A sink is a filter-like program that consumes standard input but emits nothing to standard output. Again, its actions on the input data are controlled only by startup conditions.

This interface pattern is unusual, and there are few well-known examples. One is lpr(1), the Unix print spooler. It will queue text passed to it on standard input for printing. Like many sink programs, it will also process files named to it on the command line. Another example is mail(1) in its mail-sending mode.

Many programs that might appear at first glance to be sinks take control information as well as data on standard input and are actually instances of something like the ed pattern (see below).

The term sponge is sometimes applied specifically to sink programs like sort(1) that must read their entire input before they can process any of it.

The term ‘sink’ is traditional and common.