The external program

The external program can be anything and it can be written in any language. To make it clearer however I've prepared a sample external program in perl:

examples/external.pl

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

print("Hello\n");
warn("Some warning to STDERR\n");
print("World\n");

If you run this on the command line you get the following on the screen:

examples/external.txt

Hello
Some warning to STDERR
World

Two lines are printed to STDOUT and one line to STDERR, but on the command line you can't tell them apart. (You could use SCALAR and LIST context if you'd like to know the distinction.)

examples/capture_stdout_backtick_scalar.pl

use strict;
use warnings;

my $out = `./external.pl`;
print ("==$out==\n");

The STDERR will be printed directly to the screen untouched by the backticks. If the STDOUT has multiple lines (as in our case), they will still form a single multi-line string in the capturing code:

examples/capture_stdout_backtick_scalar.txt

Some warning to STDERR
==Hello
World
==

qx in scalar context

Instead of the backticks that are hard to see and hard to search for, we can use the qx operator of perl.

At first it looks like a strangely-named function:

examples/capture_stdout_qx_scalar.pl

use strict;
use warnings;

my $out = qx(./external.pl);
print ("==$out==\n");

The results are exactly the same:

examples/capture_stdout_qx_scalar.txt

Some warning to STDERR
==Hello
World
==

However qx allows any pair of delimiters so you could use curly braces instead of parentheses and that would still do the exact same thing:

examples/capture_stdout_qx_scalar_curly.pl

use strict;
use warnings;

my $out = qx{./external.pl};
print ("==$out==\n");

Backtick `` in list context

Instead of assigning to a scalar variable we can also assign the backtick to an array, putting it in LIST context.

(See SCALAR and LIST context if you'd like to know the distinction.)

examples/capture_stdout_backtick_array.pl

use strict;
use warnings;

my @out = `./external.pl`;
print ("==$out[0]==\n");

print ("==$out[1]==\n");

In this case whatever the external program printed to STDOUT is split into rows and are assigned to the array. Each row is a separate line in the resulting array. So the number of elements in the array is the number of rows on the STDOUT.

examples/capture_stdout_backtick_array.txt

Some warning to STDERR
==Hello
==
==World
==

qx in list context

The same can be done with the qx operator.

examples/capture_stdout_qx_array.pl

use strict;
use warnings;

my @out = qx{./external.pl};
print ("==$out[0]==\n");

And the output:

examples/capture_stdout_qx_array.txt

Some warning to STDERR
==Hello
==

Use case

See How to provide STDIN to an external executable as a use-case.

Blocking

They block the main process and wait till the external program finished so it can collect all the output.

Capture::Tiny

The Capture::Tiny is an excellen alternative to the qx operator. See the example how to capture STDOUT and STDERR of an external program.

Comments

Not bad on the basics, but I was hoping for a little more detail-- like, if you're building an external command string how do you handle quoting the arguments (I tend to use the CPAN module String::ShellQuote), and when do you need to worry about security issues (untrusted input and untainting), then there are the issues with shell i/o redirection (you can get STDOUT, STDERR, or both together, but then you can't tell which was which), and then there's qx compared to system...

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how to call a .pl inside a .cgi file? thank you