Add Parse Flags From CLI Arguments as a Go TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1526 TILs and counting..._
+_1527 TILs and counting..._
 
 ---
 
@@ -403,6 +403,7 @@ _1526 TILs and counting..._
 - [Find Executables Installed By Go](go/find-executables-installed-by-go.md)
 - [Not So Random](go/not-so-random.md)
 - [Parse A String Into Individual Fields](go/parse-a-string-into-individual-fields.md)
+- [Parse Flags From CLI Arguments](go/parse-flags-from-cli-arguments.md)
 - [Replace The Current Process With An External Command](go/replace-the-current-process-with-an-external-command.md)
 - [Sleep For A Duration](go/sleep-for-a-duration.md)
 - [Upgrading From An Older Version On Mac](go/upgrading-from-an-older-version-on-mac.md)

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)