Redirect ：

Redirection operators can redirect command input and output data streams from the default device to other locations . The redirection operator itself is not a command , It is a special symbol attached to the command that can change the input and output objects of the command .

​ Output redirection operator ： > 、 >>

​ Enter the redirection operator ： < 、 <<

​ Input redirection ： It refers to reassigning the device to replace the keyboard as a new input device ;
​ Output redirection ： It refers to reassigning the device to replace the display as a new output device .
It is usually the result of the execution of a file or command instead of the keyboard as a new input device , A new output device usually refers to a file .

1、Linux Input redirection

For input redirection , The symbols and functions required are shown in the following table ：

for example ：
By default ,cat The command will accept standard input devices （ keyboard ） The input of , And display to the console , But if you use a file instead of a keyboard as an input device , Then the command will take the specified file as the input device , And read and display the contents of the file to the console .

[[email protected] ~]# cat < /etc/passwd
1

The output follows [[email protected] ~]# cat /etc/passwd equally , Only this part of the code is based on /etc/passwd File as input device .
Another example is ：

[[email protected] ~]# cat << 0
//  Here's the input （ belt > Number ）
>c.biancheng.net
>Linux
>0
//  The console prints out the information just entered 
c.biancheng.net
Linux.

As can be seen from the above code , When specifying the 0 As a boundary , As long as you don't enter 0, You can input data all the time .
But on the whole , Input redirection is rarely used .

2、Linux Output redirection

The frequency of output redirection is higher , Unlike input redirection , Output redirection can also be subdivided into standard output redirection and error output redirection .
The symbols and functions used for output redirection are as follows ：

$ ls -l > lsoutput.txt
1

This order is very important ls The output of the command is saved to a file lsoutput.txt in .

$ ps >> lsoutput.txt
1

This order is very important ps The output of the command is appended to lsoutput.txt End of file .

for example

>

[[email protected] ~]# echo 123 > file
[[email protected] ~]# more file
123

>>

[[email protected] ~]# echo 456 >> file
[[email protected] ~]# more file 
123
456

2>

[[email protected] ~]# 123 2> file
[[email protected] ~]# more file 
bash: 123: command not found...

2>>

[[email protected] ~]# 456 2> file
[[email protected] ~]# 123 2>> file
[[email protected] ~]# more file
bash: 456: command not found...
bash: 123: command not found...

&>

[[email protected] ~]# echo 123 &> fule
[[email protected] ~]# more fule 
123
[[email protected] ~]# 123 &> fule 
[[email protected] ~]# more fule
bash: 123: command not found...

>&

[[email protected] ~]# 123 >& fule 
[[email protected] ~]# echo 123 >& fule

take 1 Redirect to file , take 2 Redirect to 1

[[email protected] ~]# echo 123 1> file 2>&1
[[email protected] ~]# more file 
123

take 2 Redirect to file , take 1 Redirect to 2

[[email protected] ~]# 123 >& fule 
[[email protected] ~]# echo 123 >& fule
bash: 123: command not found...

Redirect files to 0

bash: 123: command not found...

cat Read content from standard input , Then output to a file

[[email protected] ~]# cat < file &>> file
[[email protected] ~]# more file
bash: 123: command not found...
cat: -: input file is output file

[[email protected] ~]# cat < file &>> fule
[[email protected] ~]# more fule
123
bash: 123: command not found...
cat: -: input file is output file
cat: -: input file is output file

expand

Complex usage of redirection using file descriptors

Redirection using file descriptors uses & Symbol ：

cmd >&n Send the output to the file descriptor n

cmd m>&n Output to file symbol m To the file descriptor ncmd >&- Turn off standard output

cmd <&n Input from file descriptor n

cmd m<&n m From the file descriptor n

cmd <&- Turn off standard input

cmd <&n- Move the input file descriptor n Instead of copying it

cmd >&n- Move the output file descriptor n Instead of copying it