gdb

Navigation:

c                    Continue 
n                    Next line (Step over)
s                    Step into
jump *$pc+8          Jump current program counter 8 bytes forward 

Breakpoints:

break main           Set breakpoint on functions named `main`
break test.c:12      Set breakpoint line 12 in test.c
info break           List all breakpoints
delete 1             Delete breakpoint 1

Printing:

p name               Print `name` variable
po name              Print objective-c description of `name`
info registers       Print registers for current thread
info locals          Print local variables for current frame

Stack:

bt                   Show call stack
thread apply all bt  Show call stack for all threads
frame 12             Select stack frame 12
up                   Select higher stack frame
down                 Select lower stack frame
up 2                 Select stack frame 2 above current
down 3               Select stack frame 3 below current