Info Catalog m4: Platform macros m4: Shell commands m4: Esyscmd

m4: Syscmd

 
 13.2 Executing simple commands
 ==============================
 
 Any shell command can be executed, using 'syscmd':
 
  -- Builtin: syscmd (SHELL-COMMAND)
      Executes SHELL-COMMAND as a shell command.
 
      The expansion of 'syscmd' is void, _not_ the output from
      SHELL-COMMAND!  Output or error messages from SHELL-COMMAND are not
      read by 'm4'.  ⇒Esyscmd, if you need to process the command
      output.
 
      Prior to executing the command, 'm4' flushes its buffers.  The
      default standard input, output and error of SHELL-COMMAND are the
      same as those of 'm4'.
 
      By default, the SHELL-COMMAND will be used as the argument to the
      '-c' option of the '/bin/sh' shell (or the version of 'sh'
      specified by 'command -p getconf PATH', if your system supports
      that).  If you prefer a different shell, the 'configure' script can
      be given the option '--with-syscmd-shell=LOCATION' to set the
      location of an alternative shell at GNU 'm4' installation; the
      alternative shell must still support '-c'.
 
      The macro 'syscmd' is recognized only with parameters.
 
      define(`foo', `FOO')
      =>
      syscmd(`echo foo')
      =>foo
      =>
 
    Note how the expansion of 'syscmd' keeps the trailing newline of the
 command, as well as using the newline that appeared after the macro.
 
    The following is an example of SHELL-COMMAND using the same standard
 input as 'm4':
 
      $ echo "m4wrap(\`syscmd(\`cat')')" | m4
      =>
 
    It tells 'm4' to read all of its input before executing the wrapped
 text, then hand a valid (albeit emptied) pipe as standard input for the
 'cat' subcommand.  Therefore, you should be careful when using standard
 input (either by specifying no files, or by passing '-' as a file name
 on the command line, ⇒Invoking m4 Command line files.), and also
 invoking subcommands via 'syscmd' or 'esyscmd' that consume data from
 standard input.  When standard input is a seekable file, the subprocess
 will pick up with the next character not yet processed by 'm4'; when it
 is a pipe or other non-seekable file, there is no guarantee how much
 data will already be buffered by 'm4' and thus unavailable to the child.
 

Info Catalog m4: Platform macros m4: Shell commands m4: Esyscmd