Spawns and interacts with child processes using spawn / expect commands

Usage no npm install needed!

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



nexpect is a node.js module for spawning child applications (such as ssh) and seamlessly controlling them using javascript callbacks. nexpect is based on the ideas of the expect library by Don Libes and the pexpect library by Noah Spurrier.


node.js has good built in control for spawning child processes. nexpect builds on these core methods and allows developers to easily pipe data to child processes and assert the expected response. nexpect also chains, so you can compose complex terminal interactions.


  $ npm install --save nexpect



The module exposes a single function, .spawn.

function spawn (command, [params], [options])

  • command {string|Array} The command that you wish to spawn, a string will be split on ' ' to find the params if params not provided (so do not use the string variant if any arguments have spaces in them)
  • params {Array} Optional Argv to pass to the child process
  • options {Object} Optional An object literal which may contain
    • cwd: Current working directory of the child process.
    • env: Environment variables for the child process.
    • ignoreCase: Ignores the case of any output from the child process.
    • stripColors: Strips any ANSI colors from the output for .expect() and .wait() statements.
    • stream: Expectations can be written against 'stdout', 'stderr', or 'all', which runs expectations against both stdout and stderr (defaults to 'stdout')
    • verbose: Writes the stdout for the child process to process.stdout of the current process, and any data sent with sendline to the process.stdout of the current process.

Top-level entry point for nexpect that liberally parses the arguments and then returns a new chain with the specified command, params, and options.

function expect (expectation)

  • expectation {string|RegExp} Output to assert on the target stream

Expect that the next line of output matches the expectation. Throw an error if it does not.

The expectation can be a string (the line should contain the expected value as a substring) or a RegExp (the line should match the expression).

function wait (expectation, callback)

  • expectation {string|RegExp} Output to assert on the target stream
  • callback {Function} Optional Callback to be called when output matches stream

Wait for a line of output that matches the expectation, discarding lines that do not match.

Throw an error if no such line was found.

The expectation can be a string (the line should contain the expected value as a substring) or a RegExp (the line should match the expression).

The callback will be called for every line that matches the expectation.

function sendline (line)

  • line {string} Output to write to the child process.

Adds a write line to context.process.stdin to the context.queue for the current chain.

function sendEof ()

Close child's stdin stream, let the child know there are no more data coming.

This is useful for testing apps that are using inquirer, as inquirer.prompt() calls stdin.resume() at some point, which causes the app to block on input when the input stream is a pipe.

function run (callback)

  • callback {function} Called when child process closes, with arguments
    • err {Error|null} Error if any occurred
    • output {Array} Array of lines of output examined
    • exit {Number|String} Numeric exit code, or String name of signal

Runs the context against the specified context.command and context.params.


Lets take a look at some sample usage:

  var nexpect = require('nexpect');

  nexpect.spawn("echo", ["hello"])
         .run(function (err, stdout, exitcode) {
           if (!err) {
             console.log("hello was echoed");

  nexpect.spawn("ls -la /tmp/undefined", { stream: 'stderr' })
         .expect("No such file or directory")
         .run(function (err) {
           if (!err) {
             console.log("checked that file doesn't exists");

  nexpect.spawn("node --interactive")
         .run(function (err) {
           if (!err) {
             console.log("node process started, console logged, process exited");
           else {

If you are looking for more examples take a look at the examples, and tests.


All tests are written with vows:

  $ npm test


Elijah Insua Marak Squires, and Charlie Robbins.