Go build tags

Today I learned about Go build tags. Here’s some quick notes to help me remember how to use them.

Assume you have directory like so:

$ ls -1
extra.go
go.mod
main.go

And main.go has contents:

package main

import "fmt"

var numbers = []string{
    "one",
    "two",
}

func main() {
    for _, number := range numbers {
        fmt.Println(number)
    }
}

And extra.go has contents:

//go:build extrastuff

package main

func init() {
    numbers = append(numbers, "three", "four")
}

If you build without any tags, you get this:

$ go build
$ ./buildtags
one
two

But if you specify the extrastuff tag, you get:

$ go build --tags=extrastuff
$ ./buildtags
one
two
three
four

If you want to use multiple tags, you can do so like this:

• //go:build firstTag || secondTag ← OR - builds when either tag provided
• //go:build firstTag && secondTag ← AND - builds only when both tags provided

There used to be a syntax like // +build tagName but that superseded by the //go:build syntax in Go 1.17.