', at this point all we'd get is an empty (undefined) value back. What we need to do is to step through the next executable statement with an 'B': DB<6> s main::(./data_a:5): my %data = ( main::(./data_a:6): 'this' => qw(that), main::(./data_a:7): 'tom' => qw(and jerry), main::(./data_a:8): 'welcome' => q(Hello World), main::(./data_a:9): 'zip' => q(welcome), main::(./data_a:10): ); Now we can have a look at that first ($key) variable: DB<7> p $key welcome line 13 is where the action is, so let's continue down to there via the letter 'B', which by the way, inserts a 'one-time-only' breakpoint at the given line or sub routine: DB<8> c 13 All OK main::(./data_a:13): print "$data{$key}\n"; We've gone past our check (where 'All OK' was printed) and have stopped just before the meat of our task. We could try to print out a couple of variables to see what is happening: DB<9> p $data{$key} Not much in there, lets have a look at our hash: DB<10> p %data Hello Worldziptomandwelcomejerrywelcomethisthat DB<11> p keys %data Hello Worldtomwelcomejerrythis Well, this isn't very easy to read, and using the helpful manual (B), the 'B' command looks promising: DB<12> x %data 0 'Hello World' 1 'zip' 2 'tom' 3 'and' 4 'welcome' 5 undef 6 'jerry' 7 'welcome' 8 'this' 9 'that' That's not much help, a couple of welcomes in there, but no indication of which are keys, and which are values, it's just a listed array dump and, in this case, not particularly helpful. The trick here, is to use a B to the data structure: DB<13> x \%data 0 HASH(0x8194bc4) 'Hello World' => 'zip' 'jerry' => 'welcome' 'this' => 'that' 'tom' => 'and' 'welcome' => undef The reference is truly dumped and we can finally see what we're dealing with. Our quoting was perfectly valid but wrong for our purposes, with 'and jerry' being treated as 2 separate words rather than a phrase, thus throwing the evenly paired hash structure out of alignment. The 'B<-w>' switch would have told us about this, had we used it at the start, and saved us a lot of trouble: > perl -w data Odd number of elements in hash assignment at ./data line 5. We fix our quoting: 'tom' => q(and jerry), and run it again, this time we get our expected output: > perl -w data Hello World While we're here, take a closer look at the 'B' command, it's really useful and will merrily dump out nested references, complete objects, partial objects - just about whatever you throw at it: Let's make a quick object and x-plode it, first we'll start the debugger: it wants some form of input from STDIN, so we give it something non-committal, a zero: > perl -de 0 Default die handler restored. Loading DB routines from perl5db.pl version 1.07 Editor support available. Enter h or `h h' for help, or `man perldebug' for more help. main::(-e:1): 0 Now build an on-the-fly object over a couple of lines (note the backslash): DB<1> $obj = bless({'unique_id'=>'123', 'attr'=> \ cont: {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class') And let's have a look at it: DB<2> x $obj 0 MY_class=HASH(0x828ad98) 'attr' => HASH(0x828ad68) 'col' => 'black' 'things' => ARRAY(0x828abb8) 0 'this' 1 'that' 2 'etc' 'unique_id' => 123 DB<3> Useful, huh? You can eval nearly anything in there, and experiment with bits of code or regexes until the cows come home: DB<3> @data = qw(this that the other atheism leather theory scythe) DB<4> p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data)) atheism leather other scythe the theory saw -> 6 If you want to see the command History, type an 'B': DB<5> H 4: p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data)) 3: @data = qw(this that the other atheism leather theory scythe) 2: x $obj 1: $obj = bless({'unique_id'=>'123', 'attr'=> {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class') DB<5> And if you want to repeat any previous command, use the exclamation: 'B': DB<5> !4 p 'saw -> '.($cnt += map { print "$_\n" } grep(/the/, sort @data)) atheism leather other scythe the theory saw -> 12 For more on references see L and L =head1 Stepping through code Here's a simple program which converts between Celsius and Fahrenheit, it too has a problem: #!/usr/bin/perl -w use strict; my $arg = $ARGV[0] || '-c20'; if ($arg =~ /^\-(c|f)((\-|\+)*\d+(\.\d+)*)$/) { my ($deg, $num) = ($1, $2); my ($in, $out) = ($num, $num); if ($deg eq 'c') { $deg = 'f'; $out = &c2f($num); } else { $deg = 'c'; $out = &f2c($num); } $out = sprintf('%0.2f', $out); $out =~ s/^((\-|\+)*\d+)\.0+$/$1/; print "$out $deg\n"; } else { print "Usage: $0 -[c|f] num\n"; } exit; sub f2c { my $f = shift; my $c = 5 * $f - 32 / 9; return $c; } sub c2f { my $c = shift; my $f = 9 * $c / 5 + 32; return $f; } For some reason, the Fahrenheit to Celsius conversion fails to return the expected output. This is what it does: > temp -c0.72 33.30 f > temp -f33.3 162.94 c Not very consistent! We'll set a breakpoint in the code manually and run it under the debugger to see what's going on. A breakpoint is a flag, to which the debugger will run without interruption, when it reaches the breakpoint, it will stop execution and offer a prompt for further interaction. In normal use, these debugger commands are completely ignored, and they are safe - if a little messy, to leave in production code. my ($in, $out) = ($num, $num); $DB::single=2; # insert at line 9! if ($deg eq 'c') ... > perl -d temp -f33.3 Default die handler restored. Loading DB routines from perl5db.pl version 1.07 Editor support available. Enter h or `h h' for help, or `man perldebug' for more help. main::(temp:4): my $arg = $ARGV[0] || '-c100'; We'll simply continue down to our pre-set