Add shell completion for beacon-chain and validator CLI (#16245)

**What type of PR is this?**

Feature

**What does this PR do? Why is it needed?**

Introduces a `completion` subcommand to `beacon-chain` and `validator`
that outputs shell completion scripts. Supports Bash, Zsh, and Fish
shells.

```bash
# Load completions in current session
source <(beacon-chain completion bash)

# Persist for future sessions
beacon-chain completion zsh > "${fpath[1]}/_beacon-chain"
validator completion fish > ~/.config/fish/completions/validator.fish
```

Once loaded, users can press TAB to complete subcommands, nested
commands, and flags. Flag completion supports prefix matching (e.g.,
typing `--exec<TAB>` suggests `--execution-endpoint`,
`--execution-headers`).

**Which issues(s) does this PR fix?**

Fixes #16244

**Other notes for review**

The implementation adds three files to the existing `cmd` package:
- `completion.go` - Defines `CompletionCommand()` returning a
`*cli.Command` with `bash`, `zsh`, `fish` subcommands
- `completion_scripts.go` - Contains the shell script templates
- `completion_test.go` - Unit tests for command structure and script
content

Changes to `beacon-chain` and `validator`:
- Import `cmd.CompletionCommand("binary-name")` in the Commands slice
- Set `EnableBashCompletion: true` on the cli.App to activate
urfave/cli's `--generate-bash-completion` hidden flag

The shell scripts call the binary with `--generate-bash-completion`
appended to get context-aware suggestions. This means completions
automatically reflect the current binary's flags and commands.

**Acknowledgements**

- [x] I have read
[CONTRIBUTING.md](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md).
- [x] I have included a uniquely named [changelog fragment
file](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md#maintaining-changelogmd).
- [x] I have added a description with sufficient context for reviewers
to understand this PR.
- [x] I have tested that my changes work as expected and I added a
testing plan to the PR description (if applicable).

Signed-off-by: Willian Paixao <willian@ufpa.br>

changelog/willianpaixao_shell-autocompletion.md

@@ -0,0 +1,3 @@
+### Added
+
+- Added shell completion support for `beacon-chain` and `validator` CLI tools.

cmd/BUILD.bazel

@@ -3,6 +3,8 @@ load("@prysm//tools/go:def.bzl", "go_library", "go_test")
 go_library(
     name = "go_default_library",
     srcs = [
+        "completion.go",
+        "completion_scripts.go",
         "config.go",
         "defaults.go",
         "flags.go",
@@ -28,6 +30,7 @@ go_test(
     name = "go_default_test",
     size = "small",
     srcs = [
+        "completion_test.go",
         "config_test.go",
         "flags_test.go",
         "helpers_test.go",

cmd/beacon-chain/main.go

@@ -271,9 +271,11 @@ func main() {
 		Commands: []*cli.Command{
 			dbcommands.Commands,
 			jwtcommands.Commands,
+			cmd.CompletionCommand("beacon-chain"),
 		},
-		Flags:  appFlags,
-		Before: before,
+		Flags:                appFlags,
+		Before:               before,
+		EnableBashCompletion: true,
 	}
 
 	defer func() {

cmd/completion.go

@@ -0,0 +1,63 @@
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/urfave/cli/v2"
+)
+
+// CompletionCommand returns the completion command for the given binary name.
+// The binaryName parameter should be "beacon-chain" or "validator".
+func CompletionCommand(binaryName string) *cli.Command {
+	return &cli.Command{
+		Name:     "completion",
+		Category: "completion",
+		Usage:    "Generate shell completion scripts",
+		Description: fmt.Sprintf(`Generate shell completion scripts for bash, zsh, or fish.
+
+To load completions:
+
+Bash:
+  $ source <(%[1]s completion bash)
+  # To load completions for each session, execute once:
+  $ %[1]s completion bash > /etc/bash_completion.d/%[1]s
+
+Zsh:
+  # To load completions for each session, execute once:
+  $ %[1]s completion zsh > "${fpath[1]}/_%[1]s"
+
+  # You may need to start a new shell for completions 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
+`, binaryName),
+		Subcommands: []*cli.Command{
+			{
+				Name:  "bash",
+				Usage: "Generate bash completion script",
+				Action: func(_ *cli.Context) error {
+					fmt.Println(bashCompletionScript(binaryName))
+					return nil
+				},
+			},
+			{
+				Name:  "zsh",
+				Usage: "Generate zsh completion script",
+				Action: func(_ *cli.Context) error {
+					fmt.Println(zshCompletionScript(binaryName))
+					return nil
+				},
+			},
+			{
+				Name:  "fish",
+				Usage: "Generate fish completion script",
+				Action: func(_ *cli.Context) error {
+					fmt.Println(fishCompletionScript(binaryName))
+					return nil
+				},
+			},
+		},
+	}
+}

