<, < <,>, > > Introduction in shell

One 、 File descriptor

Linux After starting , It will open by default 3 File descriptors , Namely ： The standard input standard input 0; Output correctly standard output 1; Error output ：error output 2

Linux Shell The next common input / output operator is ：

1. The standard input (stdin) ： The code is 0 , Use < or << ; /dev/stdin -> /proc/self/fd/0 0 representative ：/dev/stdin
2. standard output (stdout)： The code is 1 , Use > or >> ; /dev/stdout -> /proc/self/fd/1 1 representative ：/dev/stdout
3. Standard error output (stderr)： The code is 2 , Use 2> or 2>> ; /dev/stderr -> /proc/self/fd/2 2 representative ：/dev/stderr

Two 、> ,>>

Output redirection ： Redirect output to screen or device or file ; among , >> Means to append new information to the file

Turn off correct output and error output ：ls test.txt test.txt1 1>&- 2>&-
& Represents all outputs ：ls t.txt t.txt1 &>te.txt
&[-,1,2] respectively Close the corresponding identification , Output correctly , Error output

3、 ... and 、<,<<

Input redirection

From file test.txt Get input to cat, then cat Enter redirect to text1.txt：cat > text1.txt < test.txt

<< It means to accept the identification of the end of an input , When you enter this ID, end the input ：

cat > t.txt << eof
> q
> a
> z
> eof
#  When the console enters eof after , End input 

Four 、exec Binding redirection

Bind a new file descriptor 6, And bind standard output （ It is equivalent to saving a temporary copy）

ls /proc/self/fd
0  1  2  3
exec 6>&1
ls /proc/self/fd
0  1  2  3  6

# Bind the standard output back ：
exec 1>&6
ls /proc/self/fd
0  1  2  3  6
# Close the custom file descriptor 
exec 6>&-
ls /proc/self/fd
0  1  2  3

️ Be careful ： If you bind standard output directly to a file , So the problem is , Never come back