Here is the function which does exactly what is specified by its name:

sub runCmd_getStdoutStderrAndExitCode(@)
  my @cmd = @_;

  # fork a child
  my ($child_pid, $child_stdout, $child_stderr);
    $child_pid = open3(\*STDIN, # is /dev/null when collectl is a daemon, UPDATE: can pass 'undef' here
  if ($@ =~ /^open3:/)
    syslog(LOG_ERR, 'Error in "%s" : %s', "@cmd", $@);
    return (undef, undef, -1);

   # result buffers
  my $stdout = '';
  my $stderr = '';

  # read into result buffers avoid blocking
  my $selector = IO::Select->new();
  $selector->add($child_stdout, $child_stderr);
  while (my @ready = $selector->can_read(SOME_TIMEOUT_CONSTANT))
    foreach my $fh (@ready) {
      my $target_buf = ($fh == $child_stdout)
                     ? \$stdout
                         : \$stderr;
      if (!sysread($fh, $$target_buf, 4096, length($$target_buf)))
        close $fh;
  close $_ for ($selector->handles());

  my $finished = waitpid($child_pid, WNOHANG);
  return ($stdout, $stderr, $?) if $finished;

  if (kill 'TERM' => $child_pid) {
    # hang until SIGTERM is processed then bury child process
    waitpid($child_pid, 0);
  } else {
    syslog(LOG_WARNING, 'Error sending TERM to child process "%s"', "@cmd");
  return ($stdout, $stderr, -1);

After the script successfully completes its job and before Perl interpreter exits, I get this message in the terminal:

stty: standard input: Bad file descriptor

It would be nice to have your opinion about the code correctness, and get a piece of advice about how to avoid that message, as a bonus.

UPDATE: The message mentioned gone away when I replaced '\*STDIN' arg with 'undef'.

Major update: It appears that this code is not functioning properly. It mixes stderr and stdout from child process, and returns everything back as stdout. I have no clue, why it is so, and I started looking towards replacing my own bicycle with IPC::Run.

UPDATE 2: Finally, here is valid and working replacement of aforementioned bicycle masterpiece. The only disadvantage, in my opition, is that IPC::Run is not part of standard Perl distribution:

sub _runCmd_getStdoutStderrAndExitCode(@)
  use IPC::Run qw(run timeout);

  my @cmd = @_;
  my ($out, $err, $ok);

  my @args = (\@cmd, \*STDIN, \$out, \$err);
  eval{ $ok = run @args };
  if ($@)                    
    return ($out, $err, -1);
    return ($out, $err, ($ok? 0 : $?));

Thanks for your comments!

  • \$\begingroup\$ Could that message be related to this bug perhaps? As long as the code works as intended (which is sounds like it does) this should still be on-topic regardless of that message, I think. \$\endgroup\$
    – Phrancis
    Mar 19 '15 at 16:52
  • \$\begingroup\$ By before Perl interpreter exists, do you mean before the Perl interpreter exits? \$\endgroup\$ Mar 19 '15 at 16:59
  • \$\begingroup\$ That message makes some sense if your STDIN is /dev/null, as stated in your comment. \$\endgroup\$ Mar 19 '15 at 17:02

as it say in the IPC::Open3 documentation, the filehandle for STDERR cannot be autovivified. You need to

use Symbol qw/ gensym /; my $stderr = gensym;

to create a file handle before opening your command.


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