stsh.rb

#!/usr/bin/env ruby                                

require 'readline'
require 'pp'      

class Stsh
  def initialize
    @prompt = "-$ "
    @aliases = {}  
    @running = false
    set_traps       
  end               

  def set_traps
    Signal.trap("INT") { print "\n#{@prompt}"; "DEFAULT" }
  end                                                     

  def readcmds
    @running = true
    while @running && line = Readline.readline(@prompt, true)
      runcmd line                                            
    end                                                      
    puts                                                     
  end                                                        

  def runcmd(s, a = false)
    cmdline = s.split     
    cmd = cmdline[0]
    if @aliases[cmd] != nil && a == false
      runcmd(@aliases[cmd], true)
    else
      case cmd
      when 'cd' then Dir.chdir(File.expand_path(cmdline[1]))
      when 'alias' then add_alias(cmdline[1..-1])
      when 'paliases' then pp @aliases
      when 'exit' then @running = false
      when '!' then puts(eval(cmdline[1..-1].join(' ')))
      when 'logout' then puts "This had better not be a login shell! Try exit."
      when 'help' then print_help
      else
        pid = fork
        if pid == nil
          # Child
          exec s
        else
          Signal.trap("INT") { puts; "DEFAULT" }
          Process.wait
          set_traps
        end
      end
    end
  end

  def add_alias(a)
    @aliases[a[0]] = a[1..-1].join(' ')
  end

  def print_help
    puts <<EOHELP
Stupid Shell:
cd - change directory
alias name body - add an alias
paliases - print defined aliases
exit - exit the shell
! - evaluate the rest of the line as ruby code
help - print this message
EOHELP
  end
end

if __FILE__ == $0
  s = Stsh.new
  s.readcmds
end