cat（concatenate）拼接文件并将其输出标准输出。
SYNOPSIS
cat [OPTION]... [FILE]...
DESCRIPTION
Concatenate FILE(s) to standard output.
With no FILE, or when FILE is -, read standard input.
-A, --show-all
equivalent to -vET
-b, --number-nonblank
number nonempty output lines, overrides -n
-e equivalent to -vE
-E, --show-ends
display $ at end of each line
-n, --number
number all output lines
-s, --squeeze-blank
suppress repeated empty output lines
-t equivalent to -vT
-T, --show-tabs
display TAB characters as ^I
-u (ignored)
-v, --show-nonprinting
use ^ and M- notation, except for LFD and TAB
--help display this help and exit
--version
output version information and exit
例子：
- 查看文件内容
cat apple.txt - 查看文件内容，从1开始行（含空格）编号
cat -n apple.txt - 查看文件内容，从1开始行（不含空格）编号
cat -b apple.txt - 清空文件内容，
cat /dev/null > apple.txt - 结尾显示$(可以用来查看末尾是否含有空格)
cat -E apple.txt - 合并两个文件内容到另一个文档，
cat a.txt b.txt >> ab.txt
PS：终端中输入cat>文件，接下来终端的输入都会被写到文件中，直到用户按下ctrl+d
