21篇测试必备的Linux常用命令，每天敲一篇，每次敲三遍，每月一循环，全都可记住！！
https://www.cnblogs.com/poloyy/category/1672457.html
head -n 5 test.txt head -5 test.txt
head -c 100 test.txt
20 test.txt | tail -10
