CLI capture tool for testing command-line apps

Usage no npm install needed!

<script type="module">
  import mockCli from '';



npm version Stability Build Status

CLI capture tool for testing command-line apps


This tool allows you to simulate a separate command-line environment within the current process, automatically capturing any input/output for the simulated process. It works by temporarily faking process.argv and stdio (and optionally redirecting stdio), so your CLI app code has no idea it is actually being run in a test environment.

process.exit() and uncaught errors are handled gracefully, returning the process exit code and the captured stdio contents.


var mockCli = require('mock-cli');
var assert = require('assert');

var argv = ['node', 'hello-world-app.js', '--foo', 'bar']; // Fake argv
var stdio = {
    stdin: require('./mocks/fakeInputStream'), // Hook up a fake input stream
    stdout: process.stdout, // Display the captured output in the main console
    stderr: process.stderr // Display the captured error output in the main console
var kill = mockCli(argv, stdio, function onProcessComplete(error, result) {
    var exitCode = result.code; // Process exit code
    var stdout = result.stdout; // UTF-8 string contents of process.stdout
    var stderr = result.stderr; // UTF-8 string contents of process.stderr

    assert.equal(exitCode, 0);
    assert.equal(stdout, 'Hello, world!\n');
    assert.equal(stderr, '');

// Execute the CLI task

// Kill the task if still running after one second
setTimeout(kill, 1000);


mockCli(argv, stdio, exitCallback)

Simulate a separate command-line environment within the current process


Name Type Required Default Description
argv Array No process.argv Fake argv for the simulated process
stdio object No null Object containing streams that are used to fake stdio
stdio.stdin Stream No null Input stream that is piped into the simulated process as process.stdin
stdio.stdout Stream No null Output stream that is connected to process.stdout of the simulated process
stdio.stderr Stream No null Error stream that is connected to process.stderr of the simulated process
errorCallback function No null Callback that is invoked once the simulated process completes or throws an uncaught exception
  • errorCallback has the following signature:

    function(error, result)
    Name Type Description
    error Error Uncaught exception thrown by the simulated process
    result object Object containing information about the simulated process
    result.code number Process exit code, or 1 on an uncaught exception
    result.stdin string UTF-8 contents of process.stdin, captured during simulation
    result.stdout string UTF-8 contents of process.stdout, captured during simulation
    result.stderr string UTF-8 contents of process.stderr, captured during simulation


function Function that kills the simulated process (as if the user pressed Ctrl+C).

  • This function returns a results object, as seen in exitCallback arguments.