cmd/completion_scripts.go

@@ -0,0 +1,99 @@
+package cmd
+
+import (
+	"fmt"
+	"strings"
+)
+
+// bashCompletionScript returns the bash completion script for the given binary.
+func bashCompletionScript(binaryName string) string {
+	// Convert hyphens to underscores for bash function names
+	funcName := strings.ReplaceAll(binaryName, "-", "_")
+	return fmt.Sprintf(`#!/bin/bash
+
+_%[1]s_completions() {
+    local cur prev words cword opts
+    COMPREPLY=()
+
+    # Use bash-completion if available, otherwise set variables directly
+    if declare -F _init_completion >/dev/null 2>&1; then
+        _init_completion -n "=:" || return
+    else
+        cur="${COMP_WORDS[COMP_CWORD]}"
+        prev="${COMP_WORDS[COMP_CWORD-1]}"
+        words=("${COMP_WORDS[@]}")
+        cword=$COMP_CWORD
+    fi
+
+    # Build command array for completion - flag must be at the END
+    local -a requestComp
+    if [[ "$cur" == "-"* ]]; then
+        requestComp=("${COMP_WORDS[@]:0:COMP_CWORD}" "$cur" --generate-bash-completion)
+    else
+        requestComp=("${COMP_WORDS[@]:0:COMP_CWORD}" --generate-bash-completion)
+    fi
+
+    opts=$("${requestComp[@]}" 2>/dev/null)
+    COMPREPLY=($(compgen -W "${opts}" -- "${cur}"))
+    return 0
+}
+
+complete -o bashdefault -o default -o nospace -F _%[1]s_completions %[2]s
+`, funcName, binaryName)
+}
+
+// zshCompletionScript returns the zsh completion script for the given binary.
+func zshCompletionScript(binaryName string) string {
+	// Convert hyphens to underscores for zsh function names
+	funcName := strings.ReplaceAll(binaryName, "-", "_")
+	return fmt.Sprintf(`#compdef %[2]s
+
+_%[1]s() {
+    local curcontext="$curcontext" ret=1
+    local -a completions
+
+    # Build command array with --generate-bash-completion at the END
+    local -a requestComp
+    if [[ "${words[CURRENT]}" == -* ]]; then
+        requestComp=(${words[1,CURRENT]} --generate-bash-completion)
+    else
+        requestComp=(${words[1,CURRENT-1]} --generate-bash-completion)
+    fi
+
+    completions=($("${requestComp[@]}" 2>/dev/null))
+
+    if [[ ${#completions[@]} -gt 0 ]]; then
+        _describe -t commands '%[2]s' completions && ret=0
+    fi
+
+    # Fallback to file completion
+    _files && ret=0
+
+    return ret
+}
+
+compdef _%[1]s %[2]s
+`, funcName, binaryName)
+}
+
+// fishCompletionScript returns the fish completion script for the given binary.
+func fishCompletionScript(binaryName string) string {
+	// Convert hyphens to underscores for fish function names
+	funcName := strings.ReplaceAll(binaryName, "-", "_")
+	return fmt.Sprintf(`# Fish completion for %[2]s
+
+function __fish_%[1]s_complete
+    set -l args (commandline -opc)
+    set -l cur (commandline -ct)
+
+    # Build command with --generate-bash-completion at the END
+    if string match -q -- '-*' "$cur"
+        %[2]s $args $cur --generate-bash-completion 2>/dev/null
+    else
+        %[2]s $args --generate-bash-completion 2>/dev/null
+    end
+end
+
+complete -c %[2]s -f -a "(__fish_%[1]s_complete)"
+`, funcName, binaryName)
+}

