The programs listed here are provided as exhibitions of just how compressed a Linux ELF executable can be and still perform a function. All of them abuse the kernel's casual approach to validating ELF files in order to reduce size. The assembler source code of each of these programs includes details showing how the ELF header, the program segment header table, and the x86 instructions of the actual program overlap each other.
In order to use one of these programs, you can download the assembler source and build it with nasm. If you don't want to install nasm, and you have reason to believe that I can be trusted, you can download a pre-built binary and run it directly. (Note that you will need to use chmod +x to mark a downloaded binary as executable.)
true.asm source
true binary
45 bytes
Despite the name, this program provides binaries for both true and false. It returns an exit code of either zero or one, depending on whether it is invoked with the name "true" or "false", respectively. At 45 bytes, this program is the smallest it is possible for a Linux ELF executable to be.
keepalive.asm source
keepalive binary
56 bytes
Here's a program that I've actually used. It simply runs forever, printing a bell character every four minutes or so. When I used it, I would leave it running in the background after logging into a remote machine that times out connections when they're idle. One can then disable the terminal's bell to make the character completely invisible. Admittedly, it's a bit longer than the one-line shell script I originally used, but on the plus side it doesn't take up an extra process in order to sleep.
hello.asm source
hello binary
62 bytes
Here is the program that started me off on this whole pursuit: hello world. It is dense. With the program header table overlaid on top of the ELF header, and program itself running through both of them, some of the bytes have no less than three different purposes.
Here you can find an even smaller hello-world program, apparently written by someone named Kikuyan (菊やん). It is only 58 bytes long, and would have been 57 if the author had used the same version of the greeting as myself. The program uses the technique of loading to absolute address zero, which permits a number of tricks that further reduce code size. I have not used this technique myself, because current versions of Linux do not, by default, permit executables to use a load address of zero. It is an ingenious little program, though.
hello-x64.asm source
hello-x64 binary
85 bytes
And, for the record, here's a hello-world program for 64-bit ELF. It's quite a bit larger than the 32-bit version, but nonetheless I find it surprising that it's only five bytes larger than the smallest 64-bit ELF executable. As with that program, I credit Oleg Finkelshteyn for showing me the overlapping technique that it uses. In many ways, it is reminiscient of Kikuyan's program, in that because the assembly program starts at the very top, it becomes thoroughly interwoven with both ELF headers.
echo.asm source
echo binary
76 bytes
While echo is actually a very useful and important utility, nobody really needs a standalone binary of echo these days, so it still belongs on this page. This was inspired by a couple of blog posts written by Jan Halsema about their own attempts to use assembly to create a teensy version of echo. It's a good example of how the choice of algorithm has as much to do with size optimization as abusing the ELF standard. This program works by calling write() once per character — a terribly inefficient design choice that does wonders for its overall size.
bf.asm source
bf binary
166 bytes
Brainfuck is a very simplistic programming language, which was invented by Urban Müller solely for the purpose of being able to create a compiler that was less than 256 bytes in size, for the Amiga OS. (A description of the language can be found at my Brainfuck page, among many other places.) I decided to take up the challenge as well, and created a Brainfuck compiler for Linux using less than 256 bytes. The compiler works as a filter: redirect the source code to the compiler's input and the compiler's output to a file. (And don't forget to chmod +x the output file.) At the beginning I didn't expect that I would actually succeed, since I thought I would need almost half of that just for the headers. I think my first cut was 458 bytes in size. I was quite pleased with myself when I trimmed that down to less than 300 bytes, ecstatic when I finally reached the 255-byte goal, and downright triumphant when I later got it to work in 238 bytes. By the time I had a 198-byte version, I had to admit that I had no idea where it was going to end. It's now at 166 bytes. And though I can't quite believe it, it works just as well as the first one did. As useless as it is, I think this one would have to be the crown jewel of my collection. It is quite possibly also the ugliest and most opaque program I have ever written.