fix: improve completion cmd handling (#290)

* fix: improve completion cmd handling

The way it works today, its not possible to install the completions
automatically on Nix, as the build needs to be pure.

This is kinda hacky, but I think it'll work: we only allow the
completion cmd to run if there is good evidence that it is a completion
command, and not a prompt that happens to start with completion.

refs #287

* docs: update

* docs: wording

README.md

@@ -83,18 +83,18 @@ go install github.com/charmbracelet/mods@latest
 ```
 
 <details>
-<summary>Completions</summary>
+<summary>Shell Completions</summary>
 
 All the packages and archives come with pre-generated completion files for Bash,
 ZSH, Fish, and PowerShell.
 
 If you built it from source, you can generate them with:
 
 ```bash
-__MODS_CMP_ENABLED=1 mods completion bash -h
-__MODS_CMP_ENABLED=1 mods completion zsh -h
-__MODS_CMP_ENABLED=1 mods completion fish -h
-__MODS_CMP_ENABLED=1 mods completion powershell -h
+mods completion bash -h
+mods completion zsh -h
+mods completion fish -h
+mods completion powershell -h
 ```
 
 If you use a package (like Homebrew, Debs, etc), the completions should be set

main.go

@@ -322,12 +322,15 @@ func main() {
 	// XXX: this must come after creating the config.
 	initFlags()
 
-	// XXX: since mods doesn't have any subcommands, Cobra won't create the
-	// default `completion` command.
-	// Forcefully create the completion related subcommands by adding a fake
-	// command when completions are being used.
-	if os.Getenv("__MODS_CMP_ENABLED") == "1" || (len(os.Args) > 1 && os.Args[1] == "__complete") {
-		rootCmd.AddCommand(&cobra.Command{Use: "____fake_command_to_enable_completions"})
+	if isCompletionCmd(os.Args) {
+		// XXX: since mods doesn't have any sub-commands, Cobra won't create
+		// the default `completion` command. Forcefully create the completion
+		// related sub-commands by adding a fake command when completions are
+		// being used.
+		rootCmd.AddCommand(&cobra.Command{
+			Use:    "____fake_command_to_enable_completions",
+			Hidden: true,
+		})
 		rootCmd.InitDefaultCompletionCmd()
 	}
 
@@ -722,3 +725,35 @@ func newModelSelect() *huh.Select[string] {
 		Options(opts...).
 		Value(&config.Model)
 }
+
+func isCompletionCmd(args []string) bool {
+	if len(args) <= 1 {
+		return false
+	}
+	if args[1] == "__complete" {
+		return true
+	}
+	if args[1] != "completion" {
+		return false
+	}
+	if len(args) == 3 {
+		_, ok := map[string]any{
+			"bash":       nil,
+			"fish":       nil,
+			"zsh":        nil,
+			"powershell": nil,
+			"-h":         nil,
+			"--help":     nil,
+			"help":       nil,
+		}[args[2]]
+		return ok
+	}
+	if len(args) == 4 {
+		_, ok := map[string]any{
+			"-h":     nil,
+			"--help": nil,
+		}[args[3]]
+		return ok
+	}
+	return false
+}

main_test.go

@@ -0,0 +1,41 @@
+package main
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestIsCompletionCmd(t *testing.T) {
+	for args, is := range map[string]bool{
+		"":                                     false,
+		"something":                            false,
+		"something something":                  false,
+		"completion for my bash script how to": false,
+		"completion bash how to":               false,
+		"completion":                           false,
+		"completion -h":                        true,
+		"completion --help":                    true,
+		"completion help":                      true,
+		"completion bash":                      true,
+		"completion fish":                      true,
+		"completion zsh":                       true,
+		"completion powershell":                true,
+		"completion bash -h":                   true,
+		"completion fish -h":                   true,
+		"completion zsh -h":                    true,
+		"completion powershell -h":             true,
+		"completion bash --help":               true,
+		"completion fish --help":               true,
+		"completion zsh --help":                true,
+		"completion powershell --help":         true,
+		"__complete":                           true,
+		"__complete blah blah blah":            true,
+	} {
+		t.Run(args, func(t *testing.T) {
+			vargs := append([]string{"mods"}, strings.Fields(args)...)
+			if b := isCompletionCmd(vargs); b != is {
+				t.Errorf("%v: expected %v, got %v", vargs, is, b)
+			}
+		})
+	}
+}