If you invoke Perl with the -d switch, your script runs under the Perl source debugger. This works like an interactive Perl environment, prompting for debugger commands that let you examine source code, set breakpoints, get stack backtraces, change the values of variables, etc. This is so convenient that you often fire up the debugger all by itself just to test out Perl constructs interactively to see what they do. For example:

    $ perl -d -e 42

In Perl, the debugger is not a separate program the way it usually is in the typical compiled environment. Instead, the -d flag tells the compiler to insert source information into the parse trees it's about to hand off to the interpreter. That means your code must first compile correctly for the debugger to work on it. Then when the interpreter starts up, it preloads a special Perl library file containing the debugger.

The program will halt right before the first run-time executable statement (but see below regarding compile-time statements) and ask you to enter a debugger command. Contrary to popular expectations, whenever the debugger halts and shows you a line of code, it always displays the line it's about to execute, rather than the one it has just executed.

Any command not recognized by the debugger is directly executed (eval'd) as Perl code in the current package. (The debugger uses the DB package for keeping its own state information.)

Note that the said eval is bound by an implicit scope. As a result any newly introduced lexical variable or any modified capture buffer content is lost after the eval. The debugger is a nice environment to learn Perl, but if you interactively experiment using material which should be in the same scope, stuff it in one line.

For any text entered at the debugger prompt, leading and trailing whitespace is first stripped before further processing. If a debugger command coincides with some function in your own program, merely precede the function with something that doesn't look like a debugger command, such as a leading ; or perhaps a +, or by wrapping it with parentheses or braces.

Calling the debugger

There are several ways to call the debugger:

perl -d program_name: On the given program identified by program_name.
perl -d -e 0: Interactively supply an arbitrary expression using -e.
perl -d:Ptkdb program_name: Debug a given program via the Devel::Ptkdb GUI.
perl -dt threaded_program_name: Debug a given program using threads (experimental).

Debugger Commands

The interactive debugger understands the following commands:

h

Prints out a summary help message

h [command]

Prints out a help message for the given debugger command.

h h

The special argument of h h produces the entire help page, which is quite long.

If the output of the h h command (or any command, for that matter) scrolls past your screen, precede the command with a leading pipe symbol so that it's run through your pager, as in

    DB> |h h

You may change the pager which is used via o pager=... command.

p expr

Same as print {$DB::OUT} expr in the current package. In particular, because this is just Perl's own print function, this means that nested data structures and objects are not dumped, unlike with the x command.

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

x [maxdepth] expr

Evaluates its expression in list context and dumps out the result in a pretty-printed fashion. Nested data structures are printed out recursively, unlike the real print function in Perl. When dumping hashes, you'll probably prefer 'x \%h' rather than 'x %h'. See the Dumpvalue manpage if you'd like to do this yourself.

The output format is governed by multiple options described under Configurable Options.

If the maxdepth is included, it must be a numeral N; the value is dumped only N levels deep, as if the dumpDepth option had been temporarily set to N.

V [pkg [vars]]

Display all (or some) variables in package (defaulting to main) 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

Use ~pattern and !pattern for positive and negative regexes.

This is similar to calling the x command on each applicable var.

X [vars]

Same as V currentpackage [vars].

y [level [vars]]

Display all (or some) lexical variables (mnemonic: mY variables) in the current scope or level scopes higher. You can limit the variables that you see with vars which works exactly as it does for the V and X commands. Requires the PadWalker module version 0.08 or higher; will warn if this isn't installed. Output is pretty-printed in the same style as for V and the format is controlled by the same options.

T

Produce a stack backtrace. See below for details on its output.

s [expr]

Single step. Executes until the beginning of another statement, descending into subroutine calls. If an expression is supplied that includes function calls, it too will be single-stepped.

n [expr]

Next. Executes over subroutine calls, until the beginning of the next statement. If an expression is supplied that includes function calls, those functions will be executed with stops before each statement.

r

Continue until the return from the current subroutine. Dump the return value if the PrintRet option is set (default).

<CR>

Repeat last n or s command.

c [line|sub]

Continue, optionally inserting a one-time-only breakpoint at the specified line or subroutine.

l

List next window of lines.

l min+incr

List incr+1 lines starting at min.

l min-max

List lines min through max. l - is synonymous to -.

l line

List a single line.

l subname

List first window of lines from subroutine. subname may be a variable that contains a code reference.

