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Debuggers

Ruby comes with a debugger, accessible on the command line, for stepping through problems with programs (see Figure 9.1). Type the following to access it:

Ruby r debug MyProgramScript.rb

The debugger has a number of useful commands for, well, debugging a Ruby program. These are listed in Table 9.1.

Table 9.1. Debug Commands

Command	Use
break	Set breakpoint at specified line or method
watch	Set a watchpoint for an expression
catch	Set a catchpoint for an exception
delete	Delete breakpoints or watchpoints
display	Set display expression to be printed when program stops
undisplay	Unset display
cont	Continue program execution
step	Step forward in the program until the next source line
next	Step forward in the program until the next source line. Treat method calls as one instruction
list	List line of source code
up	Select stack frame that called current stack frame
down	Select stack frame called by current stack frame
finish	Execute until selected stack frame returns
trace	Turn trace mode on or off
quit	Exit debugger
var global	Show global variables in current stack frame
var local	Show local variables in current stack frame
var instance	Show instance variables of the given object
var const	Show constants of the given object
method instance	Show methods of the given object
method	Show instance methods of the given class or module
thread list	Show thread list
thread current	Show current thread
thread	Switch to a given thread
thread stop	Stop the given thread
thread resume	Resume the given thread
p	Evaluate the given expression in current stack frame and show its value
help	Print debug commands
else	Evaluate input in the current stack frame and show its value

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