Add Check If Cobra Flag Was Set 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).
 
-_1539 TILs and counting..._
+_1540 TILs and counting..._
 
 ---
 
@@ -403,6 +403,7 @@ _1539 TILs and counting..._
 - [Access Go Docs Offline](go/access-go-docs-offline.md)
 - [Add A Method To A Struct](go/add-a-method-to-a-struct.md)
 - [Build For A Specific OS And Architecture](go/build-for-a-specific-os-and-architecture.md)
+- [Check If Cobra Flag Was Set](go/check-if-cobra-flag-was-set.md)
 - [Combine Two Slices](go/combine-two-slices.md)
 - [Do Something N Times](go/do-something-n-times.md)
 - [Find Executables Installed By Go](go/find-executables-installed-by-go.md)

go/check-if-cobra-flag-was-set.md

@@ -0,0 +1,41 @@
+# Check If Cobra Flag Was Set
+
+When using [Cobra](https://github.com/spf13/cobra) to define a CLI, we can
+specify a flag for a command like so:
+
+```go
+var Seed int64
+myCmd.PersistentFlags().Int64VarP(&Seed, "seed", "", -1, "set a seed")
+```
+
+This `--seed` flag has a _default_ of `-1`. If the flag isn't specified, then
+when we access that flag's value, we'll get `-1`.
+
+But how do we differentiate between the _default_ `-1` and someone passing `-1`
+to the `--seed` flag when running the program?
+
+In the command definition, we can look at the flags and see, by name, if
+specific ones were changed by user input rather than being the defaults.
+
+```go
+myCommand := &cobra.Command{
+	// coommand setup ...
+	Run: func(cmd *cobra.Command, args []string) {
+		if cmd.Flags().Changed("seed") {
+			seed, err := cmd.Flags().GetInt64("seed")
+			if err != nil {
+				fmt.Println("Seed flag is missing from `cmdFlags()`")
+				os.Exit(1)
+			}
+
+			fmt.Printf("Seed was set to %d\n", seed)
+		} else {
+			fmt.Println("Seed was not set")
+		}
+	}
+}
+```
+
+If we don't want to rely on the default and instead want to specify some other
+behavior when the flag is not manually set by the user, we can detect that
+scenario like this.