The external "application"

Our solution will work for any command line application regardless the language it was written in, we are using an example written in Perl so you can easily see what it does and you can easily modify to se how our capturing solutions behave in different situations.

examples/some_app.pl

#!/usr/bin/env perl
use strict;
use warnings;

print STDERR "First message to stderr\n";
print "First message to stdout\n";
print STDERR "Second message to stderr\n";
print "Second message to stdout\n";
exit 3;

capture STDOUT and STDERR separately

We use the capture function of Capture::Tiny that will wait till the end of the external process and then return whatever was printed to STDOUT and STDERR separately as two strings. (The FindBin and File::Spec code is only there so we can easily locate the external program that can be found next to our capturing script.)

examples/capture_stdout_stderr_separately.pl

use strict;
use warnings;
use FindBin;
use File::Spec;
use Capture::Tiny qw(capture);

my $command = File::Spec->catfile($FindBin::Bin, 'some_app.pl');

my ($out, $err, $exit) = capture { system $command };

print "\n---- STDERR: --------------\n";
print $err;
print "\n---- STDOUT: --------------\n";
print $out;
print "\n---- EXIT CODE: -----------\n";
print $exit / 256;
print "\n---------------------------\n";

In our case the output will look like this:

$ perl examples/capture_stdout_stderr_separately.pl

---- STDERR: --------------
First message to stderr
Second message to stderr

---- STDOUT: --------------
First message to stdout
Second message to stdout

---- EXIT CODE: -----------
3
---------------------------

capture STDOUT and STDERR merged

In this example we capture the standard output and standard error as one stream. There is no guarantee that they will appear in this output as they would on your console as usually STDOUT is buffered and STDERR is not.

examples/capture_stdout_stderr_merged.pl

use strict;
use warnings;
use FindBin;
use File::Spec;
use Capture::Tiny qw(capture_merged);

my $command = File::Spec->catfile($FindBin::Bin, 'some_app.pl');

my ($outerr, $exit) = capture_merged { system $command };

print "\n---- STDOUT and STDERR: ---\n";
print $outerr;
print "\n---- EXIT CODE: -----------\n";
print $exit / 256;
print "\n---------------------------\n";

$ perl examples/capture_stdout_stderr_merged.pl

---- STDOUT and STDERR: ---
First message to stderr
Second message to stderr
First message to stdout
Second message to stdout

---- EXIT CODE: -----------
3
---------------------------

qx

As an alternative qx (the same as backtick) too can capture the STDOUT of your external application.