implementation of some shell cmds and features;

including:

• cmds:
  • pwd
  • ls
  • cd
  • cat
  • env
  • export
  • unset
• features:
  • \t support redundant blank(\t, spaces)
  • " ' support quote
  • \ multi-line input
  • | pipe
  • < > >> redirect
  • ; multi-cmd
  • & background
  • $ support varible: echo ".. $VAR "