-

List previous window of lines.

v [line]

View a few lines of code around the current line.

.

Return the internal debugger pointer to the line last executed, and print out that line.

f filename

Switch to viewing a different file or eval statement. If filename is not a full pathname found in the values of %INC, it is considered a regex.

evaled strings (when accessible) are considered to be filenames: f (eval 7) and f eval 7\b access the body of the 7th evaled string (in the order of execution). The bodies of the currently executed eval and of evaled strings that define subroutines are saved and thus accessible.

/pattern/

Search forwards for pattern (a Perl regex); final / is optional. The search is case-insensitive by default.

?pattern?

Search backwards for pattern; final ? is optional. The search is case-insensitive by default.

L [abw]

List (default all) actions, breakpoints and watch expressions

S [[!]regex]

List subroutine names [not] matching the regex.

t

Toggle trace mode (see also the AutoTrace option).

t expr

Trace through execution of expr. See Frame Listing Output Examples in the perldebguts manpage for examples.

b

Sets breakpoint on current line

b [line] [condition]

Set a breakpoint before the given line. If a condition is specified, it's evaluated each time the statement is reached: 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 $x > 30
    b 237 ++$count237 < 11
    b 33 /pattern/i

b subname [condition]

Set a breakpoint before the first line of the named subroutine. subname may be a variable containing a code reference (in this case condition is not supported).

b postpone subname [condition]

Set a breakpoint at first line of subroutine after it is compiled.

b load filename

Set a breakpoint before the first executed line of the filename, which should be a full pathname found amongst the %INC values.

b compile subname

Sets a breakpoint before the first statement executed after the specified subroutine is compiled.

B line

Delete a breakpoint from the specified line.

B *

Delete all installed breakpoints.

a [line] command

Set an action to be done before the line is executed. If line is omitted, set an action on the line about to be executed. The sequence of steps taken by the debugger is

  1. check for a breakpoint at this line
  2. print the line if necessary (tracing)
  3. do any actions associated with that line
  4. prompt user if at a breakpoint or in single-step
  5. evaluate line

For example, this will print out $foo every time line 53 is passed:

    a 53 print "DB FOUND $foo\n"

A line

Delete an action from the specified line.

A *

Delete all installed actions.

w expr

Add a global watch-expression. We hope you know what one of these is, because they're supposed to be obvious.

W expr

Delete watch-expression

W *

Delete all watch-expressions.

o

Display all options

o booloption ...

Set each listed Boolean option to the value 1.

o anyoption? ...

Print out the value of one or more options.

o option=value ...

Set the value of one or more options. If the value has internal whitespace, it should be quoted. For example, you could set o pager="less -MQeicsNfr" to call less with those specific options. You may use either single or double quotes, but if you do, you must escape any embedded instances of same sort of quote you began with, as well as any escaping any escapes that immediately precede that quote but which are not meant to escape the quote itself. In other words, you follow single-quoting rules irrespective of the quote; eg: o option='this isn\'t bad' or o option="She said, \"Isn't it?\"".

For historical reasons, the =value is optional, but defaults to 1 only where it is safe to do so--that is, mostly for Boolean options. It is always better to assign a specific value using =. The option can be abbreviated, but for clarity probably should not be. Several options can be set together. See Configurable Options for a list of these.

< ? >

List out all pre-prompt Perl command actions.

< [ command ] >

Set an action (Perl command) to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines.

< * >

Delete all pre-prompt Perl command actions.

<< command >

Add an action (Perl command) to happen before every debugger prompt. A multi-line command may be entered by backwhacking the newlines.

> ? >>

List out post-prompt Perl command actions.

> command >>

Set an action (Perl command) to happen after the prompt when you've just given a command to return to executing the script. A multi-line command may be entered by backslashing the newlines (we bet you couldn't have guessed this by now).

> * >>

Delete all post-prompt Perl command actions.

>> command > >>>

Adds an action (Perl command) to happen after the prompt when you've just given a command to return to executing the script. A multi-line command may be entered by backslashing the newlines.

{ ?

List out pre-prompt debugger commands.

{ [ command ]

Set an action (debugger command) to happen before every debugger prompt. A multi-line command may be entered in the customary fashion.

