redirecting STDERR, then back again

[melguin]

I got this program I've been handed to make work right. What I need to do is redirect STDERR in the middle of it to someplace else, the a little while later redirect it back.

I remember reading about it somewhere but haven't been able to find it again. Any ideas or pointers to references?

as always, many thanks,

melguin.

[danger]

The documentation for open() perlfunc:open has an example of saving, redirecting, and restoring STDOUT and STDERR that you might find helpful.

[princepawn]

search for the first part of your question by typing "dup STDERR" into the search box.

I presume that if you saved a copy of the filehandle you wanted to restore:

my $restore = \*original;

that you could use the dup method discussed in the posts you will find to restore it.

[grinder]

The trick is to use a local'ized version of STDERR, in the scope that interests you:

#! /usr/bin/perl -w

use strict;

warn "initial write to STDERR\n";

# open a scope
{
  local *STDERR;
  open STDERR, '>tmp.stderr' or die "Cannot open tmp.stderr for output
+: $!\n";
  warn "redirected write to STDERR\n";
  close STDERR;
}

warn "final write to STDERR\n";

You could probably get away without having the close STDERR at the end of the scope, but it's much cleaner (easier to understand what's going on) to do so explicitly.

Later: I just went and checked the example as per danger's suggestion. The technique there seems to be a little more complicated. You basically open up another file handle to cache a copy of STDERR, and then when you are finished you use that to reset the original STDERR back again. Which means there's some baggage you have to drag along with you (the previous STDERR descriptor). Using local hides this for you => one less damned thing to go wrong.

--

g r i n d e r

[tye]

Using local hides this for you => one less damned thing to go wrong.

Unfortunately, copying file descriptors using symbol table globs doesn't work very well with the underlying IO subsystem.

You're code reopens Perl's STDERR but the original STDERR is hanging around, hidden by local. This means the file descriptor 2 is left open so that your new Perl STDERR won't be "stderr" to C routines nor to child processes. So several things that would work as expected if you used the more complicated method recommended by the documentation won't work with your method.

        - tye (but my friends call me "Tye")

[mrmick]

You might also use select in a local block to temporarily point the STDERR filehandle someplace else.

NOTE: I've done this with STDOUT but not with STDERR (yet).

Mick

[melguin]

Here's what ended up working:

open(OLDERR, ">&STDERR");
open(STDERR, ">>/tmp/tmp.spderr") or die "Can't dup stdout";
select(STDOUT); $| = 1;     # make unbuffered
print OLDERR "";  #this fixed an error about OLDERR not being used 

## do my stuff here.

close(STDERR);
open(STDERR, ">&OLDERR");

As danger said, this is in perlfunc:open.

thanks,

melguin