Go Tidbit: Setting Variables in Go During Build

Last week I was working on a Go program. I wanted it to print the closest Git tag and build time to logs at startup.

I could write a small script that generates a Go file with these two variables set right before building the Go program.

But, Go makes it easier.

Ldflags

Say you have the following Go code:

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package main

var hello string

func main() {
  println(hello)
}

You can build it like so:

go build -ldflags "-X main.hello=HelloGo" -o a.out .

And run it:

./a.out

The program will print HelloGo.

Package Path

You can set any string variable with ldflags. Just make sure to prefix the variable name with the complete package path.

For example, if your package path is github.com/FurqanSoftware/example/cfg and the variable name is BuildTag, then your ldflags should look like this:

-X github.com/FurqanSoftware/example/cfg.BuildTag=TagGoesHere

Makefile

In the Go program that I was working on, I added a small Makefile:

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BUILD_TAG := $(shell git describe --tags)
BUILD_TIME := $(shell date -u '+%Y-%m-%dT%H:%M:%SZ')

.PHONY: printd
printd:
  go build -ldflags "-X main.buildTag=$(BUILD_TAG) -X main.buildTime=$(BUILD_TIME)" -o printd .

Now, if I run make printd, the resulting binary will have the two variables buildTag and buildTime set to the closest Git tag and the time when the binary was built.

Here’s what the Go program now prints to the log at startup:

(The line “Release” has the build tag and time.)

This post is 16th of my #100DaysToOffload challenge. Want to get involved? Find out more at 100daystooffload.com.