cvs commit: perlfaq perlfaq8.pod

cvsuser     04/10/05 15:13:49

  Modified:    .        perlfaq8.pod
  Log:
  * new answer: How do I tell the difference between errors from the shell and perl?
  
  Revision  Changes    Path
  1.22      +69 -1     perlfaq/perlfaq8.pod
  
  Index: perlfaq8.pod
  ===================================================================
  RCS file: /cvs/public/perlfaq/perlfaq8.pod,v
  retrieving revision 1.21
  retrieving revision 1.22
  diff -u -w -r1.21 -r1.22
  --- perlfaq8.pod	5 May 2004 06:29:12 -0000	1.21
  +++ perlfaq8.pod	5 Oct 2004 22:13:49 -0000	1.22
  @@ -1,6 +1,6 @@
   =head1 NAME
   
  -perlfaq8 - System Interaction ($Revision: 1.21 $, $Date: 2004/05/05 06:29:12 $)
  +perlfaq8 - System Interaction ($Revision: 1.22 $, $Date: 2004/10/05 22:13:49 $)
   
   =head1 DESCRIPTION
   
  @@ -1065,6 +1065,74 @@
       sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
           or die "can't open /foo/somefile: $!":
   
  +=head2 How do I tell the difference between errors from the shell and perl?
  +
  +(answer contributed by brian d foy, C<< <[email protected]> >>
  +
  +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.
  +
   =head2 How do I install a module from CPAN?
   
   The easiest way is to have a module also named CPAN do it for you.