Readline

Readline.readline(prompt = nil)

The Readline module provides command-line editing capabilities for interactive programs, similar to bash or IRB. It enables features like arrow key navigation, history, and Emacs/vi editing modes.

Basic Usage

require 'readline'

# Simple prompt
input = Readline.readline("> ")
puts "You entered: #{input}"

Interactive Input

require 'readline'

# Complete line editing
while line = Readline.readline("Enter command: ", true)
  break if line.nil? || line.downcase == 'exit'
  puts "Processing: #{line}"
end

With History

require 'readline'

# Enable history
Readline::HISTORY.push("first command")
Readline::HISTORY.push("second command")

# Read with history (second param true)
line = Readline.readline("> ", true)
puts line
puts "History: #{Readline::HISTORY.to_a}"

Completion

require 'readline'

# Basic completion
words = %w[apple banana cherry date elderberry]
Readline.completion_append_character = " "

completer = lambda do |str|
  words.grep(/^#{Regexp.escape(str)}/)
end

Readline.completion_proc = completer

input = Readline.readline("Fruit: ")

Complex Completion

require 'readline'

# File completion
Readline.completion_proc = lambda do |str|
  Dir.glob("#{str}*").map { |f| f + (File.directory?(f) ? "/" : " ") }
end

Configuration

# Edit mode (Emacs or Vi)
Readline.emacs_editing_mode      # Default
Readline.vi_editing_mode         # Vi-style

# Configure
Readline.basic_word_break_characters = " \t\n\"\\'`><=|&{"
Readline.completion_append_character = " "

Practical CLI Application

#!/usr/bin/env ruby
require 'readline'

COMMANDS = %w[help status start stop restart]

def prompt
  Readline.readline("> ", true) do |line|
    Readline::HISTORY.push(line)
    process_command(line)
  end
end

def process_command(line)
  cmd = line.strip.split.first
  case cmd
  when 'help'
    puts "Available: #{COMMANDS.join(', ')}"
  when 'status'
    puts "Running"
  else
    puts "Unknown command: #{cmd}"
  end
end

prompt until Readline::HISTORY.empty? || false

Readline Extensions

# Using rb-readline for cross-platform
# gem install rb-readline

require 'rbreadline'
# Same API, works on Windows

Return Values

# Returns string, or nil on EOF (Ctrl+D)
line = Readline.readline("> ")
if line
  puts "Got: #{line}"
else
  puts "Goodbye!"
end

The Readline module is essential for building polished interactive command-line applications in Ruby.

See Also

• Date
• Time
• YAML