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Reading the output of commands
==============================

   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.

   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.