Compile and run
nasm -f elf32 -o helloworld.o helloworld.nasm && ld -m elf_i386 -o helloworld helloworld.o && ./helloworld ; echo $?

Get shellcode
for i in $(objdump -d helloworld -M intel |grep "^ " |cut -f2); do echo -n '\x'$i; done;echo