问题
How to pipe stderr, and not stdout? perfecly captures my problem, and the first answer is exactly how I initially tried to solve it:
(echo stdout; echo 1>&2 stderr) 2>&1 >/dev/null | less
(The subshell command with echos is a minimal placeholder thanks to user1934428 to demonstrate the issue; my actual problem has a more useful command, but one that everyone else can't run.)
However, it doesn't work: It's showing both stdout and stderr together. If I remove the pipe, it works as expected, only showing stderr. Eventually I realized this might be a shell thing, and tried bash: It worked perfectly there. I'm using Zsh: what about Zsh makes this fail?
回答1:
This is due to MULTIOS, which duplicates the output streams when you supply multiple redirections. Pipes are implicit redirections.
Type ls >/dev/null | less in your zsh and you will see output even though you redirected it. That's multios duplicating the stream.
unsetopt multios will provide the desired behaviour. This is quite a handy feature if I wouldn't always forget about it until it irritates me.
http://zsh.sourceforge.net/Doc/Release/Redirection.html#Multios
回答2:
Both behave the same for me, in that they only show stderr.
For testing, I created a command which outputs to stdout and stderr, and works the same in Zsh and bash:
(echo stdout; echo 1>&2 stderr)
prints
stdout
stderr
Now I do your redirection:
(echo stdout; echo 1>&2 stderr) 2>&1 >/dev/null
Here, I see on bash and zsh
stderr
but of course written to standard output, because 2>&1 first redirects the stderr of the command to what is at the moment standard output, and then discards the standard output of the command.
来源：https://stackoverflow.com/questions/58019928/pipe-stderr-and-not-stdout-not-working-in-zsh