== A Bourne shell irritation: piping just stderr

I generally like the Bourne shell, but I will easily admit that it has
a number of things that are less than ideal. One of those less than
ideal things is a peculiar omission: you cannot easily redirect *just*
standard error into a pipeline.

This sounds like a peculiar thing to want, but there are [[situations
../sysadmin/LighttpdCGIStderr]] where you do need to process stderr
separately; for example, you might need to timestamp all stderr output
and log it. In a hypothetical shell with this feature you could have a
'timestamper' program and just write:

> _process |[2] timestamper >>error-log_

In the Bourne shell, not so much. You can do it, but it is what you
would call intricate:

> _exec 3>&1_ \\
> _process 2>&1 >&3 3>&- | timestamper >>error-log_

What's going on here is that first we save a copy of the real
stdout on file descriptor 3, and then in the command itself
we:
* send stdout to the timestamper process (remember, pipelines
  are set up *before* redirection)
* redirect stderr to stdout, which is the pipeline
* redirect stdout to fd 3, which is our original stdout
* close fd 3 so that _process_ does not inherit a surprise file
  descriptor.

(We must save the original stdout separately via _exec_ because
of the pipeline vs redirection timing issue; if we wrote '_process
3>&1 ..._' we would be capturing the pipe'd stdout, not the
original stdout.)

(This incantation is not original to me; I've seen it written up
elsewhere on the Internet, but I never really understood it until
I wrote out everything here (sort of [[cf UnixHistoryDiscoveries]],
especially the comment).)