open and STDERR

[grocery_stocker]

"Found in /usr/lib/perl5/5.8.3/pod/perlfaq8.pod
How can I capture STDERR from an external command?

There are three basic ways of running external commands:

system $cmd; # using system()
$output = ‘$cmd‘; # using backticks (‘‘)
open (PIPE, "cmd │"); # using open()

With system(), both STDOUT and STDERR will go the same
place as the script’s STDOUT and STDERR, unless the sys­
tem() command redirects them. Backticks and open() read
only the STDOUT of your command."

If open() reads only to STDOUT, Then how when I do something like the
following:

#!/usr/bin/perl -w

open(STDERR, "whore.txt") or die "Cant: $! \n";

I get:
miss_xtc@linux:~/perl> ./op.pl
Cant: No such file or directory

Wouldn't this be writing to STDERR in this case?

 

[anno4000]

"Found in /usr/lib/perl5/5.8.3/pod/perlfaq8.pod
How can I capture STDERR from an external command?

There are three basic ways of running external commands:

system $cmd; # using system()
$output = ‘$cmd‘; # using backticks (‘‘)
open (PIPE, "cmd │"); # using open()

With system(), both STDOUT and STDERR will go the same
place as the script’s STDOUT and STDERR, unless the sys­
tem() command redirects them. Backticks and open() read
only the STDOUT of your command."

If open() reads only to STDOUT, Then how when I do something like the
following:

#!/usr/bin/perl -w

open(STDERR, "whore.txt") or die "Cant: $! \n";

That tries to open "whore.txt" for reading, which fails unless the
file already exists.

I get:
miss_xtc@linux:~/perl> ./op.pl
Cant: No such file or directory

Wouldn't this be writing to STDERR in this case?

Why should it? You have to open the file for writing:

open STDERR, '>', 'whore.txt' or die "Can't create 'whore.txt': $!";

Anno

 

[grocery_stocker]

That tries to open "whore.txt" for reading, which fails unless the
file already exists.


Why should it? You have to open the file for writing:

open STDERR, '>', 'whore.txt' or die "Can't create 'whore.txt': $!";

Anno

If I understand the faq, something like
$output = `$cmd`;

Reads only the STDOUT. Where is STDERR going in this case?

 

[Eric Amick]

If I understand the faq, something like
$output = `$cmd`;

Reads only the STDOUT. Where is STDERR going in this case?

Nowhere, i.e., the bit bucket. If you want it to go somewhere, redirect
stderr in the command according to the command shell's conventions. For
example,

$output = `$cmd 2>/some/file/or/other`

on *nix sends stderr to /some/file/or/other. (This is all in perlop.)

 

[Joe Smith]

If I understand the faq, something like
$output = `$cmd`;

Reads only the STDOUT. Where is STDERR going in this case?

STDERR from $cmd goes to the same place that your perl program's
STDERR is going. Use

$output = `$cmd 2>&1`;

if you want STDERR to go to the same place as STDOUT. In that
case, $output will have both, and your program may have to
search for and/or remove the error messages from it.
-Joe

 

[grocery_stocker]

STDERR from $cmd goes to the same place that your perl program's
STDERR is going. Use

$output = `$cmd 2>&1`;

if you want STDERR to go to the same place as STDOUT. In that
case, $output will have both, and your program may have to
search for and/or remove the error messages from it.
-Joe

Further down the perl faq, they give the following solution
To capture a program's STDOUT, but discard its STDERR:

use IPC::Open3;
use File::Spec;
use Symbol qw(gensym);
open(NULL, ">", File::Spec->devnull);
my $pid = open3(gensym, \*PH, ">&NULL", "cmd");
while( <PH> ) { }
waitpid($pid, 0);

they don't use
$output = `$cmd 2>&1`;

I'm assuming the solution is perl faq done for portability purposes. I
don't have a Windows box,
otherwise I would I would see if something like

$output = `$cmd 2>&1`;

would have worked in Windows XP

 

[Joe Smith]

don't have a Windows box,
otherwise I would I would see if something like

$output = `$cmd 2>&1`;

would have worked in Windows XP

It does.

C:\>perl -le "$_=`net file 2>&1`;s/\s*$//;print'>>>',$_,'<<<'"
-Joe