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bats-mock

Mocking/stubbing library for BATS (Bash Automated Testing System)

A maintained fork of https://github.com/jasonkarns/bats-mock, which is dormant.

There are great things happening in the bats ecosystem! Anyone actively using it should be installing from [bats-core]: https://github.com/bats-core.

Installation

Recommended installation is via git submodule. Assuming your project's bats tests are in test:

git submodule add https://github.com/buildkite-plugins/bats-mock test/helpers/mocks
git commit -am 'added bats-mock module'

then in test/test_helper.bash:

load helpers/mocks/stub

Usage

After loading bats-mock/stub you have two new functions defined:

• stub: for creating new stubs, along with a plan with expected args and the results to return when called.
• unstub: for cleaning up, and also verifying that the plan was fullfilled.

Stubbing

The stub function takes a program name as its first argument, and any remaining arguments goes into the stub plan, one line per arg.

Each plan line represents an expected invocation, with a list of expected arguments followed by a command to execute in case the arguments matched, separated with a colon:

arg1 arg2 ... : only_run if args matched

The expected args (and the colon) is optional.

So, in order to stub date, we could use something like this in a test case (where format_date is the function under test, relying on data from the date command):

load helper

# this is the "code under test"
# it would normally be in another file
format_date() {
  date -r 222
}

setup() {
  stub date \
      "-r 222 : echo 'I am stubbed!'" \
      "-r \* : echo 'Wed Dec 31 18:03:42 CST 1969'"
}

teardown() {
  unstub date
}

@test "date format util formats date with expected arguments" {
  result="$(format_date)"
  [ "$result" == 'I am stubbed!' ]

  result="$(format_date)"
  [ "$result" == 'Wed Dec 31 18:03:42 CST 1969' ]
}

This verifies that format_date indeed called date using the args defined in each specified stub, and made proper use of the output of it. Note that `* will match any one argument.

The plan is verified, one by one, as the calls come in, but the final check that there are no remaining un-met plans at the end is left until the stub is removed with unstub.

Unstubbing

Once the test case is done, you should call unstub <program> in order to clean up the temporary files, and make a final check that all the plans have been met for the stub.

Verifying stub input

If you want to verify that your stub was passed the correct data in STDIN, you can redirect its content to a temporary file and check it.

@test "send_message" {
  stub curl \
    "${_CURL_ARGS} : cat > ${_TMP_DIR}/actual-input ; cat ${_RESOURCES_DIR}/mock-output"

  run send_message
  assert_success
  diff "${_TMP_DIR}/actual-input" "${_RESOURCES_DIR}/expected-input"
  unstub curl
}

Verifying stub arguments

If you want to verify that your stub was called with the correct arguments, you can use (escaped) positional variables in the command. They will be interpreted correctly ($1, $2, ...).

@test "send_message" {
  stub curl \
    "-X \* \* : echo \$3 > ${_TMP_DIR}/curl-url; echo \$2 > ${_TMP_DIR}/curl-method"

  run send_message
  
  assert_success
  diff "${_TMP_DIR}/curl-url" "${_RESOURCES_DIR}/expected-url"
  called_method="$(cat ${_TMP_DIR}/curl-method)"
  [ "$called_method" == "POST" ]

  unstub curl
}

Accepting any (or no) arguments

Sometimes the argument is too complicated to determine in advance or it would make the stubbing really long and convoluted. In those cases you can use \* to ensure that an argument is given.

@test "send_message" {

	stub grep \
    '\* \* : echo OK' \
    '\* \* : echo OK'

  # matches because there are exactly 2 arguments
  grep "$complicated_pattern" /home/user/file
  # this does not because there are 3 arguments :(
  grep -ri "$complicated_pattern" /home/user/file

}

If you do not care about the amount of arguments, not having any colons whatsoever means accepting any amount of arguments:

@test "send_message" {

	stub grep \
    'exit 0' \
    'exit 1' \
    'exit 2'
  
  # Will match the first stub line and exit with code 0
  grep "$complicated_pattern" /home/user/file

  # Matches the second one and exits witch code 1
  grep -E -i "$some_pattern" "$some_file"

  # No arguments also match, the third one exits with code 2 :)
  grep

If you want to ensure no arguments whatsoever, you add a single colon at the very beginning:

@test "send_message" {
  # Note that a single colon at the start is interpreted as "no arguments"
  stub cat ': echo "OK"'
  ! cat foo # `cat` stub fails as an argument was passed

  # But don't forget the space!
  stub cat':echo "OK"'
  # Will accept any arguments and execute `:echo "OK"` -> Fails
  !cat foo # command `:echo` not found

  # If your command contains ' : ' just start with double-colon
  stub cat '::echo "Hello : World"'
  # Prints "Hello : World"
  cat foo bar

Incremental Stubbing

In some case it might be preferable to define the invocation plan incrementally to mirror the actual behavior of the program under test. This can be done by invocing stub multiple times with the same command.
In case you want to to start with a new plan call unstub first.

# Function to test
function install() {
  apt-get update
  pt-add-repository -y myrepo
  apt-get update
}

@test "test installation" {
  stub apt-get "update : "
  stub apt-add-repository "-y myrepo : "
  stub apt-get "update : " # Appends to existing plan
  run install
  unstub apt-get # Verifies plan and removes all remaining files
  stub apt-get "upgrade" # Start with a new plan
}

Troubleshooting

It can be difficult to figure out why your mock has failed. You can enable debugging setting an environment variable called after the command being stubbed (all in underscore-separeted, uppercase) with the STUB_DEBUG suffix. The value of the variable needs to be a device or file descriptor where to redirect the debugging output. Recommended value is 3, which should make the output compatible with tap's expectation but you can also use /dev/tty.

If you have stubbed the date command, you can do something like:

export DATE_STUB_DEBUG=3

How it works

(You may want to know this, if you get weird results there may be stray files lingering about messing with your state.)

Under the covers, bats-mock uses three scripts to manage the stubbed programs/functions.

First, it is the command (or program) itself, which when the stub is created is placed in (or rather, the binstub script is sym-linked to) ${BATS_MOCK_BINDIR}/${program} (which is added to your PATH when loading the stub library). Secondly, it creates a stub plan, based on the arguments passed when creating the stub, and finally, during execution, the command invocations are tracked in a stub run file which is checked once the command is unstub'ed. The ${program}-stub-[plan|run] files are both in ${BATS_MOCK_TMPDIR}.

Caveat

If you stub functions, make sure to unset them, or the stub script wan't be called, as the function will shadow the binstub script on the PATH.

Credits

Forked from https://github.com/jasonkarns/bats-mock originally with thanks to @jasonkarns.

Originally extracted from the ruby-build test suite. Many thanks to its author and contributors: Sam Stephenson and Mislav Marohnić.