!cmd !desc
!controlling execution  
r arg1 ... run program
s step through program
si step one instruction exactly
c continue running program
b funcname (or line) set breakpoint at function
! ...  
bt backtrace
l list surrounding source
info locals show local variables
info args show arguments
up/down n go up or down n frames in the stack
disassemble disassemble section of memory
p/x variable print variable in hexadecimal
x addr Examine memory at addr
show env Show environment
set env FOO = bar Set environment variable FOO to bar
whatis variable show variable type
set var variable=value Set variable to value
set env variable=value Set environment variable to value
attach pid Attach gdb to already running process
set print pretty on pretty print structures
set print array on readable array output
set print array-indexes on show array indices
set print demangle on demangle C++ names
info registers show registers (excluding floating point)
macro expand macro expands a macro (requires -gdwarf-2 -g3 compiler flags)
code print type
x hexadecimal
u unsigned int
d signed int

Check a core dump


gdb executable coredump