Write shell completions directly to a file

cmd/completion.go

@@ -2,6 +2,9 @@ package cmd
 
 import (
 	"fmt"
+	"io"
+	"os"
+	"path/filepath"
 	"strings"
 
 	"github.com/spf13/cobra"
@@ -10,13 +13,21 @@ import (
 const shellPowerShell = "powershell"
 
 func newCompletionCmd() *cobra.Command {
-	return &cobra.Command{
+	var output string
+
+	cmd := &cobra.Command{
 		Use:       "completion [bash|zsh|fish|powershell]",
 		Short:     "Generate shell completion scripts",
 		Args:      cobra.ExactArgs(1),
 		ValidArgs: []string{"bash", "zsh", "fish", shellPowerShell},
 		RunE: func(cmd *cobra.Command, args []string) error {
-			out := cmd.OutOrStdout()
+			out, closeOut, err := completionOutput(cmd, output)
+			if err != nil {
+				return err
+			}
+			if closeOut != nil {
+				defer func() { _ = closeOut() }()
+			}
 			root := cmd.Root()
 
 			switch shell := strings.ToLower(args[0]); shell {
@@ -33,4 +44,23 @@ func newCompletionCmd() *cobra.Command {
 			}
 		},
 	}
+	cmd.Flags().StringVarP(&output, "output", "o", "", "Write the completion script to a file")
+	return cmd
+}
+
+func completionOutput(cmd *cobra.Command, output string) (io.Writer, func() error, error) {
+	if strings.TrimSpace(output) == "" {
+		return cmd.OutOrStdout(), nil, nil
+	}
+	dir := filepath.Dir(output)
+	if dir != "." && dir != "" {
+		if err := os.MkdirAll(dir, 0o755); err != nil {
+			return nil, nil, fmt.Errorf("create completion output directory: %w", err)
+		}
+	}
+	f, err := os.Create(output)
+	if err != nil {
+		return nil, nil, fmt.Errorf("create completion output file: %w", err)
+	}
+	return f, f.Close, nil
 }

cmd/completion_test.go

@@ -2,6 +2,8 @@ package cmd
 
 import (
 	"bytes"
+	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/spf13/cobra"
@@ -24,6 +26,25 @@ func TestCompletionCmd_GeneratesPowerShell(t *testing.T) {
 	assert.Contains(t, out.String(), "Register-ArgumentCompleter")
 }
 
+func TestCompletionCmd_WritesOutputFile(t *testing.T) {
+	root := &cobra.Command{Use: "app"}
+	root.AddCommand(newCompletionCmd())
+	outPath := filepath.Join(t.TempDir(), "nested", "app.ps1")
+
+	var out bytes.Buffer
+	root.SetOut(&out)
+	root.SetErr(&out)
+	root.SetArgs([]string{"completion", "powershell", "--output", outPath})
+
+	err := root.Execute()
+
+	require.NoError(t, err)
+	assert.Empty(t, out.String())
+	data, err := os.ReadFile(outPath)
+	require.NoError(t, err)
+	assert.Contains(t, string(data), "Register-ArgumentCompleter")
+}
+
 func TestCompletionCmd_RejectsUnsupportedShell(t *testing.T) {
 	root := &cobra.Command{Use: "app"}
 	root.AddCommand(newCompletionCmd())