A Unix Shell In Ruby

shirt

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

def main
  loop do
    $stdout.print ENV['PROMPT']
    line = $stdin.gets

    if line
      line.strip!
    else
      # in the case, EOF was received on $stdin
      exit
    end

    command, *arguments = Shellwords.shellsplit(line)

    if BUILTINS[command]
      BUILTINS[command].call(*arguments)
    else
      pid = fork {
        exec command, *arguments
      }

      Process.wait pid
    end
  end
end

def main2
  loop do
    $stdout.print ENV['PROMPT']
    line = $stdin.gets

    commands = split_on_pipes(line)

    placeholder_in = $stdin
    placeholder_out = $stdout
    pipe = []

    commands.each_with_index do |command, index|
      program, *arguments = Shellwords.shellsplit(command)

      if builtin?(program)
        call_builtin(program, *arguments)
      else
        if index+1 < commands.size
          pipe = IO.pipe
          placeholder_out = pipe.last
        else
          placeholder_out = $stdout
        end

        spawn_program(program, *arguments, placeholder_out, placeholder_in)

        placeholder_out.close unless placeholder_out == $stdout
        placeholder_in.close unless placeholder_in == $stdin
        placeholder_in = pipe.first
      end
    end

    Process.waitall
  end
end

def split_on_pipes(line)
  line.scan( /([^"'|]+)|["']([^"']*)["']/ ).flatten.compact
end

def spawn_program(program, *arguments, placeholder_out, placeholder_in)
  fork {
    unless placeholder_out == $stdout
      $stdout.reopen(placeholder_out)
      placeholdder_out.close
    end

    unless placeholder_in == $stdin
      $stdin.reopen(placeholder_in)
      placeholder_in.close
    end

    exec program, *arguments
  }
end

BUILTINS = {
  'cd'    => lambda { |dir = ENV['HOME']| Dir.chdir(dir) },
  'exit'  => lambda { |code = 0| exit(code.to_i) },
  'exec'  => lambda { |*command| exec *command },
  'set'   => lambda { |args|
    key, value = args.split('=')
    ENV[key] = value
  }
}

ENV['PROMPT'] = '> '

main