perldebug - Perl debugging


First of all, have you tried using the -w switch?


If you invoke Perl with a -d switch, your script will be run under the debugger. However, the Perl debugger is not a separate program as it is in a C environment. Instead, the -d flag tells the compiler to insert source information into the pseudocode it's about to hand to the interpreter. (That means your code must compile correctly for the debugger to work on it.) Then when the interpreter starts up, it pre-loads a Perl library file containing the debugger itself. The program will halt before the first executable statement (but see below) and ask you for one of the following commands:

Prints out a help message.

Stack trace. If you do bizarre things to your @_ arguments in a subroutine, the stack backtrace will not always show the original values.

Single step. Executes until it reaches the beginning of another statement.

Next. Executes over subroutine calls, until it reaches the beginning of the next statement.

Finish. Executes statements until it has finished the current subroutine.

Continue. Executes until the next breakpoint is reached.

c line
Continue to the specified line. Inserts a one-time-only breakpoint at the specified line.

Repeat last n or s.

l min+incr
List incr+1 lines starting at min. If min is omitted, starts where last listing left off. If incr is omitted, previous value of incr is used.

l min-max
List lines in the indicated range.

l line
List just the indicated line.

List next window.

List previous window.

w line
List window (a few lines worth of code) around line.

l subname
List subroutine. If it's a long subroutine it just lists the beginning. Use ``l'' to list more.

Regular expression search forward in the source code for pattern; the final / is optional.

Regular expression search backward in the source code for pattern; the final ? is optional.

List lines that have breakpoints or actions.

Lists the names of all subroutines.

Toggle trace mode on or off.

b line [ condition ]
Set a breakpoint. If line is omitted, sets a breakpoint on the line that is about to be executed. If a condition is specified, it is evaluated each time the statement is reached and a breakpoint is taken only if the condition is true. Breakpoints may only be set on lines that begin an executable statement. Conditions don't use if:

b 237 $x > 30 b 33 /pattern/i

b subname [ condition ]
Set breakpoint at first executable line of subroutine.

d line
Delete breakpoint. If line is omitted, deletes the breakpoint on the line that is about to be executed.

Delete all breakpoints.

a line command
Set an action for line. A multiline command may be entered by backslashing the newlines. This command is Perl code, not another debugger command.

Delete all line actions.

< command
Set an action to happen before every debugger prompt. A multiline command may be entered by backslashing the newlines.

> command
Set an action to happen after the prompt when you've just given a command to return to executing the script. A multiline command may be entered by backslashing the newlines.

V package [symbols]
Display all (or some) variables in package (defaulting to the main package) using a data pretty-printer (hashes show their keys and values so you see what's what, control characters are made printable, etc.). Make sure you don't put the type specifier (like $) there, just the symbol names, like this:

V DB filename line

X [symbols]
Same as as ``V'' command, but within the current package.

! number
Redo a debugging command. If number is omitted, redoes the previous command.

! -number
Redo the command that was that many commands ago.

H -number
Display last n commands. Only commands longer than one character are listed. If number is omitted, lists them all.

q or ^D
Quit. (``quit'' doesn't work for this.)

Execute command as a Perl statement. A missing semicolon will be supplied.

p expr
Same as print DB::OUT expr . The DB::OUT filehandle is opened to /dev/tty, regardless of where STDOUT may be redirected to.

Any command you type in that isn't recognized by the debugger will be.directly executed ( eval 'd) as Perl code. Leading white space will cause the debugger to think it's NOT a debugger command.

If you have any compile-time executable statements (code within a BEGIN block or a use statement), these will NOT be stopped by debugger, although require s will. From your own code, however, you can transfer control back to the debugger using the following statement, which is harmless if the debugger is not running:

$DB::single = 1;


If you want to modify the debugger, copy from the Perl library to another name and modify it as necessary. You'll also want to set environment variable PERL5DB to say something like this:

BEGIN { require "" }

You can do some customization by setting up a .perldb file which contains initialization code. For instance, you could make aliases like these (the last one in particular most people seem to expect to be there):

$DB::alias{'len'} = 's/^len(.*)/p length($1)/'; $DB::alias{'stop'} = 's/^stop (at|in)/b/'; $DB::alias{'.'} = 's/^\./p ' . '"\$DB::sub(\$DB::filename:\$DB::line):\t"' . ',\$DB::dbline[\$DB::line]/' ;

Other resources

You did try the -w switch, didn't you?


If your program exit() s or die() s, so does the debugger.

There's no builtin way to restart the debugger without exiting and coming back into it. You could use an alias like this:

$DB::alias{'rerun'} = 'exec "perl -d $DB::filename"';

But you'd lose any pending breakpoint information, and that might not be the right path, etc.