Because this command is in some senses new, a warning is issued if you appear to have accidentally entered a block instead. If that's what you mean to do, write it as with ;{ ... } or even do { ... }.

{ *

Delete all pre-prompt debugger commands.

{{ command

Add an action (debugger command) to happen before every debugger prompt. A multi-line command may be entered, if you can guess how: see above.

! number

Redo a previous command (defaults to the previous command).

! -number

Redo number'th previous command.

! pattern

Redo last command that started with pattern. See o recallCommand, too.

!! cmd

Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See o shellBang, also. Note that the user's current shell (well, their $ENV{SHELL} variable) will be used, which can interfere with proper interpretation of exit status or signal and coredump information.

source file

Read and execute debugger commands from file. file may itself contain source commands.

H -number

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

q or ^D

Quit. ("quit" doesn't work for this, unless you've made an alias) This is the only supported way to exit the debugger, though typing exit twice might work.

Set the inhibit_exit option to 0 if you want to be able to step off the end the script. You may also need to set $finished to 0 if you want to step through global destruction.

R

Restart the debugger by exec()ing a new session. We try to maintain your history across this, but internal settings and command-line options may be lost.

The following setting are currently preserved: history, breakpoints, actions, debugger options, and the Perl command-line options -w, -I, and -e.

|dbcmd

Run the debugger command, piping DB::OUT into your current pager.

||dbcmd

Same as |dbcmd but DB::OUT is temporarily selected as well.

= [alias value]

Define a command alias, like

    = quit q

or list current aliases.

command

Execute command as a Perl statement. A trailing semicolon will be supplied. If the Perl statement would otherwise be confused for a Perl debugger, use a leading semicolon, too.

m expr

List which methods may be called on the result of the evaluated expression. The expression may evaluated to a reference to a blessed object, or to a package name.

M

Displays all loaded modules and their versions

man [manpage]

Despite its name, this calls your system's default documentation viewer on the given page, or on the viewer itself if manpage is omitted. If that viewer is man, the current Config information is used to invoke man using the proper MANPATH or -M manpath option. Failed lookups of the form XXX that match known manpages of the form perlXXX will be retried. This lets you type man debug or man op from the debugger.

On systems traditionally bereft of a usable man command, the debugger invokes perldoc. Occasionally this determination is incorrect due to recalcitrant vendors or rather more felicitously, to enterprising users. If you fall into either category, just manually set the $DB::doccmd variable to whatever viewer to view the Perl documentation on your system. This may be set in an rc file, or through direct assignment. We're still waiting for a working example of something along the lines of:

    $DB::doccmd = 'netscape -remote http://something.here/';

Configurable Options

The debugger has numerous options settable using the o command, either interactively or from the environment or an rc file. (./.perldb or ~/.perldb under Unix.)

recallCommand, ShellBang: The characters used to recall command or spawn shell. By default, both are set to !, which is unfortunate.
pager: Program to use for output of pager-piped commands (those beginning with a | character.) By default, $ENV{PAGER} will be used. Because the debugger uses your current terminal characteristics for bold and underlining, if the chosen pager does not pass escape sequences through unchanged, the output of some debugger commands will not be readable when sent through the pager.
tkRunning: Run Tk while prompting (with ReadLine).
signalLevel, warnLevel, dieLevel: Level of verbosity. By default, the debugger leaves your exceptions and warnings alone, because altering them can break correctly running programs. It will attempt to print a message when uncaught INT, BUS, or SEGV signals arrive. (But see the mention of signals in BUGS below.); To disable this default safe mode, set these values to something higher than 0. At a level of 1, you get backtraces upon receiving any kind of warning (this is often annoying) or exception (this is often valuable). Unfortunately, the debugger cannot discern fatal exceptions from non-fatal ones. If dieLevel is even 1, then your non-fatal exceptions are also traced and unceremoniously altered if they came from eval'ed strings or from any kind of eval within modules you're attempting to load. If dieLevel is 2, the debugger doesn't care where they came from: It usurps your exception handler and prints out a trace, then modifies all exceptions with its own embellishments. This may perhaps be useful for some tracing purposes, but tends to hopelessly destroy any program that takes its exception handling seriously.
AutoTrace: Trace mode (similar to t command, but can be put into PERLDB_OPTS).
LineInfo: File or pipe to print line number info to. If it is a pipe (say, |visual_perl_db), then a short messag