A
filter
takes
data from stdin,
performs a conceptually simple
transformation
on the data, taking instructions, if any, as command line
options and arguments,
and outputs the resulting
data on stdout.
[This allows filters to be combined/pipelined and data to come
from files or devices.]
Errors
are written to
stderr
[so that they do not merge with data out].
A usage message should be written to stderr if there are errors with
options or arguments.
If the filter completes successfully a 0
exit status
is returned to the operating system. A nonzero exit status (usually in
range 1 - 255) is used to indicate various abnormal conditions.
Last update: 2000 December 29