Linux学习教程-获取可用命令行参数 or 文件上下翻转和左右翻转

终端运行man ls可以查看ls所有可用的参数，上下箭头翻页，按q退出查看。(man: manual, 手册)

ct@ehbio:~/data$ man ls
NAME
 ls - list directory contents
SYNOPSIS
 ls [OPTION]... [FILE]...

DESCRIPTION
 List  information  about  the  FILEs  (the current directory by default).
 Sort entries alphabetically if none of -cftuvSUX nor --sort is specified.

 Mandatory arguments to long options are mandatory for short options too.

 -a, --all
  do not ignore entries starting with .

 -A, --almost-all
  do not list implied . and ..

 --author
  with -l, print the author of each file

 -b, --escape
  print C-style escapes for nongraphic characters
 ....

两个有意思的命令，tac: 文件翻转，第一行变为最后一行，第二行变为倒数第二行；rev每列反转，第一个字符变为最后一个字符，第二个字符变为倒数第二个字符。

ct@ehbio:~/data$ cat <  first
> second
> third
> END
third
second
first
ct@ehbio:~/data$ cat <  abcde
> xyz
> END
edcba
zyx