Redirection of STDERR

Redirection of STDERR

[Christoph Pleger]

Hello,

In my initialization scripts for hotplug (written for bash) the
following command is used to redirect output which normally goes to
stderr to the system logger:

"exec 2> >(logger -t $0[$$])"

With kernel 2.4 this command works fine, but with kernel version 2.6.3
it leads to a system hang.

Can anybody help me to solve that problem?

Thanks in advance
  Christoph 

Re: Redirection of STDERR

[Stephen Samuel]

Christoph Pleger wrote:
> Hello,
> 
> In my initialization scripts for hotplug (written for bash) the
> following command is used to redirect output which normally goes to
> stderr to the system logger:
> 
> "exec 2> >(logger -t $0[$$])"
I don't remember this syntax as legal.
what I'd use would be:
exec some_command 2>&1  | logger -t "$0[$$]"


On the other hand, this could replace the script you're
running with something that may never exit.. (sepending
on what the command does)

> 
> With kernel 2.4 this command works fine, but with kernel version 2.6.3
> it leads to a system hang.

-- 
Stephen Samuel +1(604)876-0426                samuel@bcgreen.com
		   http://www.bcgreen.com/~samuel/
    Powerful committed communication. Transformation touching
      the jewel within each person and bringing it to light.

Re: Redirection of STDERR

[Andreas Schwab]

Stephen Samuel <samuel@bcgreen.com> writes:

> Christoph Pleger wrote:
>> Hello,
>> In my initialization scripts for hotplug (written for bash) the
>> following command is used to redirect output which normally goes to
>> stderr to the system logger:
>> "exec 2> >(logger -t $0[$$])"
> I don't remember this syntax as legal.

That's the process substitution feature of bash, quite handy when you want
to get an fd connected to a pipe.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux AG, Maxfeldstraße 5, 90409 Nürnberg, Germany
Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."
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Re: Redirection of STDERR

[Andreas Schwab]

Stephen Samuel <samuel@bcgreen.com> writes:

> Christoph Pleger wrote:
>> Hello,
>> In my initialization scripts for hotplug (written for bash) the
>> following command is used to redirect output which normally goes to
>> stderr to the system logger:
>> "exec 2> >(logger -t $0[$$])"
> I don't remember this syntax as legal.

That's the process substitution feature of bash, quite handy when you want
to get an fd connected to a pipe.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux AG, Maxfeldstraße 5, 90409 Nürnberg, Germany
Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Re: Redirection of STDERR

[Christoph Pleger]

Hello,

> >> In my initialization scripts for hotplug (written for bash) the
> >> following command is used to redirect output which normally goes to
> >> stderr to the system logger:
> >> "exec 2> >(logger -t $0[$$])"
> > I don't remember this syntax as legal.
> 
> That's the process substitution feature of bash, quite handy when you
> want to get an fd connected to a pipe.

I found out that the problem exists with bash 2.05b, but not with 2.05a.
The reason is that with 2.05a the command uses the file descriptors
under /dev/fd0 for the pipe, but with 2.05b the command creates a pipe
under /tmp. Obviously, the 2.05b mechanism worked with Kernel 2.4, but
not with 2.6.

Christoph 

Re: Redirection of STDERR

[vda]

On Wednesday 10 March 2004 09:50, Christoph Pleger wrote:
> Hello,
>
> > >> In my initialization scripts for hotplug (written for bash) the
> > >> following command is used to redirect output which normally goes to
> > >> stderr to the system logger:
> > >> "exec 2> >(logger -t $0[$$])"
> > >
> > > I don't remember this syntax as legal.
> >
> > That's the process substitution feature of bash, quite handy when you
> > want to get an fd connected to a pipe.
>
> I found out that the problem exists with bash 2.05b, but not with 2.05a.
> The reason is that with 2.05a the command uses the file descriptors
> under /dev/fd0 for the pipe, but with 2.05b the command creates a pipe
> under /tmp. Obviously, the 2.05b mechanism worked with Kernel 2.4, but
> not with 2.6.

I always wondered what prevents bash from NOT using
pipes in the filesystem. It does not use them for
ps | less 
and friends.
-- 
vda

Re: Redirection of STDERR

[Andreas Schwab]

Christoph Pleger <Christoph.Pleger@uni-dortmund.de> writes:

> I found out that the problem exists with bash 2.05b, but not with 2.05a.
> The reason is that with 2.05a the command uses the file descriptors
> under /dev/fd0 for the pipe, but with 2.05b the command creates a pipe
> under /tmp. Obviously, the 2.05b mechanism worked with Kernel 2.4, but
> not with 2.6.

Works fine here, please make sure that your bash isn't misconfigured.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux AG, Maxfeldstraße 5, 90409 Nürnberg, Germany
Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Re: Redirection of STDERR

[Stephen Samuel]

Yeah, you're right.  I was tired when I wrote it,

however, the exec command still isn't quite
complete.  Normally, you'd want a command to be
exec'ing.

In theory, the exec command by itself should (I think) just
redirect for the rest of your commands... However, it would
appear that there's a bug in bash for that syntax.
(i.e. If I type it in on the command line, It doesn't work for me, either).

In this case, I think you're going to be better off to just start
your processes with the output going direct  logger...

my (more successful) way of doing this is:

{
	command 1
	command2
	command3
} | logger -t $0[$$]

(needs a semicolon before the '}' if it's a one-liner.

It's also more portable (compatible with old bourne shells)
I prefer to save things like the named pipe syntax for the really
ornery situations where bourne-compatible syntax jusr doesn't
do the job.

Andreas Schwab wrote:
 > Stephen Samuel <samuel@bcgreen.com> writes:
 >>Christoph Pleger wrote:
 >>>Hello,
 >>>In my initialization scripts for hotplug (written for bash) the
 >>>following command is used to redirect output which normally goes to
 >>>stderr to the system logger:
 >>>"exec 2> >(logger -t $0[$$])"
 >>
 >>I don't remember this syntax as legal.
 >
 > That's the process substitution feature of bash, quite handy when you want
 > to get an fd connected to a pipe.


-- 
Stephen Samuel +1(604)876-0426                samuel@bcgreen.com
		   http://www.bcgreen.com/~samuel/
    Powerful committed communication. Transformation touching
      the jewel within each person and bringing it to light.