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shell 编程之2>&1

Jun 07, 2016 pm 03:20 PM
amp shell programming Script

经常可以在一些脚本,尤其是在crontab调用时发现如下形式的命令调用 /tmp/test.sh /tmp/test.log 21 前半部分/tmp/test.sh /tmp/test.log很容易理解,那么后面的21是怎么回事呢? 要解释这个问题,还是得提到文件重定向。我们知道和是文件重定向符。那么1和2

经常可以在一些脚本,尤其是在crontab调用时发现如下形式的命令调用

/tmp/test.sh > /tmp/test.log 2>&1

前半部分/tmp/test.sh > /tmp/test.log很容易理解,那么后面的2>&1是怎么回事呢?

要解释这个问题,还是得提到文件重定向。我们知道>和&1的意思就是将标准错误也输出到标准输出当中。

下面通过一个例子来展示2>&1有什么作用:

$ cat test.sh
t
date

test.sh中包含两个命令,其中t是一个不存在的命令,执行会报错,默认情况下,错误会输出到stderr。date则能正确执行,并且输出时间信息,默认输出到stdout

./test.sh > test1.log
./test.sh: line 1: t: command not found

$ cat test1.log
Tue Oct 9 20:51:50 CST 2007

可以看到,date的执行结果被重定向到log文件中了,而t无法执行的错误则只打印在屏幕上。

$ ./test.sh > test2.log 2>&1

$ cat test2.log
./test.sh: line 1: t: command not found
Tue Oct 9 20:53:44 CST 2007

这次,stderr和stdout的内容都被重定向到log文件中了。

实际上, > 就相当于 1> 也就是重定向标准输出,不包括标准错误。通过2>&1,就将标准错误重定向到标准输出了,那么再使用>重定向就会将标准输出和标准错误信息一同重定向了。如果只想重定向标准错误到文件中,则可以使用2> file

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