Search notes:

GDB (GNU debugger)

Some commands:

start - start the program
p - print values of variables (might use frame to change the context)
x - examine (print) low level data structers: memory, registers …
s - step an instruction
n - step over a function
display/i $pc - Show the current instruction.
watch - interrupt the program when the watched variable changes
rwatch (?) - interrupt the program when the value of a variable is read
list - show the program source code (if compiled with -g)
break - set a breakpoint
bt - backtrace
set var varName = 42 - set the value of varName to 42.
tui enable: start TUI mode (Text User Interface).
quit

Setting the syntax of disassembled code:

set disassembly-flavor intel

Compare with

The -M option of objdump chooses the assembly dialect.
.intel_syntax noprefix in the GNU assembler (as)

Python

gdb is built with a python interpreter.

For example, existing breakpoints can be displayed with

(gdb) python print(gdb.breakpoints())
(gdb) python print(gdb.breakpoints()[0].location)

See also

/usr/include/sys/user.h

Apparently, on Linux, gdb used ptrace to debug a program.