go-embed-version

This is a quick tutorial on embedding build-time information, such as version, into a Go binary. I'm using make in this case but it's not required. I'm also not very adept with make so take that into consideration.

-ldflags -X

go build will accept a flag -ldflags to pass specific arguments to the linker. The link tool will accept a -X flag to set the value of an exported variable. It will only set variables, not constants. The full import path to the variable must be specified.

A Version variable

In my projects I tend to create a <package>/cmd/version.go file. Each binary can import this package. The location for the variable can be in any package you like.

cmd/version.go:

package cmd

import (
	"flag"
	"fmt"
	"os"

       )

var (
	Version = "" // set at compile time with -ldflags "-X versserv/cmd.Version=x.y.yz"

	FVersion = flag.Bool("version", false, "show version and exit")

    )

func ShowVersion() {
	fmt.Println("version:", Version)
	os.Exit(0)

}

I also provide a flag and utility function to show the version and exit. If the version isn't set ont he command line during go build the variable is unchanged:

cmd$ go build -o /tmp/versserv server/server.go 
cmd$ /tmp/versserv -version
version: 

You can set it to an arbitrary value by hand:

cmd$ go build -o /tmp/versserv -ldflags "-X github.com/AgentZombie/go-embed-version/cmd.Version=foo" server/server.go 
cmd$ /tmp/versserv -version
version: foo

Setting the variable automatically

I'm using make and I have it create the version string based on git tag or a timestamp, depending on if it's a dev or release binary.

Makefile:

VERSION := $(shell git tag | grep ^v | sort -V | tail -n 1)
LDFLAGS = -ldflags "-X github.com/AgentZombie/go-embed-version/cmd.Version=${VERSION}"
TIMESTAMP := $(shell date +%Y%m%d-%H%M)
DEVLDFLAGS = -ldflags "-X github.com/AgentZombie/go-embed-version/cmd.Version=dev-${TIMESTAMP}"

SERVBIN=/tmp/versserv

dev-serv: server/*
        go build ${DEVLDFLAGS} -o ${SERVBIN} server/*.go

serv: server/*
        go build ${LDFLAGS} -o ${SERVBIN} server/*.go

The VERSION pipeline in the Makefile lists all tags visible from this commit, selects those that being with v assuming those are semver tags. It sorts those using sort's version sorting, then keeps only the latest one.'

cmd$ make dev-serv
go build -ldflags "-X github.com/AgentZombie/go-embed-version/cmd.Version=dev-20190608-1945" -o /tmp/versserv server/*.go
cmd$ /tmp/versserv -version
version: dev-20190608-1945
cmd$ rm /tmp/versserv 
cmd$ make serv
go build -ldflags "-X github.com/AgentZombie/go-embed-version/cmd.Version=v0.2.0" -o /tmp/versserv server/*.go
cmd$ /tmp/versserv -version
version: v0.2.0

Tagging a version

In git tags are arbitrary labels but it's a common convention to use tags with Semantic Versioning. Tags apply to commits so any changes you want a tag to apply to must be committed. A simple tag operation looks like this:

git tag -a v0.3.0

You can also apply a message with the tag and it's good to use this to communicate what's captured in this version:

git tag -a v0.3.0 -m 'Add tutorial in README.md'

By default, tags aren't pushed upstream with other commits. To push with tags add --follow-tags:

git push --follow-tags

You can also configure git to always push tags with commits but it's not the best idea.

Additional use for the version

I also build docker images using make and include the version tag in the build:

docker-image:
	docker build -t versserv:${VERSION} dockerdir

Special Thanks

Special thanks to subcon for gently nudging me to publish this.