How do I mock Perl's built-in backticks operator?

Question

I'd like to unit test a Perl program of mine that is using backticks. Is there a way to mock the backticks so that they would do something different from executing the external command?

Another question shows what I need, but in Ruby. Unfortunately, I cannot choose to use Ruby for this project, nor do I want to avoid the backticks.

Answer 1

You can^* mock the built-in readpipe function. Perl will call your mock function when it encounters a backticks or qx expression.

BEGIN {
  *CORE::GLOBAL::readpipe = \&mock_readpipe
};

sub mock_readpipe {
  wantarray ? ("foo\n") : "foo\n";
}

print readpipe("ls -R");
print `ls -R`;
print qx(ls -R);

---

$ perl mock-readpipe.pl
foo
foo
foo

^* - if you have perl version 5.8.9 or later.

Answer 2

Instead of using backticks, you can use capture from IPC::System::Simple, and then write a mock version of capture() in your unit test.

# application
use IPC::System::Simple qw(capture);
my $stuff = capture("some command");

# test script
{
     package IPC::System::Simple;
     sub capture
     {
         # do something else; perhaps a call to ok()
     }
}

# ... rest of unit test here