How to cancel redirection in a shell program

/*************************************** ******************************
* Author: Samson
* Date: 08/18/2012
* Test Platform:
* GNU Linux version 2.6.29.4
* GNU bash, version 4.0.16 (1)-release (i386-redhat-linux-gnu)

**************************************** ****************************/

In the previous example about shell programming, we talked about extending the shell script and redirecting it to a file for viewing and debugging.

Http://blog.csdn.net/yygydjkthh/article/details/7879690. However, if there is another situation: only redirection is performed in some places, but not elsewhere. Let's modify the example in the previous article to meet this requirement:

#! /Bin/bash
Function setlogfile
{
If! [-Z "$1"]; then
Echo "logfilename is not empty! "> Kthh
Exec 3> & 1
Exec 4> & 2
Exec 2> $1
Exec 1> $1
Fi
}

Num1 = $1
Logfile = $2
Execlogfile = $3

Setlogfile $ {execlogfile}
Set-x
If [$ num1-EQ 0]; then
Echo "num1 is 0" >$ {logfile}
Elif [$ num1-GE 0]; then
Echo "num1 is grate 0" >$ {logfile}
Else
Echo "num1 is less 0" >$ {logfile}
Fi
Exec 2> & 4
Exec 1> & 3
If [$ num1-EQ 0]; then
Echo "num1 is 0 again" >$ {logfile}
Fi

In the setlogfile function, exec 3> & 1; Exec 4> & 2 is added. 3, 1, 4, and 2 indicate file descriptors, 1 is the standard output, that is, stdout, 2 is the standard error output, that is, stderr, 1 and 2 are originally output on the display, and 3 and 4 are two file descriptors, it can be understood as two temporary variables that store 1 and 2. 3> & 1 indicates assigning the value of file descriptor 1 to file descriptor 3, it can also be understood that the rule of file descriptor 1 (current standard output) is saved to file descriptor 3, and then the output of 1 is redirected to the file, in the following shellProgramIn, we use 1> & 3, which means that the original standard output rules saved in file descriptor 3 are assigned to file descriptor 1, the original standard output rule is output to the display, rather than output to a file. This is the entire process of redirecting and canceling redirection restoration. The same is true for 2 and 4.

Let's talk about the running result first:
[Root @ UFO shellprogram] #./testexecutelog. SH 0 msglog execlog
+ Exec
+ '['0-EQ 0']'
+ Echo 'num1 is 0 again'

[Root @ UFO shellprogram] # Cat execlog
+ '['0-EQ 0']'
+ Echo 'num1 is 0'
+ Exec

Have you seen the result ??? Didn't you see it ??? Take a closer look, ^ _ ^