IO重定向-交换标准输出和标准错误 [英] IO Redirection - Swapping stdout and stderr

问题描述

给出一个shell脚本:

Given a shell script:

#!/bin/sh

echo "I'm stdout";
echo "I'm stderr" >&2;

有没有一种方法可以调用该脚本，以便当命令的最后一部分为2>/dev/null(即

Is there a way to call that script such that only stderr would print out, when the last part of the command is 2>/dev/null, ie

$ > sh myscript.sh SOME_OPTIONS_HERE 2>/dev/null
I'm stderr

或者，或者:

$ > sh myscript.sh SOME_OPTIONS_HERE >/dev/null
I'm stdout

这是一组幻灯片的结尾处的一个问题，但是经过将近一天的工作之后，我几乎可以肯定这是一种错字.旋转不起作用. 2>&-不起作用.我没主意了！

It's a question at the end of a set of lecture slides, but after nearly a day working at this, I'm nearly certain it's some sort of typo. Pivoting doesn't work. 2>&- doesn't work. I'm out of ideas!

推荐答案

% (sh myscript.sh 3>&2 2>&1 1>&3) 2>/dev/null
I'm stderr
% (sh myscript.sh 3>&2 2>&1 1>&3) >/dev/null 
I'm stdout

3>&2 2>&1 1>&3的解释:

• 3>&2表示制作文件描述符2(fd 2)(stderr)的副本，名为fd 3(文件描述符3).它会复制文件描述符，不会像tee那样复制流.
• 2>&1表示sh myscript.sh的fd 2成为其fd 1(stdout)的副本.现在，当myscript写入stderr(即fd 2)时，我们会在stdout(我们的fd 1)上收到它.
• 1>&3表示sh myscript.sh的fd 1成为fd 3(stderr)的副本.现在，当myscript将其写入stdout(即fd 1)时，我们会在stderr(我们的fd 2)上收到它.

• 3>&2 means make a copy of file descriptor 2 (fd 2) (stderr), named fd 3 (file descriptor 3). It copies the file descriptor, it doesn't duplicate the stream as tee does.
• 2>&1 means that fd 2 of sh myscript.sh becomes a copy of it's fd 1 (stdout). Now, when myscript writes to it's stderr (it's fd 2), we receive it on stdout (our fd 1).
• 1>&3 means that fd 1 of sh myscript.sh becomes a copy of fd 3 (stderr). Now, when myscript writes to it's stdout (it's fd 1), we receive it on stderr (our fd 2).

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