sponge

sponge n.  [UNIX] A special case of a {filter} that reads its
   entire input before writing any output; the canonical example is a
   sort utility.  Unlike most filters, a sponge can conveniently
   overwrite the input file with the output data stream.  If a file
   system has versioning (as ITS did and VMS does now) the
   sponge/filter distinction loses its usefulness, because directing
   filter output would just write a new version.  See also {slurp}.