[frontend] Add --help=page option for paginated help output (#20961)

Signed-off-by: reidliu41 <reid201711@gmail.com>

docs/cli/README.md

@@ -37,6 +37,9 @@ Start the vLLM OpenAI Compatible API server.
 
     # To search by keyword
     vllm serve --help=max
+
+    # To view full help with pager (less/more)
+    vllm serve --help=page
     ```
 
 ## chat

vllm/entrypoints/utils.py

@@ -5,6 +5,7 @@ import argparse
 import asyncio
 import functools
 import os
+import subprocess
 import sys
 from typing import Any, Optional, Union
 
@@ -25,7 +26,8 @@ VLLM_SUBCMD_PARSER_EPILOG = (
     "   - To view a argument group:     --help=ModelConfig\n"
     "   - To view a single argument:    --help=max-num-seqs\n"
     "   - To search by keyword:         --help=max\n"
-    "   - To list all groups:           --help=listgroup")
+    "   - To list all groups:           --help=listgroup\n"
+    "   - To view help with pager:      --help=page")
 
 
 async def listen_for_disconnect(request: Request) -> None:
@@ -190,6 +192,24 @@ def _validate_truncation_size(
     return truncate_prompt_tokens
 
 
+def _output_with_pager(text: str):
+    """Output text using scrolling view if available and appropriate."""
+
+    pagers = ['less -R', 'more']
+    for pager_cmd in pagers:
+        try:
+            proc = subprocess.Popen(pager_cmd.split(),
+                                    stdin=subprocess.PIPE,
+                                    text=True)
+            proc.communicate(input=text)
+            return
+        except (subprocess.SubprocessError, OSError, FileNotFoundError):
+            continue
+
+    # No pager worked, fall back to normal print
+    print(text)
+
+
 def show_filtered_argument_or_group_from_help(parser: argparse.ArgumentParser,
                                               subcommand_name: list[str]):
 
@@ -208,16 +228,24 @@ def show_filtered_argument_or_group_from_help(parser: argparse.ArgumentParser,
         if arg.startswith('--help='):
             search_keyword = arg.split('=', 1)[1]
 
+            # Enable paged view for full help
+            if search_keyword == 'page':
+                help_text = parser.format_help()
+                _output_with_pager(help_text)
+                sys.exit(0)
+
             # List available groups
             if search_keyword == 'listgroup':
-                print("\nAvailable argument groups:")
+                output_lines = ["\nAvailable argument groups:"]
                 for group in parser._action_groups:
                     if group.title and not group.title.startswith(
                             "positional arguments"):
-                        print(f"  - {group.title}")
+                        output_lines.append(f"  - {group.title}")
                         if group.description:
-                            print("    " + group.description.strip())
-                        print()
+                            output_lines.append("    " +
+                                                group.description.strip())
+                        output_lines.append("")
+                _output_with_pager("\n".join(output_lines))
                 sys.exit(0)
 
             # For group search
@@ -229,7 +257,7 @@ def show_filtered_argument_or_group_from_help(parser: argparse.ArgumentParser,
                     formatter.add_text(group.description)
                     formatter.add_arguments(group._group_actions)
                     formatter.end_section()
-                    print(formatter.format_help())
+                    _output_with_pager(formatter.format_help())
                     sys.exit(0)
 
             # For single arg
@@ -243,10 +271,10 @@ def show_filtered_argument_or_group_from_help(parser: argparse.ArgumentParser,
                         matched_actions.append(action)
 
             if matched_actions:
-                print(f"\nParameters matching '{search_keyword}':\n")
+                header = f"\nParameters matching '{search_keyword}':\n"
                 formatter = parser._get_formatter()
                 formatter.add_arguments(matched_actions)
-                print(formatter.format_help())
+                _output_with_pager(header + formatter.format_help())
                 sys.exit(0)
 
             print(f"\nNo group or parameter matching '{search_keyword}'")