory whose ‘sticky bit’ is set becomes an append-only directory, or, more accurately, a directory in which the deletion of files is restricted. 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 super-user. 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.

A sticky command appeared in

Neither open(2) nor mkdir(2) will create a file with the sticky bit set.