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

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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.



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