system/backticks output not as expected

Hi,

I have a problem getting the output of a system call. It's a simple
code:

my $return=`$command`;
print "$?\n";
print "result:$return\n";

(I am calling a command which actually doesn't exist, to check error
handling)

as result on the console I have:

Das System kann das angegebene Laufwerk nicht finden.
256
result:

The first error says it can't find the program. Actually I expected this
in $return.

compared to system():

my $return=system("$command");
print "$?\n";
print "result:$return\n";

Das System kann das angegebene Laufwerk nicht finden.
256
result:256

So obviously system() returns the error code. But why don't the
backticks return anything at all? Ideally I'd want all the output
catched to log, but still be able to react on errors...

Lars

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RE: system/backticks output not as expected

I usually capture the output for backticks to an array rather than a
string so try

my @return=`$command`;
Foreach my $line(@output){print "$line\n"}

Regards,
Conor Lillis
---------------------------------------------
Senior Systems Administrator,
Group Network Services,
Phone +353-1-616-2540

Anglo Irish Bank
www.angloirishbank.ie
mailto:conorlillis@angloirishbank.ie
---------------------------------------------

-----Original Message-----
From: activeperl-bounces@listserv.ActiveState.com
[mailto:activeperl-bounces@listserv.ActiveState.com] On Behalf Of
Oeschey, Lars (I/ET-83, extern)
Sent: 30 October 2009 13:38
To: activeperl@listserv.ActiveState.com
Subject: system/backticks output not as expected

Hi,

I have a problem getting the output of a system call. It's a simple
code:

my $return=`$command`;
print "$?\n";
print "result:$return\n";

(I am calling a command which actually doesn't exist, to check error
handling)

as result on the console I have:

Das System kann das angegebene Laufwerk nicht finden.
256
result:

The first error says it can't find the program. Actually I expected this
in $return.

compared to system():

my $return=system("$command");
print "$?\n";
print "result:$return\n";

Das System kann das angegebene Laufwerk nicht finden.
256
result:256

So obviously system() returns the error code. But why don't the
backticks return anything at all? Ideally I'd want all the output
catched to log, but still be able to react on errors...

Lars

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RE: system/backticks output not as expected

Oeschey, Lars (I/ET-83, extern) <> wrote:
> Hi,
>
> I have a problem getting the output of a system call. It's a simple
> code:
>
> my $return=`$command`;
> print "$?\n";
> print "result:$return\n";
>
> (I am calling a command which actually doesn't exist, to check error
> handling)
>
> as result on the console I have:
>
> Das System kann das angegebene Laufwerk nicht finden.
> 256
> result:
>
> The first error says it can't find the program. Actually I expected
> this in $return.
>
> compared to system():
>
> my $return=system("$command");
> print "$?\n";
> print "result:$return\n";
>
> Das System kann das angegebene Laufwerk nicht finden.
> 256
> result:256
>
> So obviously system() returns the error code. But why don't the
> backticks return anything at all? Ideally I'd want all the output
> catched to log, but still be able to react on errors...

Backticks only capture output from stdout, while error messages are
generally output on stderr. In unix-like shells it usual to redirect
stderr to stdout by adding 2>&1 to the command line. I don't know if it
works the same on win32, but it might be worth a try, if that is your
platform.

Also, you could always try verifying that the command exists before
trying to run it.

HTH

--
Brian Raven
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Re: system/backticks output not as expected

Brian Raven wrote:

> Backticks only capture output from stdout, while error messages are
> generally output on stderr. In unix-like shells it usual to redirect
> stderr to stdout by adding 2>&1 to the command line. I don't know if it
> works the same on win32, but it might be worth a try, if that is your
> platform.

Yes, it works.

--
Serguei Trouchelle
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