commons:

　　all of them could use regular-expression to match the result.

differences:

　　1)grep:

　　　　search for files with patterns.

　　2)awk:

　　　　anylyst for 'fields'; add addition message.

　　3)sed:

　　　　focus on lines of a file,including 'append,insert,delete',or even a range of lines.