How to set package variable using -ldflags -X in Golang build

Question

I am creating an app using Go 1.9.2 and I am trying to add a version string variable to it using the ldflags -X options during the build.

I've managed to set a Version variable in my main package by using: -ldflags "-X main.Version=1.0.0", however what I really need is to set the Version variable inside my config package instead of the main one. Is this possible?

Here is my build command:

go build -ldflags "-X config.Version=1.0.0" -o $(MY_BIN) $(MY_SRC)

Answer 1

Quoting from doc of Command link:

 -X importpath.name=value     Set the value of the string variable in importpath named name to value.     Note that before Go 1.5 this option took two separate arguments.     Now it takes one argument split on the first = sign. 

So it can be used for any package, not just for the main package. But you must specify the full import path, not just the package name.

E.g. if your config package is located at $GOPATH/src/my/package/config, then use the following command:

go build -ldflags "-X my/package/config.Version=1.0.0" -o $(MY_BIN) $(MY_SRC) 

Answer 2

Here's a simple example, hopefully it would clarify and help how to do it easily:

Create a directory for your application:

$ mkdir app && cd app 

Create a sub directory config:

$ mkdir config  

Add the following file. it should be under app/config/vars.go:

package config  var Version string   var BuildTime string  //todo: can add as many as build vars  

In the root app/, add main package main.go:

package main  import (     "fmt"     "app/config" )  func main() {     fmt.Println("build.Version:\t", Version)     fmt.Println("build.Time:\t", build.BuildTime) } 

Now it is time to build:

go build -ldflags "-X 'app/config.Version=0.0.1' -X 'app/config.BuildTime=$(date)'"

Once it's built, you can run now the app:

$ ./app  Version:     0.0.1 build.Time:  Sat Jul  4 19:49:19 UTC 2020 

Finally, you may need sometimes to explore what does specific app you didn't code yourself provide in ldflags, you can do this by using nm that comes with go tool to list all ldflags.

Just build the app then use go tool nm to list all linker flags.

$ go build -o app  $ go tool nm ./app  

Still have any questions or anything to be more clarified? please feel free to leave a comment below and I will get back to you as soon as I can.

Answer 3

Add your (unitialized string) variable, build your binary, then do a

$ go tool nm <your binary> | grep <your variable>

it will display something like

101aa3820 B <import path>/config.Version

That’s the import path your looking for, to set in

go build -ldflags "-X <import path>/config.Version=1.0.0" -o $(MY_BIN) $(MY_SRC)