cat命令:
查看文件内容                                      
-n 显示行号                                     
-b 忽略空行,显示非空行行号                     
-E 在每一行末尾显示$符号                        
-s 如果有多个连续空行,将多个空行压缩至一行显示 
-A 显示一些不可见的内容                         
cat >>a.txt<<EOF 多行追加                       
111                                             
222                                             
EOF结束                                    
cat /dev/null > file.txt 清空文件               
echo > file.txt                                 
> file.txt
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