Add Shell Completion script generation

`dnsutils shell-completion <shell>` will generate a shell completion
script for the specified shell (bash or zsh). If no shell is specified,
the script will be generated for the current shell, using `$SHELL`.

commands/completion.go

@@ -0,0 +1,91 @@
+package commands
+
+import (
+	"embed"
+	"errors"
+	"fmt"
+	"github.com/urfave/cli/v2"
+	"os"
+	"path"
+	"strings"
+	"text/template"
+)
+
+//go:embed completion-scripts/completion.*.gotmpl
+var completionScripts embed.FS
+
+func shellCompletionCommand() *cli.Command {
+	supportedShells, templates, err := getCompletionSupportedShells()
+	if err != nil {
+		panic(err)
+	}
+	return &cli.Command{
+		Name:        "shell-completion",
+		Usage:       "generate shell completion scripts",
+		Hidden:      true,
+		ArgsUsage:   fmt.Sprintf("[ %s ]", strings.Join(supportedShells, " | ")),
+		Description: fmt.Sprintf("Generate shell completion script for [ %s ]", strings.Join(supportedShells, " | ")),
+		BashComplete: func(ctx *cli.Context) {
+			for _, shell := range supportedShells {
+				if strings.HasPrefix(shell, ctx.Args().First()) {
+					ctx.App.Writer.Write([]byte(shell + "\n"))
+				}
+			}
+		},
+		Action: func(ctx *cli.Context) error {
+			var inputShell string
+			if inputShell = ctx.Args().First(); inputShell == "" {
+				if inputShell = os.Getenv("SHELL"); inputShell == "" {
+					return cli.Exit(errors.New("shell not specified"), 1)
+				}
+			}
+			shellName := path.Base(inputShell) // necessary if using $SHELL, noop otherwise
+
+			template := templates[shellName]
+			if template == nil {
+				return cli.Exit(fmt.Errorf("unknown shell: %s", inputShell), 1)
+			}
+
+			err = template.Execute(ctx.App.Writer, struct {
+				App *cli.App
+			}{ctx.App})
+			if err != nil {
+				return cli.Exit(fmt.Errorf("failed to print completion script: %w", err), 1)
+			}
+			return nil
+		},
+	}
+}
+
+var _ = cmd(catUtils, shellCompletionCommand())
+
+// getCompletionSupportedShells returns a list of shells with available completions.
+// The list is generated from the embedded completion scripts.
+func getCompletionSupportedShells() (shells []string, shellCompletionScripts map[string]*template.Template, err error) {
+	scripts, err := completionScripts.ReadDir("completion-scripts")
+	if err != nil {
+		return nil, nil, fmt.Errorf("failed to read completion scripts: %w", err)
+	}
+
+	shellCompletionScripts = make(map[string]*template.Template)
+
+	for _, f := range scripts {
+		fNameWithoutExtension := strings.TrimSuffix(f.Name(), ".gotmpl")
+		shellName := strings.TrimPrefix(path.Ext(fNameWithoutExtension), ".")
+
+		content, err := completionScripts.ReadFile(path.Join("completion-scripts", f.Name()))
+		if err != nil {
+			return nil, nil, fmt.Errorf("failed to read completion script %s", f.Name())
+		}
+
+		t := template.New(shellName)
+		t, err = t.Parse(string(content))
+		if err != nil {
+			return nil, nil, fmt.Errorf("failed to parse template %s", f.Name())
+		}
+
+		shells = append(shells, shellName)
+		shellCompletionScripts[shellName] = t
+	}
+	return shells, shellCompletionScripts, nil
+}