A common use-case for build-time variables is to inject the Git version information in the binary. There are several approaches on how to do this, but this one is the one I found the most useful one.
Let's start with creating a sample project using Go modules:
$ mkdir example-go-build-time-variables
$ cd example-go-build-time-variables
$ go mod init github.com/pieterclaerhout/example-go-build-time-variables
go: creating new go.mod: module github.com/pieterclaerhout/example-go-build-time-variables
As we want to play with the Git revision and branch info, let's also initialize it as a Git repository:
$ echo "# example-go-build-time-variables" >> README.md
$ git init
Initialized empty Git repository in ~/example-go-build-time-variables/.git/
$ git add .
$ git commit -m "first commit"
[master (root-commit) 47c55c2] first commit
2 files changed, 4 insertions(+)
create mode 100644 README.md
create mode 100644 go.mod
Let's start with creating the most simple app we can think of by adding a main.go file with the following contents:
main.go
package main
import (
"fmt"
)
func main() {
fmt.Println("Hello stranger!")
}
Let's also create a package called version in which the version information is going to be stored:
version/version.go
package version
// GitRevision will be injected with the current git commit hash
var GitRevision string
// GitBranch will be injected with the current git branch name
var GitBranch string
I prefer to create a non-related package just containing this information. This makes it easy to access the version information from anywhere.
We can then update the main.go file to use this information:
main.go
package main
import (
"fmt"
"github.com/pieterclaerhout/example-go-build-time-variables/version"
)
func main() {
fmt.Println("Git Revision:", version.GitRevision)
fmt.Println("Git Branch:", version.GitBranch)
}
If we build and run it, you'll get the following output:
$ go build -o example-go-build-time-variables .
$ ./example-go-build-time-variables
Git Revision:
Git Branch:
That's the expected behaviour as we didn't inject the version information yet.
To inject the information, you need to specify the -ldflags option during the go build process. You can test it manually by changing the build command to:
$ go build -o example-go-build-time-variables -ldflags "-X github.com/pieterclaerhout/example-go-build-time-variables/version.GitRevision=revision -X github.com/pieterclaerhout/example-go-build-time-variables/version.GitBranch=branch" .
$ ./example-go-build-time-variables
Git Revision: revision
Git Branch: branch
Now, the only thing we need to do is to get the proper Git revision and branch name. The revision can be found with the following command:
$ git rev-parse --short HEAD
47c55c2
The branch name is obtained by the following command:
$ git rev-parse --abbrev-ref HEAD | tr -d '\040\011\012\015\n'
master
To make it easier, I usually combine everything in a simple Makefile which looks as follows:
APPNAME := example-go-build-time-variables
PACKAGE := github.com/pieterclaerhout/example-go-build-time-variables/version
REVISION := $(shell git rev-parse --short HEAD)
BRANCH := $(shell git rev-parse --abbrev-ref HEAD | tr -d '\040\011\012\015\n')
build:
go build -ldflags "-X $(PACKAGE).GitRevision=$(REVISION) -X $(PACKAGE).GitBranch=$(BRANCH)" -o $(APPNAME)
run: build
./$(APPNAME)
You can then simply run either make build to create the build or make run to build and run the application:
make run
go build -ldflags "-X github.com/pieterclaerhout/example-go-build-time-variables/version.GitRevision=47c55c2 -X github.com/pieterclaerhout/example-go-build-time-variables/version.GitBranch=master" -o example-go-build-time-variables
./example-go-build-time-variables server
Git Revision: 47c55c2
Git Branch: master
The complete source code for this example can be found here.