(m4.info.gz) Syscmd

 
 12.2 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'.
 
      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.
 
    As an example of SHELL-COMMAND using the same standard input as
 `m4', the command line `echo "m4wrap(\`syscmd(\`cat')')" | m4' will
 tell `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), 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.info.gz) Platform macros
(m4.info.gz) Shell commands
(m4.info.gz) Esyscmd
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