cat VS tac
cat是查看文本文件的内容，tac是cat反过来，反向查看文件
$cat 1.txt
ls: cannot access ee: No such file or directory
1line
2line
3line
4line$tac 1.txt
4line
3line
2line
1line
ls: cannot access ee: No such file or directory
more
查看，扩展翻页,回车下一行，空格下一页，q退出
$man ls | more
。。。。。。。。。。。--More--
