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永久重定向
1.脚本中有大量数据需要重定向，那么逐条重定向所有的 echo 语句会很烦琐。
这时可以用 exec 命令，它会告诉 shell 在脚本执行期间重定向某个特定文件描述符：

$ cat test10 
#!/bin/bash 
# redirecting all output to a file 
exec 1>testout 
echo "This is a test of redirecting all output" 
echo "from a script to another file." 
echo "without having to redirect every individual line" 
$ ./test10 
$ cat testout 
This is a test of redirecting all output 
from a script to another file. 
without having to redirect every individual line 
$

2.exec 命令会启动一个新 shell 并将 STDOUT 文件描述符重定向到指定文件。脚本中送往STDOUT 的所有输出都会被重定向。
也可以在脚本执行过程中重定向 STDOUT：

$ cat test11 
#!/bin/bash 
# redirecting output to different locations 
exec 2>testerror 
echo "This is the start of the script" 
echo "now redirecting all output to another location" 
exec 1>testout 
echo "This output should go to the testout file" 
echo "but this should go to the testerror file" >&2 
$
$ ./test11 
This is the start of the script 
now redirecting all output to another location 
$ cat testout 
This output should go to the testout file 
$ cat testerror 
but this should go to the testerror file 
$