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.
原文：http://www.cnblogs.com/listened/p/3929349.html