cmd/completion_test.go

@@ -0,0 +1,105 @@
+package cmd
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/OffchainLabs/prysm/v7/testing/assert"
+	"github.com/OffchainLabs/prysm/v7/testing/require"
+	"github.com/urfave/cli/v2"
+)
+
+func TestCompletionCommand(t *testing.T) {
+	t.Run("creates command with correct name", func(t *testing.T) {
+		cmd := CompletionCommand("beacon-chain")
+		require.Equal(t, "completion", cmd.Name)
+	})
+
+	t.Run("has three subcommands", func(t *testing.T) {
+		cmd := CompletionCommand("beacon-chain")
+		require.Equal(t, 3, len(cmd.Subcommands))
+
+		names := make([]string, len(cmd.Subcommands))
+		for i, sub := range cmd.Subcommands {
+			names[i] = sub.Name
+		}
+		assert.DeepEqual(t, []string{"bash", "zsh", "fish"}, names)
+	})
+
+	t.Run("description contains binary name", func(t *testing.T) {
+		cmd := CompletionCommand("validator")
+		assert.Equal(t, true, strings.Contains(cmd.Description, "validator"))
+	})
+}
+
+func TestBashCompletionScript(t *testing.T) {
+	script := bashCompletionScript("beacon-chain")
+
+	assert.Equal(t, true, strings.Contains(script, "beacon-chain"), "script should contain binary name")
+	assert.Equal(t, true, strings.Contains(script, "_beacon_chain_completions"), "script should contain function name with underscores")
+	assert.Equal(t, true, strings.Contains(script, "complete -o bashdefault"), "script should contain complete command")
+	assert.Equal(t, true, strings.Contains(script, "--generate-bash-completion"), "script should use generate-bash-completion flag")
+}
+
+func TestZshCompletionScript(t *testing.T) {
+	script := zshCompletionScript("validator")
+
+	assert.Equal(t, true, strings.Contains(script, "#compdef validator"), "script should contain compdef directive")
+	assert.Equal(t, true, strings.Contains(script, "_validator"), "script should contain function name")
+	assert.Equal(t, true, strings.Contains(script, "--generate-bash-completion"), "script should use generate-bash-completion flag")
+}
+
+func TestFishCompletionScript(t *testing.T) {
+	script := fishCompletionScript("beacon-chain")
+
+	assert.Equal(t, true, strings.Contains(script, "complete -c beacon-chain"), "script should contain complete command")
+	assert.Equal(t, true, strings.Contains(script, "__fish_beacon_chain_complete"), "script should contain function name with underscores")
+	assert.Equal(t, true, strings.Contains(script, "--generate-bash-completion"), "script should use generate-bash-completion flag")
+}
+
+func TestScriptFunctionNames(t *testing.T) {
+	// Test that hyphens are converted to underscores in function names
+	bashScript := bashCompletionScript("beacon-chain")
+	assert.Equal(t, true, strings.Contains(bashScript, "_beacon_chain_completions"))
+	assert.Equal(t, false, strings.Contains(bashScript, "_beacon-chain_completions"))
+
+	zshScript := zshCompletionScript("beacon-chain")
+	assert.Equal(t, true, strings.Contains(zshScript, "_beacon_chain"))
+
+	fishScript := fishCompletionScript("beacon-chain")
+	assert.Equal(t, true, strings.Contains(fishScript, "__fish_beacon_chain_complete"))
+}
+
+func TestCompletionSubcommandActions(t *testing.T) {
+	// Test that Action functions execute without errors
+	cmd := CompletionCommand("beacon-chain")
+
+	tests := []struct {
+		name       string
+		subcommand string
+	}{
+		{"bash action executes", "bash"},
+		{"zsh action executes", "zsh"},
+		{"fish action executes", "fish"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var subCmd *cli.Command
+			for _, sub := range cmd.Subcommands {
+				if sub.Name == tt.subcommand {
+					subCmd = sub
+					break
+				}
+			}
+			require.NotNil(t, subCmd, "subcommand should exist")
+			require.NotNil(t, subCmd.Action, "subcommand should have an action")
+
+			// Action should not return an error; use a real cli.Context
+			app := &cli.App{}
+			ctx := cli.NewContext(app, nil, nil)
+			err := subCmd.Action(ctx)
+			require.NoError(t, err)
+		})
+	}
+}

cmd/validator/main.go

@@ -140,8 +140,10 @@ func main() {
 			slashingprotectioncommands.Commands,
 			dbcommands.Commands,
 			web.Commands,
+			cmd.CompletionCommand("validator"),
 		},
-		Flags: appFlags,
+		Flags:                appFlags,
+		EnableBashCompletion: true,
 		Before: func(ctx *cli.Context) error {
 			// Load flags from config file, if specified.
 			if err := cmd.LoadFlagsFromConfig(ctx, appFlags); err != nil {