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(m4.info)Esyscmd

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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.

   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.