If you want m4 to read the output of a UNIX command, use
esyscmd:
esyscmd(shell-command)
which expands to the standard output of the shell command shell-command.
Prior to executing the command, m4 flushes its output buffers.
The default standard input and error output of shell-command are
the same as those of m4. The error output of shell-command
is not a part of the expansion: it will appear along with the error
output of m4.
Assume you are positioned into the `checks' directory of GNU
m4 distribution, then:
define(`vice', `esyscmd(grep Vice ../COPYING)') => vice => Ty Coon, President of Vice =>
Note how the expansion of esyscmd has a trailing newline.
The builtin macro esyscmd is recognized only when given
arguments.
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