Basic GDB commands.

Break points:

b F                     Set a break-point at function F.
b *A                    Set a break-point at absolute address A
b N                     Set a break-point at line number N.
b N:F                   Set a break-point at line number N at file F.


info b                  Lists break-points.
cond B cond             Set a condition to a break-point B.
cond B                  Remove the condition of a break-point B.
delete B                Delete a break-point B.

Stepping:

stepi or si             Execute one machine instruction (follows a call).
step or s               Execute one C-program statement (steps into functions).
stepi N                 Do N machine instructions.
nexti or ni             Same as si but execute calls as one instructions.
next or n               Same as ni but execute functions as one statement.
bt                      Show names of the stack frames.
up                      Go up one stack frame.
down                    Go down one stack frame.

Examining:

info reg                List contents of registers.
p V                     Print contents of a variable V.
x /CT A                 Examining memory where:
                        C   number of units to display. 
                        T   x hex integer.
                            d dec integer.
                            u unsigned dec integer.
                            o octal integer.
                            c character.
                            s null terminated string.
                            i as machine instruction.
                        A   an absolute address or 
                            $reg pointed by some register.