Best Practices for Using Mage To Build Your Project

On my team at Mattel, we have a magefile for every Go project (and we have several Go projects). Our use of mage has grown with the team and the projects, and it has been a big help keeping our dev practices uniform and shareable. Here’s how we do it.


One of the great things about Go is the tooling, not just the official tooling but all the community tooling. The only downside is keeping track of everything you need to build your complicated projects. That’s where retool comes in. It’s like dep but for go tools, pinning revisions so that all your developers run the exact same version.

Mage makes it easy to use retool – a Tools target ensures you have retool installed (go get won’t hit the internet if the code already resides locally, so it’s ok to run it every time), and then calls retool sync to make sure you’re up to date. Any other target that uses a build tool should define the Tools target as a dependency - mg.Deps(Tools). Thanks to Mage, no matter how many places that gets called in your build infrastructure, it’ll only do the tools check once.

func Tools() error {

	update, err := envBool("UPDATE")
	if err != nil {
		return err
	retool := ""

	args := []string{"get", retool}
	if update {
		args = []string{"get", "-u", retool}

	if err := sh.Run("go", args...); err != nil {
		return err

	return sh.Run("retool", "sync")

Now that you have your tools synced, you need to make sure you run the version retool caches, instead of the one that is in your PATH. This is easy with a little wrapper function:

// retool runs a command using a retool-cached binary.
func retool(cmd string, args ...string) error {
	return sh.Run("retool", append([]string{"do", cmd}, args...)...)

Here, we use mage’s sh package to output nice error messages when things fail (instead of just “exiting with code 1”), support verbose or quiet output, and automatic expansion of $FOO style environment variables. Prepending retool do on the front of the command, so makes it use retool to run the tool from retool’s cache.


Have you ever forgotten to update your dependencies before rebuilding? I think we all have. With mage, mg.Deps(Dep) at the beginning of any build target ensures that we’ve run dep ensure and our code isn’t out of date. (Yeah, we should move to modules at some point.)


We generate releases uses the wonderful goreleaser, which automates creating releases for a multitude of release platforms, with a single simple configuration file. Now, go releaser is easy to run, but I’ll be damned if I can remember how to create a tag and push it to a remote in git. Now I can do both in one simple command like TAG=v1.2.3 mage release and we even clean up the tag if something goes wrong.

func Release() (err error) {
	if os.Getenv("TAG") == "" {
		return errors.New("TAG environment variable is required")
	if err := sh.RunV("git", "tag", "-a", "$TAG"); err != nil {
		return err
	if err := sh.RunV("git", "push", "origin", "$TAG"); err != nil {
		return err
	defer func() {
		if err != nil {
			sh.RunV("git", "tag", "--delete", "$TAG")
			sh.RunV("git", "push", "--delete", "origin", "$TAG")
	return retool("goreleaser")

Along the same lines, sometimes you just want to produce a binary and not do the whole release process. But who can remember how to format ldflags to embed build info into your binary? Mage remembers -

func Build() error {
    return sh.RunV("go", "build", "-o", "project", "-ldflags="+ldflags(), "")

func ldflags() string {
	timestamp := time.Now().Format(time.RFC3339)
	hash := hash()
	tag := tag()
	if tag == "" {
		tag = "dev"
	return fmt.Sprintf(`-X "" ` +
    `-X "" ` +
    `-X ""`, timestamp, hash, tag)

// tag returns the git tag for the current branch or "" if none.
func tag() string {
	s, _ := sh.Output("git", "describe", "--tags")
	return s

// hash returns the git hash for the current repo or "" if none.
func hash() string {
	hash, _ := sh.Output("git", "rev-parse", "--short", "HEAD")
	return hash

Reusing Mage Code

Of course, all this code is mostly the same across projects, so sharing it between projects should be a priority. That’s easy with the new mage:import directive. This will bring in all the mage targets from the given package as targets for anyone that runs this magefile. If you import it without the _ you can use common helper functions from the package as well. An easy way to customize that code across projects is to use simple init function in your local magefile to set the current repo name, and then use that in your imported code:

import (
    _ ""

func init() {
    os.Setenv("MATTEL_REPO", "projname")

Then your common build target might look like this:

func Build() error {
    return sh.RunV("go", "build", "-o", "$MATTEL_REPO", "-ldflags="+ldflags(), "$MATTEL_REPO")

At Mattel, using mage has solved a lot of common dev problems, like mismatched binaries, forgetting processes, and just time spent wrangling the command line. Plus, standardizing across projects makes it easier to jump into a new project and figure out how everything fits together.

I’d love to hear how others are using mage (you’d be surprised at the variety of ways people use it). Come talk on #mage on gopher slack, or post on the new magefile google group.

comments powered by Disqus