FAQ 8.44 How do I tell the difference between errors from the shell and perl?

Discussion in 'Perl Misc' started by PerlFAQ Server, Feb 20, 2011.

  1. This is an excerpt from the latest version perlfaq8.pod, which
    comes with the standard Perl distribution. These postings aim to
    reduce the number of repeated questions as well as allow the community
    to review and update the answers. The latest version of the complete
    perlfaq is at http://faq.perl.org .

    --------------------------------------------------------------------

    8.44: How do I tell the difference between errors from the shell and perl?

    (answer contributed by brian d foy)

    When you run a Perl script, something else is running the script for
    you, and that something else may output error messages. The script might
    emit its own warnings and error messages. Most of the time you cannot
    tell who said what.

    You probably cannot fix the thing that runs perl, but you can change how
    perl outputs its warnings by defining a custom warning and die
    functions.

    Consider this script, which has an error you may not notice immediately.

    #!/usr/locl/bin/perl

    print "Hello World\n";

    I get an error when I run this from my shell (which happens to be bash).
    That may look like perl forgot it has a "print()" function, but my
    shebang line is not the path to perl, so the shell runs the script, and
    I get the error.

    $ ./test
    ./test: line 3: print: command not found

    A quick and dirty fix involves a little bit of code, but this may be all
    you need to figure out the problem.

    #!/usr/bin/perl -w

    BEGIN {
    $SIG{__WARN__} = sub{ print STDERR "Perl: ", @_; };
    $SIG{__DIE__} = sub{ print STDERR "Perl: ", @_; exit 1};
    }

    $a = 1 + undef;
    $x / 0;
    __END__

    The perl message comes out with "Perl" in front. The "BEGIN" block works
    at compile time so all of the compilation errors and warnings get the
    "Perl:" prefix too.

    Perl: Useless use of division (/) in void context at ./test line 9.
    Perl: Name "main::a" used only once: possible typo at ./test line 8.
    Perl: Name "main::x" used only once: possible typo at ./test line 9.
    Perl: Use of uninitialized value in addition (+) at ./test line 8.
    Perl: Use of uninitialized value in division (/) at ./test line 9.
    Perl: Illegal division by zero at ./test line 9.
    Perl: Illegal division by zero at -e line 3.

    If I don't see that "Perl:", it's not from perl.

    You could also just know all the perl errors, and although there are
    some people who may know all of them, you probably don't. However, they
    all should be in the perldiag manpage. If you don't find the error in
    there, it probably isn't a perl error.

    Looking up every message is not the easiest way, so let perl to do it
    for you. Use the diagnostics pragma with turns perl's normal messages
    into longer discussions on the topic.

    use diagnostics;

    If you don't get a paragraph or two of expanded discussion, it might not
    be perl's message.



    --------------------------------------------------------------------

    The perlfaq-workers, a group of volunteers, maintain the perlfaq. They
    are not necessarily experts in every domain where Perl might show up,
    so please include as much information as possible and relevant in any
    corrections. The perlfaq-workers also don't have access to every
    operating system or platform, so please include relevant details for
    corrections to examples that do not work on particular platforms.
    Working code is greatly appreciated.

    If you'd like to help maintain the perlfaq, see the details in
    perlfaq.pod.
    PerlFAQ Server, Feb 20, 2011
    #1
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