### 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`. See Reading the output of commands, 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, see 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.