Compile Time Variables

A very handy build flag allows you to set symbol values from the command line go build tool. Check out the example below:

package main

import "fmt"

var (
    version string

func main() {
    fmt.Println("version: ", version)

This example has an empty string called version. when we compile and run this program we get the following output:

go build && ./main

go tool nm main | grep vers
  5942d0 D main.version

Great, we were able to print an empty version variable! The tool nm shows you the symbol table of your executable. That is pretty neat, we can see our main.version variable defined.

What if

What if we could make our CI build system embed the version and other meta data about the build of this executable into the application, so the application is aware of the version??

With the below command, we are able to populate that variable statically inside the executable at compile time:

[husobee@localhost ugh]$ go build -ldflags "-X main.version=1.0.1" main.go
[husobee@localhost ugh]$ ./main
version:  1.0.1

Go’s Linker allows you to set string symbols with the -X flag, which is exposed through go build’s -ldflags.

In real life

In real life at my company we use this technique to embed commit hash, semver versions and other meta data into our executables at build time. This has been handy as we have a route we use in our web application we can GET from that will display this type of meta data information.