Simply shellcode is opcode in machine language.
This can be extract from low level languages such as assembly, c
Lets say there is a c file which do something called dosomething.c
gcc -o dosomething.o dosomething.c
When execute shellcode.o it will do something. We need to get machine code of this process.
C:\Users\Ali\Desktop>objdump -D shellcode.o
Now we can see machine code and asm instructions. Using this we can build the shellcode.
00000000 <.text>:
0: 31 c0 xor %eax,%eax
2: 50 push %eax
3: b8 41 41 41 64 mov $0x64414141,%eax
8: c1 e8 08 shr $0x8,%eax
b: c1 e8 08 shr $0x8,%eax
e: c1 e8 08 shr $0x8,%eax
