sed function detail

  FIRST  YOU MUST UNDSTAND  WHAT IS MEANT OF PATTERN SPACE /HOLD SPACE .THEN TRY SOME PRACTICE TO FIND OUT A WAY THAT IT IS TRULY 

AS SAME AS YOU THINK.    

       d      Delete pattern space.  Start next cycle.

 

       D      Delete  up  to  the first embedded newline in the pattern space.

              Start next cycle, but skip reading from the input  if  there  is

              still data in the pattern space.

 

       h H    Copy/append pattern space to hold space.

 

       g G    Copy/append hold space to pattern space.

 

       x      Exchange the contents of the hold and pattern spaces.

 

       l      List out the current line in a ``visually unambiguous'' form.

 

       n N    Read/append the next line of input into the pattern space.