Add info at build time to go binaries

Introduction

Sometimes it can be useful to capture information about your environment at build time, and have this information injected into the binary that you’re building. Some examples centre around versioning, where it might make sense to capture git commit hashes or build serials.

An example program

package main

import "fmt"

var gitCommit string
var buildSerial string

func main() {
  fmt.Printf("git hash: %s, build serial: %s", gitCommit, buildSerial)
}

Two variables in this module gitCommit, and buildSerial are going to hold some version information for us. Running this program yields some rather uninteresting results.

go run main.go
git hash: , build serial: 

-X switch

While building a program, you can use the -X linker switch which will allow you to supply information into module variables from the build process.

We can obtain the latest build hash using git with the following:

git rev-list -1 HEAD

We can even synthesize a build number involving the date, perhaps?

date +%s

Using the -ldflags switch, we can now specify these at the console.

go build -ldflags "-X main.gitCommit=`git rev-list -1 HEAD` -X main.buildSerial=`date +%s`" main.go

Closing up

Now that we have a binary built, it’s had its build information applied - these variables now magically receive these values.

./main
git hash: f6f62d9a759a03afffb913a1d24fb64a1bc5507d, build serial: 1564573076

Remember, these switches can be buried behid a Makefile also, so you don’t need to be typing these things over and over.

GOOS=linux
GOARCH=386

.PHONY: build

GIT_COMMIT := $(shell git rev-list -1 HEAD)
BUILD_NO := $(shell date +%s)

build:
  GOOS=$(GOOS) GOARCH=$(GOARCH) go build -ldflags "-X main.gitCommit=$(GIT_COMMIT) -X main.buildSerial=$(BUILD_NO)" .