Programming Perl, 4th Edition

Using the Debugger

The debugger prompt is something like:

DB<8>

or even:

DB<<17>>

where the number shows how many commands you’ve executed. A csh-like history mechanism allows you to access previous commands by number. For example, !17 would repeat command number 17. The number of angle brackets indicates the depth of the debugger. For example, you get more than one set of brackets if you’re already at a breakpoint and then print out the result of a function call that itself also has a breakpoint.

If you want to enter a multiline command, such as a subroutine definition with several statements, you may escape the newline that would normally end the debugger command with a backslash. Here’s an example:

DB<1> for (1..3) {         \
  cont:     print "ok\n";    \
  cont: }
  ok
  ok
  ok

Let’s say you want to fire up the debugger on a little program of yours (let’s call it camel_flea) and stop it as soon as it gets down to a function named infested. Here’s how you’d do that:

% perl –d camel_flea
  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::(camel_flea:2):     pests('bactrian', 4);
DB<1>

The debugger halts your program right before the first runtime executable statement (but see below about compile-time statements) and asks you to enter a command. Again, whenever the debugger stops to show you a line of code, it displays the line it’s about to execute, not the one it just executed. The line displayed may not look exactly like ...