is a filter-like program that
requires no input; its output is controlled only by startup
conditions. The paradigmatic example would be
the Unix directory lister. Other classic examples include
Under Unix, report generators like
tend strongly to obey the source pattern, so their output can be
filtered with standard tools.
The term ‘source’ is, as Doug McIlroy noted, very
traditional. It is less common than it might be because
‘source’ has other important meanings.