Mock system call in ruby

Question

Know of a way to mock %[]? I'm writing tests for code that makes a few system calls, for example:

def log(file)
  %x[git log #{file}]
end

and would like to avoid actually executing system calls while testing this method. Ideally I'd like to mock %x[..] and assert that the correct shell command is passed to it.

Answer 1

%x{…} is Ruby built-in syntax that will actually call Kernel method Backtick (`). So you can redefine that method. As backtick method returns the standard output of running cmd in a subshell, your redefined method should return something similar to that ,for example, a string.

module Kernel
    def `(cmd)
        "call #{cmd}"
    end
end

puts %x(ls)
puts `ls`
# output
# call ls
# call ls

Answer 2

Using Mocha, if you want to mock to following class:

class Test
  def method_under_test
    system "echo 'Hello World!"
    `ls -l`
  end
end

your test would look something like:

def test_method_under_test
  Test.any_instance.expects(:system).with("echo 'Hello World!'").returns('Hello World!').once
  Test.any_instance.expects(:`).with("ls -l").once
end

This works because every object inherits methods like system and ` from the Kernel object.

Answer 3

I don't know of a way to mock a module, I'm afraid. With Mocha at least, Kernel.expects doesn't help. You could always wrap the calling in a class and mock that, something like this:

require 'test/unit'
require 'mocha'

class SystemCaller
  def self.call(cmd)
    system cmd
  end
end

class TestMockingSystem < Test::Unit::TestCase
  def test_mocked_out_system_call
    SystemCaller.expects(:call).with('dir')
    SystemCaller.call "dir"
  end
end

which gives me what I'd hope for:

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