Debugging

If you invoke perl with a `-d' switch, your script will be run under a debugging monitor. It will halt before the first executable statement and ask you for a command, such as:

`h'

Prints out a help message.

`T'

Stack trace.

`s'

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

`n'

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

`f'

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

`c'

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.

`<CR>'

Repeat last n or s.

`n'

Single step around subroutine call.

`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.

`l'

List next window.

`-'

List previous window.

`w line'

List window around line

`l subname'

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

`/pattern/'

Regular expression search forward for pattern; the final `/' is optional.

`?pattern?'

Regular expression search backward for pattern; the final `?' is optional.

`L'

List lines that have breakpoints or actions.

`S'

Lists the names of all subroutines.

`t'

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.

`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.

`D'

Delete all breakpoints.

`a line command'

Set an action for line. A multi-line command may be entered by backslashing the newlines.

`A'

Delete all line actions.

`< command'

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

`V package'

List all variables in package. Default is `main' package.

`! number'

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

`! -number'

Redo the command that was that many (number) commands ago.

`H -number'

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

`q'

`^D'

Quit.

`command'

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.

If you want to modify the debugger, copy `perldb.pl' from the perl library to your current directory and modify it as necessary. (You'll also have to put `-I.' on your command line.) You can do some customization by setting up a `.perldb' file which contains initialization code. For instance, you could make aliases like these:

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