breakpoint with a 'B': DB<1> c main::(temp:10): if ($deg eq 'c') { Followed by a view command to see where we are: DB<1> v 7: my ($deg, $num) = ($1, $2); 8: my ($in, $out) = ($num, $num); 9: $DB::single=2; 10==> if ($deg eq 'c') { 11: $deg = 'f'; 12: $out = &c2f($num); 13 } else { 14: $deg = 'c'; 15: $out = &f2c($num); 16 } And a print to show what values we're currently using: DB<1> p $deg, $num f33.3 We can put another break point on any line beginning with a colon, we'll use line 17 as that's just as we come out of the subroutine, and we'd like to pause there later on: DB<2> b 17 There's no feedback from this, but you can see what breakpoints are set by using the list 'L' command: DB<3> L temp: 17: print "$out $deg\n"; break if (1) Note that to delete a breakpoint you use 'd' or 'D'. Now we'll continue down into our subroutine, this time rather than by line number, we'll use the subroutine name, followed by the now familiar 'v': DB<3> c f2c main::f2c(temp:30): my $f = shift; DB<4> v 24: exit; 25 26 sub f2c { 27==> my $f = shift; 28: my $c = 5 * $f - 32 / 9; 29: return $c; 30 } 31 32 sub c2f { 33: my $c = shift; Note that if there was a subroutine call between us and line 29, and we wanted to B through it, we could use the 'B' command, and to step over it we would use 'B' which would execute the sub, but not descend into it for inspection. In this case though, we simply continue down to line 29: DB<4> c 29 main::f2c(temp:29): return $c; And have a look at the return value: DB<5> p $c 162.944444444444 This is not the right answer at all, but the sum looks correct. I wonder if it's anything to do with operator precedence? We'll try a couple of other possibilities with our sum: DB<6> p (5 * $f - 32 / 9) 162.944444444444 DB<7> p 5 * $f - (32 / 9) 162.944444444444 DB<8> p (5 * $f) - 32 / 9 162.944444444444 DB<9> p 5 * ($f - 32) / 9 0.722222222222221 :-) that's more like it! Ok, now we can set our return variable and we'll return out of the sub with an 'r': DB<10> $c = 5 * ($f - 32) / 9 DB<11> r scalar context return from main::f2c: 0.722222222222221 Looks good, let's just continue off the end of the script: DB<12> c 0.72 c Debugged program terminated. Use q to quit or R to restart, use O inhibit_exit to avoid stopping after program termination, h q, h R or h O to get additional info. A quick fix to the offending line (insert the missing parentheses) in the actual program and we're finished. =head1 Placeholder for a, w, t, T Actions, watch variables, stack traces etc.: on the TODO list. a w t T =head1 REGULAR EXPRESSIONS Ever wanted to know what a regex looked like? You'll need perl compiled with the DEBUGGING flag for this one: > perl -Dr -e '/^pe(a)*rl$/i' Compiling REx `^pe(a)*rl$' size 17 first at 2 rarest char at 0 1: BOL(2) 2: EXACTF (4) 4: CURLYN[1] {0,32767}(14) 6: NOTHING(8) 8: EXACTF (0) 12: WHILEM(0) 13: NOTHING(14) 14: EXACTF (16) 16: EOL(17) 17: END(0) floating `'$ at 4..2147483647 (checking floating) stclass `EXACTF ' anchored(BOL) minlen 4 Omitting $` $& $' support. EXECUTING... Freeing REx: `^pe(a)*rl$' Did you really want to know? :-) For more gory details on getting regular expressions to work, have a look at L, L, and to decode the mysterious labels (BOL and CURLYN, etc. above), see L. =head1 OUTPUT TIPS To get all the output from your error log, and not miss any messages via helpful operating system buffering, insert a line like this, at the start of your script: $|=1; To watch the tail of a dynamically growing logfile, (from the command line): tail -f $error_log Wrapping all die calls in a handler routine can be useful to see how, and from where, they're being called, L has more information: BEGIN { $SIG{__DIE__} = sub { require Carp; Carp::confess(@_) } } Various useful techniques for the redirection of STDOUT and STDERR filehandles are explained in L and L. =head1 CGI Just a quick hint here for all those CGI programmers who can't figure out how on earth to get past that 'waiting for input' prompt, when running their CGI script from the command-line, try something like this: > perl -d my_cgi.pl -nodebug Of course L and L will tell you more. =head1 GUIs The command line interface is tightly integrated with an B extension and there's a B interface too. You don't have to do this all on the command line, though, there are a few GUI options out there. The nice thing about these is you can wave a mouse over a variable and a dump of its data will appear in an appropriate window, or in a popup balloon, no more tiresome typing of 'x $varname' :-) In particular have a hunt around for the following: B perlTK based wrapper for the built-in debugger B data display debugger B and B are NT specific NB. (more info on these and others would be appreciated). =head1 SUMMARY We've seen how to encourage good coding practices with B and B<-w>. We can run the perl debugger B to inspect your data from within the perl debugger with the B

and B commands. You can walk through your code, set breakpoints with B and step through that code with B or B, continue with B and return from a sub with B. Fairly intuitive stuff when you get down to it. There is of course lots more to find out about, this has just scratched the surface. The best way to learn more is to use perldoc to find out more about the language, to read the on-line help (L is probably the next place to go), and of course, experiment. =head1 SEE ALSO L, L, L, L, L =head1 AUTHOR Richard Foley Copyright (c) 2000 =head1 CONTRIBUTORS Various people have made helpful suggestions and contributions, in particular: Ronald J Kimball Hugo van der Sanden Peter Scott