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WebMock for shell commands
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What is ShellMock?

It’s webmock for shell commands. It’s pretty simple. You can do things like this:

require 'shell_mock/rspec'

RSpec.describe "shelling out to run 'ls'" do
  before { ShellMock.enable } # (1)
  after  { ShellMock.disable }

  let(:stub) { ShellMock.stub_command('ls') } # (2)

  it "works"
    system('ls') # (3)

    expect(stub).to have_run # (4)
  1. enables ShellMock’s monkey patches during the test

  2. creates a command stub that will match the command "ls" (by default it will exit 0 and have no output)

  3. shells out to run "ls" (in this case using Kernel#system)

  4. correctly expects that our command stub for "ls" will have recorded an invocation

Using ShellMock

You can narrow what invocations are matched to your command stub:

Match env vars as well as the command: ShellMock.stub_command('ls').with_env({'FOO' ⇒ 'bar'})

Provide a more complete invocation: ShellMock.stub_command('ls $HOME')

Shelling out to run "ls" won’t match this command stub, but shelling out to run "ls $HOME" will.

ShellMock always matches as strictly as possible, so if you stubbed both "ls" and "ls $HOME", invocations of "ls $HOME' will only ever match against the "ls $HOME" stub and never the "ls" stub.

Setting the behavior of the command invocation:

Have the mock command invocation write to stdout: ShellMock.stub_command('ls').to_output("\n")

Set the mock command invocation’s exit status: ShellMock.stub_command('ls').to_exit(2)

Set the mock command invocation’s exit status to 0: ShellMock.stub_command('ls').to_succeed

Set the mock command invocation’s exit status to 1: ShellMock.stub_command('ls').to_fail

If you want to both write to stdout and set the exit code (a common pair), ShellMock.stub_command('ls').to_return("\n") will both have the command invocation write the passed string to stdout, and will set the mock command invocation’s exit status to 0.

Specifying the expected number of command invocations:

Called exactly once: expect(stub).to have_run.once

Not called: expect(stub).to have_run.never

Not called (using RSpec expectation negation): expect(stub).to_not have_run

Called exactly n times: expect(stub).to have_run.times(n)

Called more than n times: expect(stub).to have_run.more_than(n)

Called fewer than n times: expect(stub).to have_run.fewer_than(n)

less_than can be used as an alias for fewer_than


Currently, only exact string matches of the stubbed command string are supported. Basic regex support or more complex matching for arguments and flags may be added later.

ShellMock supports stubbing these ways of shelling out in Ruby:

ShellMock currently DOES NOT support stubbing these ways of shelling out in Ruby (but will):


Add this line to your application’s Gemfile:

gem 'shell_mock'

And then execute:

$ bundle

Or install it yourself as:

$ gem install shell_mock


Bug reports and pull requests are welcome on GitHub at

Code of Conduct

This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Convenant code of conduct.


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install.


To run all the tests, run

$ bundle exec rspec

Pull Requests

Pull requests should be well-scoped and include tests appropriate to the changes.

When submitting a pull request that changes user-facing behavior, add release note lines to the commit message body like this. You can preview your release lines by running

$ bundle exec rake changelog:preview


Releasing a new version is a 2-step process.

First, run

$ bundle exec rake changelog:compile

This will add a new release section before the other release sections. It will contain all the release notes in the commit messages since the last release, and will be prepopulated with the minimum possible version given those changes. Proof-read it and reorder the notes if you think doing so would be necessary or clearer. Feel free to increase the version if necessary (to force a major release, for example).

Once you’re satisfied, run

$ bundle exec rake changelog:release

This will create a git tag for the version, push git commits and tags, and push the .gem file to


The gem is available as open source under the terms of the MIT License.

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