I was running some tests recently using Mocha and I wanted to re-run the tests when files changed. Typically this kind of functionality is built into test runners, but a) I'm not that familiar with Mocha (meh, it's just a tool) and b) I've got nodemon (which I wrote) 🙌.

The thing is: when I ran a failing test with nodemon and mocha, the way mocha exits (in this particular case) makes nodemon think that the command totally failed with process failed, unhandled exit code (2). Cleaning the exit is a good trick to know.

Exit codes

When a process runs (in a unix-based system…I'm unsure how much applies to Windows), it will exit with a code. There's only a handful of standardised exit codes, but the ones of most interest are:

  • exit 0 → success
  • exit 1 → failure

If you try this in your terminal, the terminal will just close (since you're running exit). But to test this, you can run the following:

$ sh -c "exit 0"
$ echo $?
0
$ sh -c "exit 1"
$ echo $?
1

Running sh -c "…" runs the string as a bash command and returns the result. Then the shell value $? is the numerical status of the exit code from the last executed command.

Changing exit codes

When I ran mocha, it was returning an exit code of 2. This is a weirdness of mocha that (misuses) the exit status reporting the number of failing tests.

The "fix" is only a few characters though. When I run mocha inside of nodemon, I use an bash or statement that reads "if this fails, fail with an exit 1":

$ nodemon --exec "mocha bad.test.js || exit 1"

Now if mocha fails with exit 2 it'll exit nodemon's exec with an exit 1 which nodemon sees as a failure.

Wait, that's not how or works!

If you've used to an or statement in code then you (like me) might think that exit 0 || exit 1 would result in 1 - since 0 is generally falsy. Except in exit codes, remember, 0 is success (truthy), and 1 is failure (falsy).

So || (or) reads as: if it failed then do X. You can see the results here:

$ sh -c 'sh -c "exit 0" || exit 1'; echo $?
0 # the OR was not used

$ sh -c 'sh -c "exit 1" || exit 1'; echo $?
1 # the OR _was_ used

$ sh -c 'sh -c "exit 2" || exit 1'; echo $?
1 # again, the OR _was_ used with exit 2 - failure

Explanation of the lines above: the inner sh -c is a script that exits with the given code. The outer sh -c will run the first script (the inner sh -c) and if that fails, it will run exit 1 as specified with the || operator.

Elsewhere

I've used this same trick to fix a postinstall problem where the postinstall command in npm was failing which causes the entire npm install process to blow up.

So I change the package.json from:

{
  "scripts": {
    "postinstall": "node -e \"console.log('Hi there')"
  }
}

To:

{
  "scripts": {
    "postinstall": "node -e \"console.log('Hi there') || exit 0"
  }
}

The || exit 0 will cleanly exit the postinstall if there's any error. Of course, if the node command is successful it'll exit with 0 and the || exit 0 does not run and is ignored.

So it's always good to understand exit codes and how to manipulate them if you need to.