Why do I get a "bad filedescriptor" here? (redirection problem)

Why do I get a "bad filedescriptor" here? (redirection problem)

[Com MN PG P E B Consultant 3]

zsh 4.0.7

$ ls >& -
/bin/ls: write error: Bad file descriptor
$ cat >&- <<<d
cat: standard output: Bad file descriptor
$ ls >/dev/null
$ echo x >& -
$ 

Why do I get a "bad file descriptor" in the first two cases?
Note that closing stdout with echo works fine, as does
(of course) redirection of ls output to the bit bucket.

And in case this matters:

$ type ls
ls is an alias for /bin/ls --almost-all --color --classify
$ type cat
cat is /bin/cat

Ronald
-- 
Ronald Fischer (phone +49-89-63676431)
mailto:mn-pg-p-e-b-consultant-3.com@siemens.com

Re: Why do I get a "bad filedescriptor" here? (redirection problem)

[Phil Pennock]

On 2006-10-25 at 10:52 +0200, Com MN PG P E B Consultant 3 wrote:
> zsh 4.0.7
> 
> $ ls >& -
> /bin/ls: write error: Bad file descriptor
> $ cat >&- <<<d
> cat: standard output: Bad file descriptor
> $ ls >/dev/null
> $ echo x >& -
> $ 
> 
> Why do I get a "bad file descriptor" in the first two cases?

Because you've closed it, so it's not available for writing.  Most tools
just assume that file descriptors 0, 1 and 2 are available for use and
will just write to stdout without checking, so they'll get errors and
complain like that when it's closed.

> Note that closing stdout with echo works fine, as does
> (of course) redirection of ls output to the bit bucket.

echo as a shell built-in is handling it gracefully, because zsh is
handling what you meant, not what you wrote.  Try it with /bin/echo and
you'll see it fail in the same way as the other commands.

If you want to discard output, redirect it to the bitbucket.

Regards,
-Phil

RE: Why do I get a "bad filedescriptor" here? (redirection problem)

[Com MN PG P E B Consultant 3]

> > Why do I get a "bad file descriptor" in the first two cases?
> 
> Because you've closed it, so it's not available for writing.  
> Most tools
> just assume that file descriptors 0, 1 and 2 are available for use and
> will just write to stdout without checking, so they'll get errors and
> complain like that when it's closed.

I see! And I thought that >&- is just a short cut for >/dev/null !!!
Thank you for explaining this.

Ronald
-- 
Ronald Fischer (phone +49-89-63676431)
mailto:mn-pg-p-e-b-consultant-3.com@siemens.com

Re: Why do I get a "bad filedescriptor" here? (redirection problem)

[Phil Pennock]

On 2006-10-25 at 13:05 +0200, Com MN PG P E B Consultant 3 wrote:
> I see! And I thought that >&- is just a short cut for >/dev/null !!!

zshmisc(1), section REDIRECTION, covers all of the options here.

-Phil