FAQ 8.44 How do I tell the difference between errors from the shell and perl?
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From: PerlFAQ Server on 31 Jul 2010 18:00 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.
From: brian d foy on 10 Aug 2010 09:10 In article <slrni59df1.v04.hjp-usenet2(a)hrunkner.hjp.at>, Peter J. Holzer <hjp-usenet2(a)hjp.at> wrote: > On 2010-07-31 22:00, PerlFAQ Server <brian(a)theperlreview.com> wrote: > > -------------------------------------------------------------------- > > > > 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. > > I find this paragraph confusing. The "something else" that "is running > the script for" me is the perl interpreter. But from the rest of this > entry I guess that "something else" means the shell, which doesn't run > the script, it merely starts it. Starting it is the same as running it, when most people consider "running" to be typing at the command line and hitting enter to kick something off. It sounds like you've sussed it out just fine by reading the whole thing. :)
From: Ilya Zakharevich on 10 Aug 2010 17:16 On 2010-08-10, brian d foy <brian.d.foy(a)gmail.com> wrote: > In article <slrni59df1.v04.hjp-usenet2(a)hrunkner.hjp.at>, Peter J. > Holzer <hjp-usenet2(a)hjp.at> wrote: > >> On 2010-07-31 22:00, PerlFAQ Server <brian(a)theperlreview.com> wrote: >> > -------------------------------------------------------------------- >> > >> > 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. >> >> I find this paragraph confusing. The "something else" that "is running >> the script for" me is the perl interpreter. But from the rest of this >> entry I guess that "something else" means the shell, which doesn't run >> the script, it merely starts it. > > Starting it is the same as running it, when most people consider > "running" to be typing at the command line and hitting enter to kick > something off. > > It sounds like you've sussed it out just fine by reading the whole > thing. :) "When you start a Perl script, there may be an intermediate agent (such shell) between your action and actual execution of Perl interpreter." etc What about somesuch. [As written above, it looks majorly confusing to me as well...] Yours, Ilya
From: brian d foy on 13 Aug 2010 05:57 In article <slrni68klo.8n9.hjp-usenet2(a)hrunkner.hjp.at>, Peter J. Holzer <hjp-usenet2(a)hjp.at> wrote: > The target audience of this FAQ is not "most people", but Perl > programmers. Actually, it's "most people" in the sense that it's not what I would classify as the "programmer" crowd. A lot of these questions are directed at people with little technical or programming experience.
From: brian d foy on 13 Aug 2010 06:00
In article <slrni63gdi.qgu.nospam-abuse(a)powdermilk.math.berkeley.edu>, Ilya Zakharevich <nospam-abuse(a)ilyaz.org> wrote: > "When you start a Perl script, there may be an intermediate agent > (such shell) between your action and actual execution of Perl > interpreter." etc > > What about somesuch. I've been thinking about that as well, but I don't thing it makes it any clearer for people who don't already understand what a shell (agent, whatever) is. This is why I used such loose language in the original answer. A better answer probably changes the verbs instead of the nouns. |