Devel::Trepan -- A new modular Perl debugger


A modular, testable, gdb-like debugger in the family of the Ruby trepanning debuggers.


  • extensive online-help

  • syntax highlighting of Perl code

  • context-sensitive command completion

  • out-of-process and remote debugging

  • interactive shell support

  • code disassembly

  • gdb syntax

  • easy extensibility at several levels: aliases, commands, and plugins

  • comes with extensive tests

  • is not as ugly as perl5db

Some of the features above require additional modules before they take effect. See "Plugins" and "Recommended Modules" below.



From a shell:

    $ [trepan-opts] -- perl-program [perl-program-opts]

For out-of-process (and possibly out-of server) debugging:

    $ --server [trepan-opts] -- perl-program [perl-program-opts]

and then from another process or computer:

    $ --client [--host DNS-NAME-OR-IP]

Calling the debugger from inside your Perl program using Joshua ben Jore's Enbugger:

    # This needs to be done once and could even be in some sort of
    # conditional code
    require Enbugger; Enbugger->load_debugger( 'trepan' );

    # Alternatively, to unconditionally load Enbugger and trepan:
    use Enbugger 'trepan';

    # work, work, work...
    # Oops! there was an error! Enable the debugger now!
    Enbugger->stop;  # or Enbugger->stop if ...

Or if you just want POSIX-shell-like set -x line tracing:

    $ -x -- perl-program [perl-program-opts]

Inside the debugger tracing is turned on using the command set trace print. There is extensive help from the help command.

Basic Commands

The help system follows the gdb classificiation. Below is not a full list of commands, nor does it contain the full list of options on each command, but rather some of the more basic commands and options.

Commands involving running the program

The commands in the section involve controlling execution of the program, either by kinds of stepping (step into, step over, step out) restarting or termintating the program altogether. However setting breakpoints is in "Making the program stop at certain points".

Step into (step)

step[<+|-] [into] [count]

Execute the current line, stopping at the next event. Sometimes this is called "step into".

With an integer argument, step that many times. With an 'until' expression that expression is evaluated and we stop the first time it is true.

A suffix of + in a command or an alias forces a move to another position, while a suffix of - disables this requirement. A suffix of > will continue until the next call. (finish will run run until the return for that call.)

If no suffix is given, the debugger setting different determines this behavior.


    step        # step 1 event, *any* event obeying 'set different' setting
    step 1      # same as above
    step+       # same but force stopping on a new line
    step over   # same as 'next'
    step out    # same as 'finish'

Related and similar is the next (step over) and finish (step out) commands. All of these are slower than running to a breakpoint.

Step over (next)


Step one statement ignoring steps into function calls at this level. Sometimes this is called "step over".

Continue execution (continue)

continue [location]

Leave the debugger loop and continue execution. Subsequent entry to the debugger however may occur via breakpoints or explicit calls, or exceptions.

If a parameter is given, a temporary breakpoint is set at that position before continuing.


 continue 10    # continue to line 10
 continue gcd   # continue to first instruction of method gcd

See also step, next, finish commands and help location.

Step out (finish)


Continue execution until the program is about to leave the current function. Sometimes this is called 'step out'.

Gently exit debugged program (quit)

quit[!] [unconditionally] [exit-code]

Gently exit the debugger and debugged program.

The program being debugged is exited via exit() which runs the Kernel at_exit() finalizers. If a return code is given, that is the return code passed to exit() - presumably the return code that will be passed back to the OS. If no exit code is given, 0 is used.


 quit                 # quit prompting if we are interactive
 quit unconditionally # quit without prompting
 quit!                # same as above
 quit 0               # same as "quit"
 quit! 1              # unconditional quit setting exit code 1

See also set confirm and kill.

Hard termination (kill)

kill[!] [signal-number|signal-name]

Kill execution of program being debugged.

Equivalent of kill('KILL', $$). This is an unmaskable signal. Use this when all else fails, e.g. in thread code, use this.

If you are in interactive mode, you are prompted to confirm killing. However when this command is aliased from a command ending in !, no questions are asked.


 kill KILL # same as above
 kill -9   # same as above
 kill  9   # same as above
 kill! 9   # same as above, but no questions asked
 kill unconditionally # same as above
 kill TERM # Send "TERM" signal

See also quit

Restart execution (restart)


Restart debugger and program via an exec call.

See also show args for the exact invocation that will be used.

Examining data

Evaluate Perl code (eval)

eval[@$][?] [Perl-code]

Run Perl-code in the context of the current frame.

If no string is given after the word "eval", we run the string from the current source code about to be run. If the "eval" command ends ? (via an alias) and no string is given we try to pick out a useful expression in the line.

