Debugger interaction typically follows a loop pattern. First, you identify the location(s) you want to inspect, and tell the debugger to stop execution once one of these breakpoints is reached. Here's a command that could instruct a command-line debugger to stop at Line 239:

(debugger) break 239
(debugger) _

Then you have the debugger resume or start execution. The debugger will stop at the given location.

(debugger) continue
Line 239: s = x
(debugger) _

When it stops at the given location, you use debugger commands to inspect the state (and check whether things are as expected).

(debugger) print s
s = 'abc'
(debugger) _

You can then step through the program, executing more lines.

(debugger) step
Line 240: c = s[0]
(debugger) print c
c = 'a'
(debugger) _

You can also define new stop conditions, investigating other locations, variables, and conditions.

break      -- Set a breakpoint in given line. If no line is given, list all breakpoints
continue   -- Resume execution
delete     -- Delete breakpoint in line given by `arg`.
           Without given line, clear all breakpoints
help       -- Give help on given `command`. If no command is given, give help on all
list       -- Show current function. If `arg` is given, show its source code.
print      -- Print an expression. If no expression is given, print all variables
quit       -- Finish execution
step       -- Execute up to the next line