Write a self-replicating program.
Assembly for x86 Linux, 106 bytes
BITS 32
org 0x2E620000
db 0x7F, "ELF", 1, 1, 1, 0 ; e_ident
dd 0, 0
dw 2 ; e_type
dw 3 ; e_machine
dd 1 ; e_version
dd _start ; e_entry
dd phdr - $$ ; e_phoff
dd 0 ; e_shoff
dd 0 ; e_flags
dw 0x34 ; e_ehsize
dw 0x20 ; e_phentsize
phdr: dd 1 ; e_phnum ; p_type
; e_shentsize
dd 0 ; e_shnum ; p_offset
; e_shstrndx
dd $$ ; p_vaddr
fname equ $ - 2
db 'out', 0 ; p_paddr
dd filesize ; p_filesz
dd filesize ; p_memsz
dd 5 ; p_flags
dd 0x1000 ; p_align
_start: mov al, 5 ; 5 = open syscall
mov ebx, fname
mov cl, 65 ; 65 = O_WRONLY | O_CREAT
mov dx, 666q
int 0x80
lea edx, [byte ecx + filesize - 65]
xchg eax, ebx
xchg eax, ecx
mov cl, 0
mov al, 4 ; 4 = write syscall
int 0x80
mov al, 1 ; 1 = exit syscall
int 0x80
filesize equ $ - $$
This is for the nasm assembler. Build the binary with the command line: nasm -f bin -o a.out selfrep.asm && chmod +x a.out
Here's the same file as a hex dump: 7F 45 4C 46 01 01 01 00 00 00 00 00 00 00 00 00 02 00 03 00 01 00 00 00 4C 00 62 2E 2C 00 00 00 00 00 00 00 00 00 00 00 34 00 20 00 01 00 00 00 00 00 00 00 00 00 62 2E 6F 75 74 00 6A 00 00 00 6A 00 00 00 05 00 00 00 00 10 00 00 B0 05 BB 36 00 62 2E B1 41 66 BA B6 01 CD 80 8D 51 29 93 91 B1 00 B0 04 CD 80 B0 01 CD 80
As requested, the program copies itself to a separate file. (The program could have been significantly shorter if it had been allowed to just write to stdout and let the user redirect to a file.)
I avoided using any borderline tricks to reduce the size. This should be a fully conformant 32-bit ELF binary.
Edited to add: In the above version the created file is just a plain file, but it occurs to me that for a couple of bytes (and a tiny bend of the rules), you can create something a little more interesting. This version is only two bytes longer, at 108 bytes:
BITS 32
org 0x00010000
db 0x7F, "ELF", 1, 1, 1, 0 ; e_ident
dd 0, 0
dw 2 ; e_type
dw 3 ; e_machine
dd 1 ; e_version
dd _start ; e_entry
dd phdr - $$ ; e_phoff
dd 0 ; e_shoff
dd 0 ; e_flags
dw 0x34 ; e_ehsize
dw 0x20 ; e_phentsize
phdr: dd 1 ; e_phnum ; p_type
; e_shentsize
dd 0 ; e_shnum ; p_offset
; e_shstrndx
dd $$ ; p_vaddr
fname: db 'asr', 0 ; p_paddr
dd filesize ; p_filesz
dd filesize ; p_memsz
dd 7 ; p_flags
dd 0x1000 ; p_align
_start: mov al, 5 ; 5 = open syscall
mov ebx, fname
inc byte [ebx]
mov cl, 65 ; 65 = O_WRONLY | O_CREAT
mov dx, 777q
int 0x80
lea edx, [byte ecx + filesize - 65]
xchg eax, ebx
xchg eax, ecx
mov cl, 0
mov al, 4 ; 4 = write syscall
int 0x80
mov al, 1 ; 1 = exit syscall
int 0x80
filesize equ $ - $$
Name this version asr, for "a self-replicator": nasm -f bin -o asr asr.asm && chmod +x asr
Hex dump version for the nasm-impaired:
7F 45 4C 46 01 01 01 00 00 00 00 00 00 00 00 00 02 00 03 00 01 00 00 00 4C 00 01 00 2C 00 00 00 00 00 00 00 00 00 00 00 34 00 20 00 01 00 00 00 00 00 00 00 00 00 01 00 61 73 72 00 6C 00 00 00 6C 00 00 00 07 00 00 00 00 10 00 00 B0 05 BB 38 00 01 00 FE 03 B1 41 66 BA FF 01 CD 80 8D 51 2B 93 91 B1 00 B0 04 CD 80 B0 01 CD 80
When you run it, it creates an almost-identical file named bsr, but one that is itself executable. Running it will create another binary file named csr. And so on.
(Note that annoying things start to happen after zsr. I considered making a version that would cascade the name change to atr and so on, but I think most people will get bored well before then, so it probably isn't worth all the extra bytes.)
