stdout bug? (redirection not honored)

stdout bug? (redirection not honored)

[J.D. Laub]

Is this a bug?
% uname -s -r -v -m -p -i -o
Linux 2.4.21-32.0.1.ELsmp #1 SMP Tue May 17 17:52:23 EDT 2005 i686 i686 i386 GNU/Linux
% echo $ZSH_VERSION > /dev/null | cat # should print nothing, but...
4.3.2

The problem becomes an issue when you have a command that generates
something on both stdout and stderr, and you want to dump the stdout
and page through the stderr:

% foo 2>&1 > /dev/null | more

I'm hoping someone says this can be addressed by setting an
option.  :-)  I was also able to duplicate it on hpux:
% uname -s -r -v -m -i -l
HP-UX B.11.11 U 9000/800 139414660 unlimited-user license

-- 
J.D. Laub (Laubster) |"Your leg's too long / Your skull's too strong /
zsh-workers@laubster.org| Suppose your nose is wrong." - Renaldo & the Loaf

Re: stdout bug? (redirection not honored)

[Peter Stephenson]

"J.D. Laub" wrote:
> Is this a bug?

No, see below.

> % uname -s -r -v -m -p -i -o
> Linux 2.4.21-32.0.1.ELsmp #1 SMP Tue May 17 17:52:23 EDT 2005 i686 i686 i386 
> GNU/Linux
> % echo $ZSH_VERSION > /dev/null | cat # should print nothing, but...
> 4.3.2
> 
> The problem becomes an issue when you have a command that generates
> something on both stdout and stderr, and you want to dump the stdout
> and page through the stderr:
> 
> % foo 2>&1 > /dev/null | more
> 
> I'm hoping someone says this can be addressed by setting an
> option.  :-)

Yes, indeed.

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'.


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Re: stdout bug? (redirection not honored)

[Danek Duvall]

On Wed, Nov 01, 2006 at 11:41:28AM -0700, J.D. Laub wrote:

> The problem becomes an issue when you have a command that generates
> something on both stdout and stderr, and you want to dump the stdout
> and page through the stderr:
> 
> % foo 2>&1 > /dev/null | more

See Peter's response re: multios, but note that this command doesn't match
what you described.  It copies stderr to stdout, and then sends it to two
places with multios.  Instead, you probably want

    % foo 2> /dev/null | more

which merely sends stderr to /dev/null and pipes stdout to more.

Danek

Re: stdout bug? (redirection not honored)

[J.D. Laub]

On Wed, Nov 01, 2006 at 06:46:07PM +0000, Peter Stephenson wrote:
>   So if you want to turn this effect off, you need
>   to unset the option `MULTIOS'.

What am I missing with the following?

% unsetopt | grep multios
nomultios
% print $ZSH_VERSION >foo1.out >foo2.out
% cat foo1.out
4.3.2
% cat foo2.out
4.3.2

> 3.26: Why is my output duplicated with `foo 2>&1 >foo.out | bar'?

I had searched for "stdout" and "redirect" in the faq index.  I need
to train myself to troll through all entries if a search doesn't
find what I'm after - sorry for the noise.

-- 
J.D. Laub (Laubster) |"Your leg's too long / Your skull's too strong /
zsh-workers@laubster.org| Suppose your nose is wrong." - Renaldo & the Loaf

Re: stdout bug? (redirection not honored)

[Wayne Davison]

On Wed, Nov 01, 2006 at 03:15:35PM -0700, J.D. Laub wrote:
> % unsetopt | grep multios
> nomultios

That means that multios is set because it indicates that nomultios
is unset.

..wayne..