From: Peter Stephenson <p.stephenson@samsung.com>
To: zsh-workers@zsh.org
Cc: Pierre Neidhardt <ambrevar@gmail.com>
Subject: Re: Redirection bug for stderr?
Date: Thu, 20 Nov 2014 16:08:16 +0000 [thread overview]
Message-ID: <20141120160816.0d259011@pwslap01u.europe.root.pri> (raw)
In-Reply-To: <20141120104440.GA23262@gmail.com>
On Thu, 20 Nov 2014 11:44:40 +0100
Pierre Neidhardt <ambrevar@gmail.com> wrote:
> I recently noticed a somewhat unexpected behaviour with a configuration-less zsh 5.0.7:
>
> $ ls /root >/dev/null 2>&1
> $ ls /root 2>/dev/null | cat
> $ ls /root >/dev/null 2>&1 | cat
> ls: cannot open directory /root: Permission denied
>
> I believe last error line should not appear.
I suspect you're hitting this.
3.26: Why is my output duplicated with `foo 2>&1 >foo.out | bar'?
This is a slightly unexpected effect of the option MULTIOS, which is
set by default. Let's look more closely:
foo 2>&1 >foo.out | bar
What you're probably expecting is that the command `foo' sends its
standard output to the pipe and so to the input of the command `bar',
while it sends its standard error to the file `foo.out'. What you
actually see is that the output is going both to the pipe and into the
file. To be more explicit, here's the same example with real commands:
% { print output; print error >&2 } 2>&1 >foo.out | sed 's/error/erratic'
erratic
output
% cat foo.out
output
and you can see `output' appears twice.
It becomes clearer what's going on if we write:
% print output >foo1.out >foo2.out
% cat foo1.out
output
% cat foo2.out
output
You might recognise this as a standard feature of zsh, called `multios'
and controlled by the option of the same name, whereby output is copied
to both files when the redirector appears twice. What's going on in the
first example is exactly the same, however the second redirector is
disguised as a pipe. So if you want to turn this effect off, you need
to unset the option `MULTIOS'.
next prev parent reply other threads:[~2014-11-20 16:18 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-11-20 10:44 Pierre Neidhardt
2014-11-20 16:08 ` Peter Stephenson [this message]
2014-11-20 16:58 ` Stephane Chazelas
2014-11-20 20:28 ` Pierre Neidhardt
2014-11-21 11:29 ` [PATCH][DOC] " Stephane Chazelas
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