Introduction to Shell I/O redirection and the use of exec

Note: The example of this article comes from the collection and collation of the network, with my comments attached. Common uses of ①i/o redirection

Symbolic             meaning
cmd1 | cmd2     pipe character, the standard output of CMD1 as the standard input for CMD2.
> FileName      writes the standard output to the file filename.
< filename      reads the contents of the file filename into the standard input.
>>filename      writes the standard output to the file filename, and if the filename file already exists, appends the standard output to the existing content of filename.
>|filename      Even if the noclobber option is turned on, the standard output is still forced to be written to the file filename, overwriting the filename file.

n>|filename     Use FD to N to open the file filename in truncate mode even if the Noclobber option is turned on.
n> filename     uses FD as n to open the file filename in truncate mode.
n< filename     uses FD as n to open the file filename in read mode.
n>>filename     Opens the file filename using FD for n in append mode.
<< delimiter    here Document (Here-document).

Introduction to the ②exec command

1. Open or close the file by file descriptor.
2, redirect the file to standard input, and redirect the standard output to a file. The
3,exec command resets the I/O redirection, to revert to the original I/O point, and to use the EXEC command to display the specified.
exec usage:
&n  : Represents a file representing a descriptor.
> <: Represents the form in which descriptors are used.
EXEC 8<&2: Descriptor 8 Opens the file corresponding to the standard error in read mode.
exec &>log: Open the file log with the standard input error.
EXEC 8<&-: Close descriptor 8.

1): run.sh script uses exec to redirect stdin (standard input) to a file

root@37c:~# cat run.sh
#!/bin/bash
exec 8<&0         # Standard input is pointing to the keyboard by default, copying one to 8, and 8 pointing to the keyboard.
exec 0< hfile     # 1. Open the file hfile using standard input. Read
a            # 2. Read reads the command from the stdin.
Read B                
echo "---------------------------"
echo $a
echo $b echo
"Close FD 8:"      
exec 0< &8 8<&-              #将FD-8 Copy to standard input (restores to the keyboard) otherwise, the standard input is still point to hfile.
echo-n "Pls. Enter Data:" #这里, the standard input is re-pointing to the keyboard.
read C                      #需要我们手工键入字符.
echo $c
exit 0

Result output:

# hfile The contents of the file.
root@37c:~# cat hfile 
value1
value2

root@37c:~#./run.sh 
---------------------------
value1
value2
Close FD 8:
Pls. Enter data:value3
value3
root@37c:~# 

2): run.sh script redirects stdout to file

root@37c:~# cat run.sh
#!/bin/bash
exec 8>&1              # Opens the FD-8 and makes it point to the monitor.
exec 1> Log            # points the standard output to the log file
echo '------Redirect to the log file-------'
# This part of the output will be stored in the log.
echo "Output of date command:"
date                   
echo "Output of DF command:"
df
sleep 5s   # sleep 5 seconds first to differentiate between two The resulting output.
exec 1>&8 8>&-
echo "------termincal display:-------"
echo "Output of Date command"
date
echo "Output of DF Command"
DF

Result output:

root@37c:~#./run.sh------termincal Display:-------Output of date command September 20, 2016 Tuesday 16:02:08 CST Output of DF C     Ommand file system 1k-block used available% mount point udev 355032 0 355032 0%/dev Tmpfs 75824    3020 72804 4%/run/dev/sda1 17673004 6600160 10152064 40%/TMPFS 379108 132 378976 1%/dev/shm tmpfs 5120 4 5116 1%/run/lock tmpfs 379108 0 379108 0%/sys/f S/cgroup tmpfs 75824 75776 1%/run/user/1000 root@37c:~# Cat log------Redirect to log file----- --Output of date command:2016 September 20 Tuesday 16:02:03 CST Output of DF command: File system 1k-block used available% mount point UD EV 355032 0 355032 0%/dev tmpfs 75824 3020 72804 4%/run/dev/sda1 17673 004 6600160 10152064 40%/Tmpfs 379108 the 378976 1%/dev/shm TMPFS 5120 4 5 1%/run/lock TmpfS 379108 0 379108 0%/sys/fs/cgroup tmpfs 75824-75776 1%/run/user/1000 ro  ot@37c:~#

3): &>file redirect stdout and stderr to file

    #!/bin/bash  
    exec 8>&1 9>&2      #FD 1 copy to FD 8,fd 2 copy to FD 9  
    exec &> log         #&> Symbols redirect stdout and stderr to file log  

    ls z*               #错误写入文件log  
    date                #输出写入文件log  
    exec 1>&8 2>&9 8< &-9<&-      #恢复关闭操作  
    echo "-----------------"  
    echo "Close FD 8 and 9:"  
    ls z*  
    date  

4): General examples

_1): Use file descriptor 3 to open the file in read mode run.py

root@37c:~# exec 3<run.py 
root@37c:~# cat <&3
#!/usr/bin/python3
#-*-Coding:utf-8  -* -
Import Logging
logging.basicconfig (level=logging.info)

s = ' 0 '
n = Int (s)
logging.info (' n =%d '% n)
print (10/n)

_2): Use file descriptor 4 to open files in truncated mode log

root@37c:~# exec 4>log
root@37c:~# echo "I think befor i am" 1>&4
root@37c:~# cat log
 I think befor I am

_3): Use file descriptor 5 to open file in Append mode log

root@37c:~# exec 5>>log
# here echo uses ' > ' rather than ' >> ' to append.
root@37c:~# Echo ' say again:i think before I am! ' >&5
root@37c:~# cat log
 I think befor I am
say Again:i Think before I am!

_4): After exec performs the observation.

# FD 8,9 also points to the file that FD 0, 1 points to.
root@37c:~# exec 9>&1
root@37c:~# exec 8<&0
root@37c:~# ls-l/proc/self/fd/ 
Total usage 0
lrwx------1 root root 64 September  21:03 0-/DEV/PTS/2
lrwx------1 root root 64 September  21:03 1-/dev/pt S/2
lrwx------1 root root 64 September  21:03 2/dev/pts/2
lr-x------1 root root 64 September  21:03 3-&G T /PROC/20455/FD
lrwx------1 root root 64 September  21:03 8/dev/pts/2
lrwx------1 root root 64 September  2 1 21:03 9-/DEV/PTS/2

③ try a question

# Try the results of the operation.
root@37c:~# exec 0<run.sh