Bash, 236
Longer than strictly necessary, but I hate long lines. The trailing newline is non-optional.
b=\\ q=\' r=\> d=\$
s='exec >$$; echo b=$b$b q=$b$q r=$b$r d=$b$d; echo s=$q$s$q'
t='echo t=$q$t$q; echo $s; echo $t; chmod 777 $$'
exec >$$; echo b=$b$b q=$b$q r=$b$r d=$b$d; echo s=$q$s$q
echo t=$q$t$q; echo $s; echo $t; chmod 777 $$
Here is a proof-of-concept (ungolfed) that shows how the Compilation services in .NET might be used to compile the source code on-the-fly to generate an identical output. The first copy is not identical to the original, but subsequent copies from subsequent runs are exactly identical with random file names:
using System;
using Microsoft.CSharp;
using System.CodeDom.Compiler;
namespace _2947
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Hello world!");
var s = @"
using System;
using System.CodeDom.Compiler;
using Microsoft.CSharp;
namespace _2947
{{
class Program
{{
static void Main(string[] args)
{{
Console.WriteLine({1}Hello world!{1});
var s = @{1}{0}{1};
s = string.Format(s, s, '{1}');
string exeName = Environment.CurrentDirectory + @{1}\{1} + new Random().Next(1000, 9999) + {1}.exe{1};
CompilerParameters cp = new CompilerParameters();
cp.GenerateExecutable = true;
cp.OutputAssembly = exeName;
cp.GenerateInMemory = false;
cp.TreatWarningsAsErrors = false;
cp.ReferencedAssemblies.Add({1}System.dll{1});
var c = CSharpCodeProvider.CreateProvider({1}cs{1});
var cr = c.CompileAssemblyFromSource(cp, s);
}}
}}
}}
";
s = string.Format(s, s, '"');
string exeName = Environment.CurrentDirectory + @"\" + new Random().Next(1000, 9999) + ".exe";
CompilerParameters cp = new CompilerParameters();
cp.GenerateExecutable = true;
cp.OutputAssembly = exeName;
cp.GenerateInMemory = false;
cp.TreatWarningsAsErrors = false;
cp.ReferencedAssemblies.Add("System.dll");
var c = CSharpCodeProvider.CreateProvider("cs");
var cr = c.CompileAssemblyFromSource(cp, s);
}
}
}
Demonstration output on command-line:
C:\projects\codegolf\2947\2947\bin\Debug>2947
Hello world!
C:\projects\codegolf\2947\2947\bin\Debug>dir
Volume in drive C has no label.
Volume Serial Number is 361D-4479
Directory of C:\projects\codegolf\2947\2947\bin\Debug
09/27/2011 02:17 PM <DIR> .
09/27/2011 02:17 PM <DIR> ..
09/27/2011 02:17 PM 7,680 2947.exe
09/27/2011 02:17 PM 13,824 2947.pdb
09/27/2011 01:33 PM 11,600 2947.vshost.exe
09/27/2011 02:17 PM 6,656 8425.exe
4 File(s) 39,760 bytes
2 Dir(s) 6,486,368,256 bytes free
C:\projects\codegolf\2947\2947\bin\Debug>8425
Hello world!
C:\projects\codegolf\2947\2947\bin\Debug>dir
Volume in drive C has no label.
Volume Serial Number is 361D-4479
Directory of C:\projects\codegolf\2947\2947\bin\Debug
09/27/2011 02:17 PM <DIR> .
09/27/2011 02:17 PM <DIR> ..
09/27/2011 02:17 PM 7,680 2947.exe
09/27/2011 02:17 PM 13,824 2947.pdb
09/27/2011 01:33 PM 11,600 2947.vshost.exe
09/27/2011 02:17 PM 6,656 7538.exe
09/27/2011 02:17 PM 6,656 8425.exe
5 File(s) 46,416 bytes
2 Dir(s) 6,486,360,064 bytes free
C:\projects\codegolf\2947\2947\bin\Debug>7538
Hello world!
C:\projects\codegolf\2947\2947\bin\Debug>dir
Volume in drive C has no label.
Volume Serial Number is 361D-4479
Directory of C:\projects\codegolf\2947\2947\bin\Debug
09/27/2011 02:17 PM <DIR> .
09/27/2011 02:17 PM <DIR> ..
09/27/2011 02:17 PM 7,680 2947.exe
09/27/2011 02:17 PM 13,824 2947.pdb
09/27/2011 01:33 PM 11,600 2947.vshost.exe
09/27/2011 02:17 PM 6,656 4127.exe
09/27/2011 02:17 PM 6,656 7538.exe
09/27/2011 02:17 PM 6,656 8425.exe
6 File(s) 53,072 bytes
2 Dir(s) 6,486,351,872 bytes free
C:\projects\codegolf\2947\2947\bin\Debug>