Capturing output, error and exit code with Perl IPC::Open3 fails in newer Perl

Question

I have a small subroutine that uses IPC::Open3 (usually I use Capture::Tiny) because I wanted to use only core Perl modules. This subroutine is part of larger installation script. It captures command output, error and exit code. It works on Centos 6 (and 5) with Perl 5.10.1 (IPC::Open3 1.04) but it fails on Ubuntu 14 LTS with Perl 5.18.1 (IPC::Open3 1.13). Could someone explain to me why it fails on newer Perl and how to fix it.

sub _capture_output {
    croak( '_capture_output() needs a $cmd and options' ) unless (@_ ==  2);
    my ($cmd, $param_href) = @_;

    my $verbose = defined $param_href->{verbose}  ? $param_href->{verbose}  : 0;   #default is silent
    print "Report: COMMAND is: $cmd\n" if $verbose;

    local $| = 1;   #autoflush
    my ( $in, $out, $err );
    open my ($in_fh),  '<', \$in;
    open my ($out_fh), '>>', \$out;
    open my ($err_fh), '>>', \$err;

    my $pid = open3($in_fh, $out_fh, $err_fh, $cmd);

    my $stdout = $out;
    my $stderr = $err;
    $stdout = '' if !defined $stdout;
    $stderr = '' if !defined $stderr;

    waitpid( $pid, 0 ) or die "$!\n";
    my $exit =  $? >> 8;

    if ($verbose == 2) {
        print 'STDOUT is: ', "$stdout", "\n", 'STDERR  is: ', "$stderr", "\n", 'EXIT   is: ', "$exit\n";
    }

    return  $stdout, $stderr, $exit;
}

It fails on line with open3 call with error:

Report: COMMAND is: plenv --version
Uncaught exception from user code:
open3: exec of plenv --version failed at ./Perlinstall.pm line 175.
IPC::Open3::_open3('open3', 'GLOB(0x27819a0)', 'GLOB(0x2781730)', 'GLOB(0x2781b68)', 'plenv --version') called at /usr/share/perl/5.18/IPC/Open3.pm line 250

When I try this command on command line I get same error code but different error. Centos6:

$ plenv --version
-bash: plenv: command not found
$ echo $?
127

Ubuntu14:

$ plenv --version
No command 'plenv' found, did you mean:
Command 'p7env' from package 'libnss3-tools' (main)
plenv: command not found
$ echo $?
127

What do you mean it works on Centos 6? You said that what you run your command from the command line on Centos 6, you get an error. — mob
You can't use different handles for STDOUT and STDERR without using select (or IO::Select). If you do, you can have deadlocks. — ikegami
I use error to check for existence of program. If I catch exit code other than zero, I install this program and try next in line. — mocnii

ikegami ikegami · Accepted Answer · 2015-11-16T16:38:49

It's not clear what you think the problem is, but open3 is behaving as documented.

It doesn't return on failure: it just raises an exception matching /^open3:/.

The program you are trying to execute doesn't exist, so open3 throws an exception. I think you are wondering why $? isn't set and why nothing was printed to the the handle in $stderr, but that shouldn't be a surprise since the program never ran.

Keep in mind that exec (used by open3) will bypass the shell if the command is a string with no shell characters except whitespace, so you'll get different results from executing

plenv --version         
# Same as: open3(..., ..., ..., 'plenv', '--version')
# Exception: Can't find plenv

and

plenv '--version'
# Same as: open3(..., ..., ..., '/bin/sh', '-c', q{plenv '--version'})
# Shell exits with an error in ($? >> 8)

You can catch exceptions using eval BLOCK.

Capturing output, error and exit code with Perl IPC::Open3 fails in newer Perl

1 Answers