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

P

PerlFAQ Server

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.
 

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments. After that, you can post your question and our members will help you out.

Ask a Question

Members online

Forum statistics

Threads
473,968
Messages
2,570,152
Members
46,698
Latest member
LydiaHalle

Latest Threads

Top