cmd: add completion command (#4994)

* cmd: add completion command

* error check

cmd/commands.go

@@ -22,6 +22,7 @@ import (
 	"strings"
 
 	"github.com/caddyserver/caddy/v2"
+	"github.com/spf13/cobra"
 	"github.com/spf13/cobra/doc"
 )
 
@@ -379,6 +380,68 @@ The manual pages are sorted into the section specified by the argument of --sect
 			return fs
 		}(),
 	})
+
+	// source: https://github.com/spf13/cobra/blob/main/shell_completions.md
+	rootCmd.AddCommand(&cobra.Command{
+		Use:   "completion [bash|zsh|fish|powershell]",
+		Short: "Generate completion script",
+		Long: fmt.Sprintf(`To load completions:
+	
+	Bash:
+	
+	  $ source <(%[1]s completion bash)
+	
+	  # To load completions for each session, execute once:
+	  # Linux:
+	  $ %[1]s completion bash > /etc/bash_completion.d/%[1]s
+	  # macOS:
+	  $ %[1]s completion bash > $(brew --prefix)/etc/bash_completion.d/%[1]s
+	
+	Zsh:
+	
+	  # If shell completion is not already enabled in your environment,
+	  # you will need to enable it.  You can execute the following once:
+	
+	  $ echo "autoload -U compinit; compinit" >> ~/.zshrc
+	
+	  # To load completions for each session, execute once:
+	  $ %[1]s completion zsh > "${fpath[1]}/_%[1]s"
+	
+	  # You will need to start a new shell for this setup to take effect.
+	
+	fish:
+	
+	  $ %[1]s completion fish | source
+	
+	  # To load completions for each session, execute once:
+	  $ %[1]s completion fish > ~/.config/fish/completions/%[1]s.fish
+	
+	PowerShell:
+	
+	  PS> %[1]s completion powershell | Out-String | Invoke-Expression
+	
+	  # To load completions for every new session, run:
+	  PS> %[1]s completion powershell > %[1]s.ps1
+	  # and source this file from your PowerShell profile.
+	`, rootCmd.Root().Name()),
+		DisableFlagsInUseLine: true,
+		ValidArgs:             []string{"bash", "zsh", "fish", "powershell"},
+		Args:                  cobra.ExactValidArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			switch args[0] {
+			case "bash":
+				return cmd.Root().GenBashCompletion(os.Stdout)
+			case "zsh":
+				return cmd.Root().GenZshCompletion(os.Stdout)
+			case "fish":
+				return cmd.Root().GenFishCompletion(os.Stdout, true)
+			case "powershell":
+				return cmd.Root().GenPowerShellCompletionWithDesc(os.Stdout)
+			default:
+				return fmt.Errorf("unrecognized shell: %s", args[0])
+			}
+		},
+	})
 }
 
 // RegisterCommand registers the command cmd.