Suppress/Reactivate STDERR

saintmike has asked for the wisdom of the Perl Monks concerning the following question:

After suppressing STDERR for a system() command, I can't seem to reactivate it afterwards, for subsequent system() commands. Here's the code:

my @cmd = ("/bin/ls", "/doesntexist");

use vars qw(*OLDERR);
open OLDERR, ">&STDERR";
open STDERR, ">/dev/null";

  # STDERR suppressed
system(@cmd);

*STDERR = *OLDERR;

print STDERR "stderr is open\n";

  # STDERR should be open - but isn't
system(@cmd);
[download]

Both system() commands produce an error message on STDERR, but the first one gets suppressed since STDERR points to nowhere land. However, after restoring STDERR, I would expect the 2nd system() to actually print the error message. Alas, what I get is this:

stderr is open
[download]

which means that the print STDERR works ok, but the system()'s STDERR channel is still blocked. Is this a bug?

Comment on Suppress/Reactivate STDERR Select or Download Code

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Re: Suppress/Reactivate STDERR by ikegami (Patriarch) on Dec 15, 2006 at 23:55 UTC
`STDERR` is not necessarily the same thing as file descriptor 2 (and it isn't after you do `STDERR = OLDERR;`). For commands executed by `system`, file descriptor 2 is stderr. They have no knowledge of the file descriptor associated with your program's `STDERR` variable. Replace `STDERR = OLDERR;` with `open STDERR, ">&OLDERR";` Update: s/3/2/g. Thanks nobull.	[reply] [d/l] [select]
Re^2: Suppress/Reactivate STDERR by nobull (Friar) on Dec 16, 2006 at 16:31 UTC
STDERR is not necessarily the same thing as file descriptor 3 s/3/2/ In recent Perl there's no need to use package variables for file handles to achieve this. `open my $olderr, '>&', \*STDERR or die $!; #... open STDERR, '>&', $olderr or die $!;` [download]	[reply] [d/l]
Re^3: Suppress/Reactivate STDERR by ikegami (Patriarch) on Dec 16, 2006 at 18:07 UTC
Re: 2 vs 3 -- Thanks, typo! Re: lexical file handles -- True. I just went along with the code that was already there. Lexicals can be used for file handles since Perl 5.6.0.	[reply]
Re: Suppress/Reactivate STDERR by almut (Canon) on Dec 15, 2006 at 23:54 UTC
In case you just want to get the code working, you can reopen STDERR `open(STDERR, ">&OLDERR"); # instead of # STDERR = OLDERR;` [download] If however, you'd like to understand why the glob assignment suffices for the print but not for the subprocess' output, you'll have to wait for wiser heads than mine to explain... :)	[reply] [d/l]