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Mocking

Overview

One of the most effective test strategies used in higher-level object-oriented languages is called mocking. It`s a powerful test strategy that is not limited to just object-oriented languages. It’s actually really powerful for low-level OS testing as well where you basically want to test just your code and not the entire OS. The typical strategy here is to essentially create a mock function or script which gets called instead of the real OS level component.

ebash supports an extensive mocking framework called emock. It integrates well into the rest of ebash and etest in particular to make it very easy to mock out real system binaries and replace them with mock instances which do something else. It is very flexible in the behavior of the mocked function utilizing the many provided option flags.

Usage

The simplest invocation of emock is to use the default of function mocking. In this mode, you simply supply the name of the binary you wish to mock, such as:

emock "dmidecode"

This will create and export a new function called dmidecode that can be invoked instead of the real dmidecode binary at /usr/bin/dmidecode. This also creates a function named dmidecode_real which can be invoked to get access to the real underlying dmidecode binary at /usr/sbin/dmidecode.

By default, this mock function will simply return 0 and produce no stdout or stderr. This behavior can be customized using the options --return-code, --stdout, and --stderr.

Mocking a real binary with a simplex name like this is the simplest, but doesn't always work. In particular, if at the call site you call it with the fully-qualified path to the binary, as in usr/sbin/dmidecode, then our mocked function won't be called. In this scenario, you need to mock it with the fully qualified path just as you would invoke it at the call site. For example:

emock "/usr/sbin/dmidecode"

Just as before, this will create and export a new function named /usr/sbin/dmidecode which will be called in place of the real dmidecode binary. It will also create a /usr/sbin/dmidecode_real function which will point to the real binary in case you need to call it instead.

emock tracks various metadata about mocked binaries for easier testability. This includes the number of times a mock is called, as well as the arguments (newline delimieted arg array), exit code, stdout, and stderr for each invocation. By default this is created in a local hidden directory named .emock and there will be a directory beneath that for each mock:

.emock/dmidecode/called
.emock/dmidecode/0/{args,exit,stdout,stderr}
.emock/dmidecode/1/{args,exit,stdout,stderr}

Mock Utility Functions

There are many utility functions to help with mocking which you can check out in the emock documentation:

  • emock
  • eunmock
  • emock_called
  • emock_stdout
  • emock_stderr
  • emock_args
  • emock_return_code
  • assert_emock_called
  • assert_emock_stdout
  • assert_emock_stderr
  • assert_emock_return_code
  • assert_emock_called_with