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Run an external command from Perl, capturing both the output and the exit value

# This is how to run an external command from Perl,
# capturing both the output and the exit value

sub run_external_command {
    my ($self, $command) = @_;
    $self->logger->debug("Running command: $command");

    my $output = '';
    $command .= ' 2>&1 | '; # capture all output
    open(my $capture, $command);
    {
        local $/ = undef;
        $output = <$capture>;
    }
    close($capture);
    chomp($output);

    my $exit_value = ${^CHILD_ERROR_NATIVE} >> 8;
    $self->logger->debug("Exit value of command: '$exit_value'");
    $self->logger->debug("Output of command: '".($output || '[no output]')."'");

    return ($exit_value, $output);
}

This is another way:

use IPC::Open3;

sub run_external_command_2 {
  local (*my_in, *my_out, *my_err);

  my $command = "some command here that might error";
  my $pid = open3(\*my_in, \*my_out, \*my_err, $command) or die($!);

  close(my_in);
  my $std_out = do { local $/; };close(my_out);
  my $std_err = do { local $/; };close(my_err);

  waitpid($pid, 0);
  my $exit_code = $? >> 8;

  return { exit_code => $exit_code, std_out => $std_out, std_err => $std_err };
}

What a solitary underscore in Perl means

It represents the previous 'stat' structure, saving a system call.
See: http://perldoc.perl.org/5.8.8/functions/-X.html
and http://perldoc.perl.org/5.8.8/functions/stat.html

Timing out a system call

as seen in perldoc perlipc

 

#!/usr/bin/perl

use warnings;

my @cmd = @ARGV;
my @ls = ();
my $t = 10;

    eval {
        local $SIG{ALRM} = sub { die "alarm went off" };
        alarm $t;
            @ls = `@cmd`;
        alarm 0;
    };

if (! @ls) {
        print "No response after $t seconds\n";
}
else {
        foreach (@ls) { print $_; }
}

# actually never gets to here
if ($@ and $@ !~ /alarm went off/) { die "Command timed out after $t seconds\n"; };

System maintenance

There is a reason we have a process of change control. It has become a dirty word in some departments, but the bare minimum is:

• making a change in the development environment
• testing it, preferably by more than one person/team
• packaging up the release and labeling it properly
• making a formal request to deploy it, including roll-back instructions

In addition, a precedent should never be set to fix something that another department has broken!