With Golang we can inject build-time variables into our binary. This can help provide users with a quick way to gather more information about the build they are using. Docker’s CLI contains information we can inject into our own project:

 Client: Docker Engine - Community
 Version:           20.10.6
 API version:       1.41
 Go version:        go1.13.15
 Git commit:        370c289
 Built:             Fri Apr  9 22:47:17 2021
 OS/Arch:           linux/amd64
 Context:           default
 Experimental:      true

Let’s say for our project we want to inject the current Git SHA of the build, so our users can let us know exactly what commit they are running.

package main

import "fmt"

var GitSHA string

func main() {
	fmt.Printf("current revision: %s", GitSHA)
}

We want the output of this binary ./build.sh && ./main to print:

current revision:

We can get our git short SHA using git rev-parse --short HEAD . The command to inject variables is -ldflags "-X main.GitSHA=<sha> as part of the go build command

Combining the two we have a small build.sh script:

#!/bin/bash

sha=$(git rev-parse --short HEAD)
go build -ldflags "-X main.GitSHA=$sha" -o main

Running this gives gives the desired result!

current revision: 9f76309

If you want to run the same in a Dockerfile instead of a bash script, you can do that as well:

FROM golang:latest

WORKDIR /go/src/github.com/klotzandrew/klotzandrew.io

COPY .git .
COPY . .

RUN GIT_SHA=$(git rev-parse --short HEAD) && \
  go build -ldflags "-X main.GitSHA=$GIT_SHA" -o main

CMD ["./main"]

And we can get the same output with

docker build -t golang-inject-git-sha .
docker run --rm golang-inject-git-sha

Some other useful pieces of information to include:

• Date of the build
• Release tag