Powerful Command-Line Applications in Go/

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Testing Commands with Mock Resources

Testing Commands with Mock Resources

Test external commands by executing them directly.

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So far, we’ve been testing the external commands by executing them directly. This is a perfectly valid approach. But sometimes it’s not desirable or possible to execute a command directly on the machine where we’re testing the code. For these cases, we’ll mock the external commands by using Go function during tests. This approach also allows us to use Go code to simulate abnormal conditions, such as timeouts, which are harder to simulate using external services. Go’s standard library applies this approach to test the function from the os/exec package. For more information, check the source code for the exec_test.go file from the standard library.

Write a test function

To use this approach, we’ll write a test function that replaces the function exec.CommandContext() from the exec package that we use to create the exec.Cmd type during tests. First, we edit the file timeoutStep.go and add a package variable command, assigning the original function exec.CommandContext():

var command = exec.CommandContext
func (s timeoutStep) execute() (string, error) {
  return s.message, nil
}
Replacing the function exec.CommandContext()

Since functions are first class types in Go, we can assign them to variables and pass them as arguments. In this case, we created a variable of type func (context.Context, string, ...string) *exec.Cmd and assigned the original exec.CommandContext() value as its initial value. Later, when testing, we’ll use this variable to override the original function with the mock function.

Then, we use the function stored in the variable to ...