how to set variables at build time

content/posts/build-time-vars-golang.md

@@ -0,0 +1,53 @@
+---
+title: "Set Variables at Build Time in Golang"
+date: 2020-01-05T21:00:00Z
+tags: [golang, programming]
+draft: false
+---
+
+For some CLI tools I use regularly I noticed that when I run the `version` flag get some version info that includes a 
+git commit hash or a git tag to the command line:
+
+```shell
+> kubectl version
+Client Version: version.Info{Major:"1", Minor:"16", GitVersion:"v1.16.3", GitCommit:"b3cbbae08ec52a7fc73d334838e18d17e8512749", GitTreeState:"clean", BuildDate:"2019-11-13T11:23:11Z", GoVersion:"go1.12.12", Compiler:"gc", Platform:"linux/amd64"}
+Server Version: version.Info{Major:"1", Minor:"16", GitVersion:"v1.16.2", GitCommit:"c97fe5036ef3df2967d086711e6c0c405941e14b", GitTreeState:"clean", BuildDate:"2019-10-15T19:09:08Z", GoVersion:"go1.12.10", Compiler:"gc", Platform:"linux/amd64"}
+```
+
+I discovered a nice way to do this for go programs. You can specify a value for a global variable at build time using 
+the `-ldflags "-X"`  flag. 
+
+For this program:
+
+```go
+package main
+
+import "fmt"
+
+var Version = "dev"
+
+func main(){
+  fmt.Printf("I'm version <%s>", Version)
+}
+```
+You can use this one liner that builds your go program while setting the *Version* variable to the *sha* hash of the latest 
+commit.
+
+```shell
+> go build -ldflags "-X main.Version=$(git rev-parse HEAD)" -o hello_app main.go
+> hello_app
+I'm version ba83f7c418c669f705a3cce0c58a1f9129a3de14
+```
+
+Note that you have to specify the full path of the package in your project, for example:
+
+```bash
+# You project looks like
+# - main.go <- module `github.com/myrepo/test`
+# - version <- `github.com/myrepo/test/version`
+#   - sha.go <- has `var Version = "dev"`
+> go build -ldflags "-X github.com/myrepo/test/version.Version=$(git rev-parse HEAD)" -o hello_app main.go
+> hello_app
+I'm version ba83f7c418c669f705a3cce0c58a1f9129a3de14
+```
+