• p1 two simple commands connected by a pipe
  the parameter of the commands are fixed
• p2 introducing variable to make it somewhat flexible
• p3 introducing arguments to be used as the parameter of the command
• p4 default value for a varible
• p4.1 introducing `read'
  introducing $#
  introducing `if'
• p5 introducing `case' statement
  introducing the backquote
  ``uname'' is used to make the program adaptive and portable.
• show-arg introducing the `for' loop
  introducing the `expr' command
  introducing the `printf' command
• sum-arg show how to use the `expr' to do arithmetic operations
• sum-arg-bc invoke `bc' to do arithmetic operations
• sum-arg-dc invoke `dc' to do arithmetic operations
• sum-stdin introducing `while'
• sum-to using `while' and 'expr'
• lower using `tr' to rename a file from uppercase to lowercase
  try ``./lower *''
• sigma Recursively invoke itself to compute the sum
• sigma in function Recursive invoke a function to compute the sum