exec operation file descriptor in linux

Every open file in Linux will be associated with a file descriptor. When needed, we can use the exec command to specify a number greater than 3 as the file

Linux default file descriptor

Every time it is opened A shell will open the default three file descriptors 0, 1, and 2, which represent standard input, standard output, and standard error output respectively.

Exec command associated file descriptor

exec 5>/tmp/a.txt reading method
exec 5exec 5<> ;/tmp/a.txt Read and write mode
exec 5<&-;exec5>&- Close the file descriptor

Application case: Reassociate the standard output in the shell script to record the log to the specified File

#!/bin/bash
exec 4>&1         #用4记录标准输出
exec 1>/tmp/abc   #重定向标准输出
echo "123"
echo "456"
exec 1>&4         #恢复标准输出
exec 4>&-         #关闭4

Run this script, the echo output in the script will be output to the /tmp/abc file
The second line of the script uses 4>&1 to record the file descriptor of the standard output for convenience after execution. To restore standard output, you can also use exec 1>/dev/tty

Note:

1. echo "hello" >&4 to the file descriptor Only use > for input content, not >>. At this time, > will not clear the file.

2. Whether to clear the file needs to be determined when the exec command associates the file descriptor to the file. Use > or >>,
   For example, there is the following script:

   #!/bin/bash
   exec 4>/tmp/abc
   echo "123" >&4
   echo "456" >&4
   exec 4>&-

   The content of the /tmp/abc file after each execution of the script is 123\n456
   If the second line If you change to exec 4>>/tmp/abc, it will be appended

• The clearing operation of the file is performed during the operation of the exec command

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