How to catch the return code of another program?

[hary536]

Hi, I have a perl script, in which I call other applications(written in C++). Each application has different return values meaning different things. For example: Return value of 0: meaning key not found. Return value of 1: key found etc,etc. Now, in my code, I have code as below:

my $exec = "$Folder/OtherTool";    
my $output = `$exec`;
print "$output\n";

Now, it prints the messages for each return value. But doesn't print the return value code like "0", "1", etc. How, can I just catch the return value instead of messages? Any advise? Thanks in advance.

[Athanasius]

Use system instead of backticks:

my $output = system($exec);
print ($output >> 8), "\n";

Update: If you need to capture the output as well as the return value of the external application, use Capture::Tiny. See the recent thread Problem logging STDOUT in perl.

Hope that helps,

[hary536]

Hi, Thanks for replying. Two questions and also, I think I may have missed some detail in my question. (1) When I tried your above code, it always prints 255. (2) What does >> 8 do? I searched online and was led to the "die"explanation, but couldn't find what >>8 means. Now, the detail I missed in my initial question was that: My code is like below: $output = "Folder/OtherTool arguments_to_the_other_tool. " I had earlier mentioned only "Folder/OtherTool", so just clarifying here.

[Athanasius]

Hello hary536,

The following little scripts will help you to see what’s going on. Put them in the same directory:

#! perl
#  File: application.pl
use strict;
use warnings;

my $code = $ARGV[0] // 0;
printf "In application: result = '%5d' (decimal) or '%016b' (binary)\n
+", $code, $code;
exit $code;

#! perl
#  File: driver.pl
use strict;
use warnings;

my $code   = $ARGV[0] // 0;
my @app    = ('perl', 'application.pl', $code);
my $result = system(@app);
printf "In driver:      result = '%5d' (decimal) or '%016b' (binary)\n
+", $result, $result;
printf "                >> 8   = '%5d' (decimal) or '%016b' (binary)\n
+", ($result >> 8), ($result >> 8);

Now, if you run, say,

perl driver.pl 255

from the command line, you should see this output:

In application: result = '  255' (decimal) or '0000000011111111' (bina
+ry)
In driver:      result = '65280' (decimal) or '1111111100000000' (bina
+ry)
                >> 8   = '  255' (decimal) or '0000000011111111' (bina
+ry)

which shows that the return value from the application called via system has been left-shifted by 8 binary digits (i.e., multiplied by 256). See the Perl documentation entry for system, and also the explanation by Perlbotics in the recent post Re: getting the bash exit code.

Note that command line arguments are fed to system as a list, in this case supplied by the array @app in this line of driver.pl:

my $result = system(@app);

When I tried your above code, it always prints 255

The application is returning -1, which usually indicates failure. What happens when you run the application directly from the command line?

Hope that helps,

[Anonymous Monk]

If you don't want to capture the output, use system . system also explains a bit about return codes and the special $? variable

[hary536]

Thanks.