Nate Finch
Dec 25, 2017 7 min read

Make Your Build Better With Mage

Many Go projects can be built using only Go’s wonderful built-in tooling. However, for many projects, these commands may not sufficient. Maybe you want to use ldflags during the build to embed the commit hash in the binary. Maybe you want to embed some files into the binary. Maybe you want to generate some code. Maybe you want to run a half dozen different linters. That’s where a build tool comes into play.

I used to propose using a normal go file to be run with go run. However, then you’re stuck building out a lot of the CLI handling yourself, which is busy work… no one wants to write another CLI parser for their project, plus error handling, plus handling output etc.

You might consider make, which handles the CLI definition for you, but then you’re stuck with writing Bash. A few months ago, I decided that neither of these build tool options were sufficient, and decided to make a third way. Thus, Mage was born. Mage is a build tool similar to make or rake, but intead of writing bash or ruby, Mage lets you write the logic in Go.

There are many reasons to choose Mage over make. The most important is the language. By definition, the contributors to your Go project already know Go. For many of them, it may be the language they’re most comfortable with. Your build system is just as important as the thing it’s building, so why not make it just as easy to contribute to? Why have a second language in your repo if you can easily avoid it? Not only is bash an esoteric language to start with, make piles even more arcane syntax on top of bash. Now you’re maintaining effectively three different languages in your repo.

One thing I love about Go is how easy it is to make cross platform applications. This is where Mage really shines. Although make is installed by default on Linux and OSX, it is not installed by default on Windows (which, as Stack Overflow notes, is the most prevalent development OS). Even if you install make on Windows, now you have to get bash running, which is non-trivial (yes, you can install the Windows Subsystem for Linux, but now you’re up to a pretty big ask just to build your Go project).

Mage, on the other hand, is a plain old Go application. If you have Go installed (and I presume you do) you can simply go get Mage has no dependencies outside the standard library, so you don’t even have to worry about a dependency manager for it. You can also download prebuilt binaries from github, if that’s preferable.

Once Mage is installed, you use Mage much like make in that you write one or more scripts (in this case, normal go files that we call magefiles) which mage then builds and runs for you. A magefile, instead of having a magic name (like Makefile), uses the go build tag //+build mage to indicate that mage should read it. Other than that, there’s nothing spceial abouit magefiles and you can name them whatever you like.

Mage includes all files that have this tag and only files that have this tag in its builds. This has several nice benefits - you can have the code for your build spread across any number of files, and those files will be ignored by the rest of your build commands. In addition, if you have platform-specific build code, you can use go’s build tags to ensure those are included or excluded as per usual. All your existing Go editor integrations, linters, and command line tools work with magefiles just like normal go files, because they are normal go files. Anything you can do with Go, any libraries you want to use, you can use with Mage.

Just like make, Mage uses build targets as CLI commands. For Mage, these targets are simply exported functions that may optionally take a context.Context and may optionally return an error. Any such function is exposed to Mage as a build target. Targets in a magefile are run just like in make

//+build mage

package main

// Creates the binary in the current directory.  It will overwrite any existing
// binary.
func Build() {

// Sends the binary to the server.
func Deploy() error {
    return nil

Running mage in the directory with the above file will list the targets:

$ mage
  build    Creates the binary in the current directory.
  deploy   Sends the binary to the server

Mage handles errors returned from targets just like you’d hope, printing errors to stderr and exiting with a non-zero exit code. Dependent targets, just like in make, will be run exactly once and starting at the leaves and moving upward through a dynamically generated dependency tree.

Mage has a ton of features - running multiple targets, default targets, target aliases, file targets and sources, shell helpers, and more. However, for this blog post I want to dive more into some of the magic behind how Mage works, not just what it does.

How it Works

When you run mage, the first thing it has to do is figure out what files it should read. It uses the normal go build heuristics (build tags, _platform in filenames, etc) with one little tweak… normally when you build, go grabs all files in a directory without tags. If you specify a tag in the build command it adds any files with that build tag… but it never takes away the files with no build tags. This won’t work for mage, since I wanted it to only include files that had a specific tag. This required some hacking. I ended up copying the entire go/build package into Mage’s repo and inserting some custom code to add the idea of a required tag… which then excludes any files that don’t explicitly specify that tag.

Once that step is done, we have a list of files with the correct build tags. Now, what to do with them? Well, we need to be able to execute the functions inside them. To do that, we need to generate some glue code to call the functions, and build the whole thing into a binary. Since this process can be time consuming the first time it’s run (on the order of 0.3 seconds on my 2017 MBP), we cache the created binary on disk whenever it’s built. Thus, after the first time it’s run, running mage for a project will start instantly like any normal Go binary (on my machine about 0.01s to print out help, for example). To ensure the cached binary exactly matches the code from the magefiles, we hash the input files and some data from the mage binary itself. If a cached version matches the hash (we just use the hash as the filename), we run that, since we know it must have been built using the exact same code.

If there’s no matching binary in the cache, we need to actually do some work. We parse the magefiles using go/types to figure out what our targets are and to look for a few other features (like if there’s a default target and if there’s any aliases). Parsing produces a struct of metadata about the binary, which is then fed into a normal go template which generates the func main() and all the code that produces the help output, the code to determine what target(s) to call, and the error handling.

This generated code is written to a file in the current directory and then it and the magefiles are run through a normal execution of go build to produce the binary, then the temp file is cleaned up.

Now that the glue code and magefiles have been compiled, it’s just a matter of running the binary and passing through the arguments sent to mage (this is the only thing that happens when the binary is cached).

From there, it’s just your go code running, same as always. No surprises, no wacky syntax. The Go we all know and love, working for you and the people on your team.

If you want some examples of magefiles, you can check out ones used by Gnorm, Hugo, and hopefully soon, dep.

Hop on the #mage channel on gopher slack to get all your questions answered, and feel free to take a look at our current issue list and pick up something to hack on.