A method instance for canceling redirection in a shell script _linux shell

In the previous example of Shell programming, it was mentioned that the shell script was extended and redirected to a file for viewing and debugging http://www.jb51.net/article/62435.htm. However, if there is another situation: redirect only in some places, and no redirects elsewhere. So let's revise the example in the previous article to meet this need:

Copy Code code as follows:

#!/bin/bash
function SetLogFile
{
if! [-Z "$"]; Then
echo "LogFileName is not empty!" >> kthh
EXEC 3>&1
EXEC 4>&2
EXEC 2>> $
EXEC 1>> $
Fi
}

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

SetLogFile ${execlogfile}
Set-x
If [$num 1-eq 0]; Then
echo "NUM1 is 0" >> ${logfile}
elif [$num 1-ge 0]; Then
echo "NUM1 is grate 0" >> ${logfile}
Else
echo "NUM1 is less 0" >> ${logfile}
Fi
EXEC 2>&4
EXEC 1>&3
If [$num 1-eq 0]; Then
echo "NUM1 is 0 again" >> ${logfile}
Fi

The exec 3>&1;exec 4>&2 is added to the function SetLogFile, where 3, 1, 4, 2 represent the file descriptor, 1 is the standard output, or the stdout,2 is the standard error output, that is, the stderr, 1 and 2 were originally output on the display, while 3 and 4 are two file descriptors, which can be understood as two and 1 of temporary variables, 3>&1 represents the assignment of the file descriptor 2 to the file descriptor 1, It is also understood that the rule of the file Descriptor 1 (current standard output) is saved to the file descriptor 3, then the output of 1 is redirected to the file, and in the following shell program we use the 1>& 3 This means that the rules of the original standard output saved in the file Descriptor 3 are assigned to the file descriptor 1, and the original standard output rule is to output to the display instead of to a file. This is the process of redirecting and canceling the redirection restore. 2 and 4 are the same.

Gossip less, first look at the results of the operation:

Copy Code code as follows:

[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

See the results??? Don't you see??? Then look carefully, ^_^