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How to catch the return code of another program?

by hary536 (Novice)
on Aug 28, 2012 at 02:45 UTC ( #990120=perlquestion: print w/replies, xml ) Need Help??
hary536 has asked for the wisdom of the Perl Monks concerning the following question:

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.

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Re: How to catch the return code of another program?
by Athanasius (Chancellor) on Aug 28, 2012 at 02:54 UTC

    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,

    Athanasius <°(((><contra mundum

      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.

        Hello hary536,

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

        #! perl # File: 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: use strict; use warnings; my $code = $ARGV[0] // 0; my @app = ('perl', '', $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 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

        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,

        Athanasius <°(((><contra mundum

Re: How to catch the return code of another program?
by Anonymous Monk on Aug 28, 2012 at 02:53 UTC
    If you don't want to capture the output, use system . system also explains a bit about return codes and the special $? variable

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