etcdctl: add command to generate shell completion

To improve the UX of etcdctl. Completion is generated by cobra
according to defined commands and flags.

Fixes #13111

etcdctl/ctlv3/command/completion_command.go

@@ -0,0 +1,84 @@
+// Copyright 2021 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package command
+
+import (
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+func NewCompletionCommand() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "completion [bash|zsh|fish|powershell]",
+		Short: "Generate completion script",
+		Long: `To load completions:
+
+Bash:
+
+  $ source <(etcdctl completion bash)
+
+  # To load completions for each session, execute once:
+  # Linux:
+  $ etcdctl completion bash > /etc/bash_completion.d/etcdctl
+  # macOS:
+  $ etcdctl completion bash > /usr/local/etc/bash_completion.d/etcdctl
+
+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:
+  $ etcdctl completion zsh > "${fpath[1]}/_etcdctl"
+
+  # You will need to start a new shell for this setup to take effect.
+
+fish:
+
+  $ etcdctl completion fish | source
+
+  # To load completions for each session, execute once:
+  $ etcdctl completion fish > ~/.config/fish/completions/etcdctl.fish
+
+PowerShell:
+
+  PS> etcdctl completion powershell | Out-String | Invoke-Expression
+
+  # To load completions for every new session, run:
+  PS> etcdctl completion powershell > etcdctl.ps1
+  # and source this file from your PowerShell profile.
+`,
+		DisableFlagsInUseLine: true,
+		ValidArgs:             []string{"bash", "zsh", "fish", "powershell"},
+		Args:                  cobra.ExactValidArgs(1),
+		Run: func(cmd *cobra.Command, args []string) {
+			switch args[0] {
+			case "bash":
+				cmd.Root().GenBashCompletion(os.Stdout)
+			case "zsh":
+				cmd.Root().GenZshCompletion(os.Stdout)
+			case "fish":
+				cmd.Root().GenFishCompletion(os.Stdout, true)
+			case "powershell":
+				cmd.Root().GenPowerShellCompletionWithDesc(os.Stdout)
+			}
+		},
+	}
+
+	return cmd
+}