Normally eval assumes you are typing a statement, not an expression; the result is a scalar value. However you can force the type of the result by adding the appropriate sigil @, or $.


    eval 1+2 # 3
    eval$ 3   # Same as above, but the return type is explicit
    $ 3       # Probably same as above if $ alias is around
    eval $^X  # Possibly /usr/bin/perl
    eval      # Run current source-code line
    eval?     # but strips off leading 'if', 'while', ..
              # from command
    eval @ARGV  # Make sure the result saved is an array rather than
                # an array converted to a scalar.
    @ @ARG       # Same as above if @ alias is around
    use English  # Note this is a statement, not an expression
    use English; # Same as above
    eval$ use English # Error because this is not a valid expression

See also set auto eval to treat unrecognized debugger commands as Perl code.

Recursively Debug into Perl code (debug)

debug Perl-code

Recursively debug Perl-code.

The level of recursive debugging is shown in the prompt. For example (( indicates one nested level of debugging.


 debug finonacci(5)   # Debug fibonacci function
 debug $x=1; $y=2;    # Kind of pointless, but doable.

Making the program stop at certain points

A Breakpoint is a way to have the program stop at a pre-determined location. A breakpoint can be perminant or one-time. A one-time breakpoint is removed as soon as it is hit. In a sense, stepping is like setting one-time breakpoints. Breakpoints can also be disabled which allows you to temporarily ignore stopping at that breakpoint while it is disabled. Finally one can control conditions under which a breakpoint is enacted upon.

Another way to force a stop is to watch to see if the value of an expression changes. Often that expression is simply examinging a variable's value.

Set a breakpont (break)

break [location] [if condition]

Set a breakpoint. If location is given use the current stopping point. An optional condition may be given.


 break                  # set a breakpoint on the current line
 break gcd              # set a breakpoint in function gcd
 break gcd if $a == 1   # set a breakpoint in function gcd with
                        # condition $a == 1
 break 10               # set breakpoint on line 10

When a breakpoint is hit the event icon is xx.

See also help breakpoints.

Set a temporary breakpoint (tbreak)

tbreak [location]

Set a one-time breakpoint. The breakpoint is removed after it is hit. If no location is given use the current stopping point.


   tbreak 10               # set breakpoint on line 10

When a breakpoint is hit the event icon is x1.

See also break and help breakpoints.

Add or modify a condition on a breakpoint (condition)

condition bp-number Perl-expression

bp-number is a breakpoint number. perl-expresion is a Perl expression which must evaluate to true before the breakpoint is honored. If perl-expression is absent, any existing condition is removed; i.e., the breakpoint is made unconditional.


   condition 5 x > 10  # Breakpoint 5 now has condition x > 10
   condition 5         # Remove above condition

See also "break", "enable" and "disable".

Delete some breakpoints (delete)

delete [bp-number [bp-number...]]

Delete some breakpoints.

Arguments are breakpoint numbers with spaces in between. To delete all breakpoints, give no arguments.

See also the clear command which clears breakpoints by line number and info break to get a list of breakpoint numbers.

Enable some breakpoints (enable)

enable num [num ...]

Enables breakpoints, watch expressions or actions given as a space separated list of numbers which may be prefaces with an 'a', 'b', or 'w'. The prefaces are interpreted as follows:

a -- action number
b -- breakpoint number
w -- watch expression number

If num is starts with a digit, num is taken to be a breakpoint number.

Disable some breakpoints (disable)

disable bp-number [bp-number ...]

Disables the breakpoints given as a space separated list of breakpoint numbers. See also info break to get a list of breakpoints

Set an action before a line is executed (action)

action position Perl-statement

Set an action to be done before the line is executed. If line is ., 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 the value of $foo every time line 53 is passed:

Stop when an expression changes value (watch)

watch Perl-expression

Stop very time Perl-expression changes from its prior value.


 watch $a  # enter debugger when the value of $a changes
 watch scalar(@ARGV))  # enter debugger if size of @ARGV changes.

Examining the call stack

The commands in this section show the call stack and let set a reference for the default call stack which other commands like list or break use as a position when one is not specified.

The most recent call stack entry is 0. Except for the relative motion commands up and down, you can refer to the oldest or top-level stack entry with -1 and negative numbers refer to the stack from the other end.

Beware that in contrast to debuggers in other programming languages, Perl really doesn't have an easy way for one to evaluate statements and expressions other than at the most recent call stack. There are ways to see lexical variables my and our, however localized variables which can hide global variables and other lexicals variables can be problematic.

backtrace [count]

Print a stack trace, with the most recent frame at the top. With a positive number, print at most many entries.

