我可以通过构建命令标志并读取内部代码吗？

Question

I want in init function to execute one of those two lines

func init() {
    log.SetPrefix(">>>>>>>>>>>>>   ")


    log.SetOutput(ioutil.Discard)
}

How to achieve something similiar to C and macro definition in make file ? I build go program like go build my_project and I don't have any custom make file. Can I pass to build command flag and read inside code ?

Answer 1

Create a string variable that you can test. Then set that string variable using -X passed to the linker. For example:

package main

var MODE string

func main() {
    if MODE == "PROD" {
        println("I'm in prod mode")
    } else {
        println("I'm NOT in prod mode")
    }

}

Now, if you build this with just go build, MODE will be "". But you can also build it this way:

go build -ldflags "-X main.MODE PROD"

And now MODE will be "PROD". You can use that to modify your logic based on build settings. I generally use this technique to set version numbers in the binary; but it can be used for all kinds of build-time configuration. Note that it can only set strings. So you couldn't make MODE a bool or integer.

---

You can also achieve this with tags, which are slightly more complicated to set up, but much more powerful. Create three files:

main.go

package main

func main() {
    doThing()
}

main_prod.go

// +build prod

package main

func doThing() {
    println("I'm in prod mode")
}

main_devel.go

// +build !prod

package main

func doThing() {
    println("I'm NOT in prod mode")
}

Now when you build with go build, you'll get your main_devel.go version (the filename doesn't matter; just the // +build !prod line at the top). But you can get a production build by building with go build -tags prod.

See the section on Build Constraints in the build documentation.