llinux shell:http://www.92csz.com/study/linux/
awk:http://www.cnblogs.com/ggjucheng/archive/2013/01/13/2858470.html
shell:http://zhizhuofl.blog.163.com/blog/static/69914779201001910641196/
shell和makefile:http://blog.csdn.net/absurd/article/details/636418
type help to get internal command