In the listing produced, an arrow indicates the 'current frame'. The current frame determines the context used for many debugger commands such as source-line listing or the edit command.


 backtrace    # Print a full stack trace
 backtrace 2  # Print only the top two entries

Select a call frame (frame)

frame [frame-number]

Change the current frame to frame frame-number if specified, or the most-recent frame, 0, if no frame number specified.

A negative number indicates the position from the other or least-recently-entered end. So frame -1 moves to the oldest frame.


    frame     # Set current frame at the current stopping point
    frame 0   # Same as above
    frame .   # Same as above. 'current thread' is explicit.
    frame . 0 # Same as above.
    frame 1   # Move to frame 1. Same as: frame 0; up
    frame -1  # The least-recent frame

Move to a more recent frame (up)

up [count]

Move the current frame up in the stack trace (to an older frame). 0 is the most recent frame. If no count is given, move up 1.

Move to a less recent frame (down)

down [count]

Move the current frame down in the stack trace (to a newer frame). 0 is the most recent frame. If no count is given, move down 1.

Support facilities

Define an alias

alias alias command

Add alias alias for a debugger command command.

Add an alias when you want to use a command abbreviation for a command that would otherwise be ambigous. For example, by default we make s be an alias of step to force it to be used. Without the alias, s might be step, show, or set, among others.


 alias cat list   # "cat" is the same as "list"
 alias s   step   # "s" is now an alias for "step".
                  # The above examples done by default.

For more complex definitions, see macro. See also unalias and show alias.

Remove an unalias

unalias alias1 [alias2 ...]

Remove alias alias1 and so on.


 unalias s  # Remove 's' as an alias for 'step'

See also alias.

Define a debugger macro

macro macro-name sub { ... }

Define macro-name as a debugger macro. Debugger macros get a list of arguments which you supply without parenthesis or commas. See below for an example.

The macro (really a Perl anonymous subroutine) should return either a string or an array reference to a list of strings. The string in both cases are strings of debugger commands. If the return is a string, that gets tokenized by a simple split(/ /, $string). Note that macro processing is done right after splitting on ;; so if the macro returns a string containing ;; this will not be handled on the string returned.

If instead, a reference to a list of strings is returned, then the first string is shifted from the array and executed. The remaining strings are pushed onto the command queue. In contrast to the first string, subsequent strings can contain other macros. Any ;; in those strings will be split into separate commands.


The below creates a macro called fin+ which issues two commands finish followed by step:

 macro fin+ sub{ ['finish', 'step']}

If you wanted to parameterize the argument of the finish command you could do it this way:

  macro fin+ sub{ \
                  ['finish', 'step ' . (shift)] \

Invoking with:

  fin+ 3

would expand to ["finish", "step 3"]

If you were to add another parameter, note that the invocation is like you use for other debugger commands, no commas or parenthesis. That is:

 fin+ 3 2

rather than fin+(3,2) or fin+ 3, 2.

See also info macro.

Gently exit debugged program (quit)

quit[!] [unconditionally] [exit-code]

Allow remote debugger connections (server)

server [options]


    -p | --port NUMBER
    -a | --address

Suspends interactive debugger session and puts debugger in server mode which opens a socket for debugger connections

Run debugger commands from a file (source)

source [options] file


    -q | --quiet | --no-quiet
    -c | --continue | --no-continue
    -Y | --yes | -N | --no
    -v | --verbose | --no-verbose

Read debugger commands from a file named file. Optional -v switch causes each command in FILE to be echoed as it is executed. Option -Y sets the default value in any confirmation command to be 'yes' and -N sets the default value to 'no'.

Option -q will turn off any debugger output that normally occurs in the running of the program.

An error in any command terminates execution of the command file unless option -c or --continue is given.

Load or Reload something Perlish (load)

Sometimes in the middle of debugging you would like to make a change to a Perl module -- perhaps you've found a bug -- and start using that. If the change is inside a Perl module, you can use the command load module:

load module {Perl-module-file}

Another thing that can occur especially if using Enbugger is that the source to Perl code is not cached inside the debugger and so you can't set a breakpoint on lines in that module. load source can be used to rectify this. However this is a bit experimenta. There may still be a problem in making sure that debugging turned on when tracing inside of that source.

load source {Perl-source_file}

Finally, if you have debugger commands of your own or if you change a debugger command, you can force a reread of that debugger command using load command.

load commmand {file-or-directory-name-1 [file-or-directory-name-2...]}

Modify parts of the Debugger Environment (set, show)

There are many parts of the debugger environment you can change, like the print line width, whether you want syntax highlighting or not and so on. These fall under set commands. show commands show you values that have been set. In fact, many of the set commands finish by runnin the corresponding "show" to echo you see what you've just set.

