Add Parse Flags From CLI Arguments as a Go TIL

go/parse-flags-from-cli-arguments.md

@@ -0,0 +1,65 @@
+# Parse Flags From CLI Arguments
+
+Though we can grab the arguments to a Go program from `os.Args`, it requires
+some manual parsing. With the built-in `flag` package, we can declare specific
+flags our program accepts, by type. When we parse them, they will be separated
+out from the rest of the positional arguments.
+
+Here is an example of the program that accepts a boolean `debug` flag. This
+will work with either `-debug` or `--debug`.
+
+```go
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+)
+
+func main() {
+	var debug bool
+	flag.BoolVar(&debug, "debug", false, "turns on debug mode, extra logging")
+	flag.Parse()
+
+	positionalArgs := flag.Args()
+
+	if len(positionalArgs) < 1 {
+		fmt.Println("Please specify which part to run: 1 or 2")
+		os.Exit(1)
+	}
+
+	if debug {
+		fmt.Println("We are in debug mode...")
+		fmt.Println("Received the following argument:", positionalArgs[0])
+	}
+
+	// ...
+}
+```
+
+We can run the program in debug mode like so:
+
+```bash
+$ go run . --debug 123
+We are in debug mode...
+Received the following argument: 123
+```
+
+We can also take advantage of the `help` flag that we get for free:
+
+```bash
+$ go run . --help
+Usage of /var/folders/62/lx9pcjbs1zbd83zg6twwym2r0000gn/T/go-build3212087168/b001/exe/test:
+  -debug
+        turns on debug mode, extra logging
+```
+
+Note: any recognized flags need to come before any of the position arguments.
+The `debug` flag won't be picked up if we run the program like this:
+
+```bash
+$ go run . 123 --debug
+```
+
+[source](https://pkg.go.dev/flag)