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Re: [Help-bash] Redirecting output to stderr

From: Bob Proulx
Subject: Re: [Help-bash] Redirecting output to stderr
Date: Sat, 15 Nov 2014 17:32:34 -0700
User-agent: Mutt/1.5.23 (2014-03-12)

Marlen Caemmerer wrote:
> I tried to output a echo to stderr.

Like this?

  echo foo 1>&2

> Unfortunatelly it does not go to stderr but to stdout.

Then you didn't actually do what you wanted to do.  :-(

> Only when I use it in a function it works.
> When I put the first statement into a script it works as expected, too.
> It works in zsh though and I guess it worked in 2003 in bash, too.
> Here is my shell output:
> address@hidden:~$
> address@hidden:~$ echo "moo" 1>&2 > /dev/null

That is equivalent to this:

  echo "moo" > /dev/null

That is not what you want.  Think of it this way:

  fd[0] = stdin
  fd[1] = stdout
  fd[2] = stderr

Thie above is how things are always initialized by the shell before
any command is invoked.  Which is why we call it a "redirection" when
we change the value of those file descriptors.  We _redirect_ them.

  fd[1] = fd[2]  # 1>&2  fd[1] now points to the same place as stderr
  fd[1] = /dev/null  # 1>/dev/null  fd[1] now points to /dev/null
  ...echo moo to fd[1] pointing to /dev/null...

> address@hidden:~$ echo "moo" 1>&2
> moo

Yes.  That works.  That redirects the stdout of the command to stderr.

  fd[1] = fd[2]  # 1>&2  fd[1] now points to the same place as stderr
  ...echo moo to fd[1] pointing to stderr...

> address@hidden:~$ echoerr() { echo "$@" 1>&2; }; echoerr "moo" > /dev/null
> moo

  fd[1] = /dev/null  # >/dev/null echoerror...
  echoerr_fd[1] = fd[2]  # 1>&2 redirects echoerror's fd[1] to stderr
  ...echo boo to echoerr_fd[1] pointing to stderr...

> But I have no idea why it is so and if this is a bug.

It is working as expected.  There is no bug there.


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