Set commands


Set to allow unique abbreviations of commands


Set controls for some "automatic" default behaviors


Set to show only file basename in showing file names


Set whether to confirm potentially dangerous operations.


Set debugging controls


Set to make sure 'next/step' move to a new position.


Set display attributes


Set whether we use terminal highlighting


Set maximum length sizes of various things


Set the value about to be returned


Influence how filenames in the debugger map to local filenames


Set to show elapsed time between debugger events


Set tracing of various sorts.


Set a my or our variable

Show commands


Show whether we allow abbreviated debugger command names


Show defined aliases


Arguments to restart program


Show controls for things with some sort of "automatic" default behavior


Show only file basename in showing file names


Show confirm potentially dangerous operations setting


Show debugging controls


Show status of 'set different'


Show display-related controls


Show whether we use terminal highlighting


Show whether debugger input is a terminal


Show "maximum length" settings


Show status of the timing hook


Set tracing of various sorts


Show debugger name and version

Syntax of debugger commands

Overall Debugger Command Syntax

If the first non-blank character of a line starts with #, the command is ignored.

Commands are split at whereever ;; appears. This process disregards any quotes or other symbols that have meaning in Perl. The strings after the leading command string are put back on a command queue.

Within a single command, tokens are then white-space split. Again, this process disregards quotes or symbols that have meaning in Perl. Some commands like eval, macro, and break have access to the untokenized string entered and make use of that rather than the tokenized list.

Resolving a command name involves possibly 4 steps. Some steps may be omitted depending on early success or some debugger settings:

1. The leading token is first looked up in the macro table. If it is in the table, the expansion is replaces the current command and possibly other commands pushed onto a command queue. See the "help macros" for help on how to define macros, and "info macro" for current macro definitions.

2. The leading token is next looked up in the debugger alias table and the name may be substituted there. See "help alias" for how to define aliases, and "show alias" for the current list of aliases.

3. After the above, The leading token is looked up a table of debugger commands. If an exact match is found, the command name and arguments are dispatched to that command. Otherwise, we may check to see the the token is a unique prefix of a valid command. For example, "dis" is not a unique prefix because there are both "display" and "disable" commands, but "disp" is a unique prefix. You can allow or disallow abbreviations for commands using "set abbrev". The default is abbreviations are on.

4. If after all of the above, we still don't find a command, the line may be evaluated as a Perl statement in the current context of the program at the point it is stoppped. However this is done only if "auto eval" is on. (It is on by default.)

If "auto eval" is not set on, or if running the Perl statement produces an error, we display an error message that the entered string is "undefined".

Debugger Command Examples


 # This line does nothing. It is a comment and is useful
 # in debugger command files.
      # any amount of leading space is also ok

Splitting Commands

The following runs two commands: info program and list

 info program;; list

The following gives a syntax error since ;; splits the line and the simple debugger parse then thinks that the quote (") is not closed.

 print "hi ;;-)\n"

If you have the Devel::Trepan::Shell plugin, you can go into a real shell and run the above.

Command Continuation

If you want to continue a command on the next line use \ at the end of the line. For example:

 eval $x = "This is \
 a multi-line string"

The string in variable $x will have a \n before the article "a".

Command suffixes which have special meaning

Some commands like step, or list do different things when an alias to the command ends in a particular suffix like ">".

Here are a list of commands and the special suffixes:

    command   suffix
    -------   ------
    list       >
    step       +,-,<,>
    next       +,-,<,>
    quit       !
    kill       !
    eval       ?

See help on the commands listed above for the specific meaning of the suffix.


Because this should be useful in all sorts of environments such as back to perl 5.008, we often can make use of newer Perlisms nor can we require by default all of the modules, say for data printing, stack inspection, or interactive terminal handling. That said, if you have a newer Perl or the recommended modules or install plugins, you'll get more.

Although modular, this program is even larger than perl5db and so it loads a little slower. I think part of the slowness is the fact that there are over 70 or so (smallish) files (rather than one nearly 10K file) and because relative linking via rlib is used to glue them together.


Rocky Bernstein


My Devel::Trepan blogs and wiki


Other Debuggers

  • perldebug is perl's built-in tried-and-true debugger that other debuggers will ultimately be compared with

  • Devel::ebug

  • DB is a somewhat abandoned debugger API interface. I've tried to use some parts of this along with perl5db.


Copyright (C) 2011, 2012 Rocky Bernstein <>

This program is distributed WITHOUT ANY WARRANTY, including but not limited to the implied warranties of merchantability or fitness for a particular purpose.

The program is free software. You may distribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation (either version 2 or any later version) and the Perl Artistic License as published by O'Reilly Media, Inc. Please open the files named gpl-2.0.txt and Artistic for a copy of these licenses.