The programs listed here are provided as exhibitions of just how compressed a Linux ELF executable can be and still accomplish something useful. Because they are potentially useful, they all adhere to the ELF specification (though most of them still bypass libc in favor of making direct system calls).
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.)
cls.asm source
cls binary
98 bytes
Admittedly, this program is trivial, no more than a variant on hello-world. Yet the fact is that I use this program almost daily. It just clears the terminal screen, in the process also clearing any odd persistent state such as reverse video. It's both shorter to type than "clear" and faster than "reset". (For some reason the standard reset utility takes a full second to complete.) Admittedly this program assumes an ANSI-compatible terminal without testing for same, or even if stdout is connected to a terminal, but I have yet to find such shortcuts to be problematic. The name "cls", short for "clear screen", is deeply embedded in my muscle memory, thanks to the TRS-80 and MS-DOS both using it, and I find myself typing it habitually.
hexdump.asm source
hexdump binary
202 bytes
This is your classic hexdump program. I wrote it one day in a fit of pique, after trying (and failing) to convince the standard Linux programs to output bytes in the formatting style that I've been accustomed to since before reaching my majority. (I've since stopped using this program, as the default output of xxd(1) is nearly identical.) My original version simply read from standard input, but I quickly found this irritating enough that I sacrificed a few more bytes in order to allow it to accept a filename on the command line.
base64.asm source
base64 binary
256 bytes
This is a simple base64 decoder utility. Like hexdump, it can either accept a filename on the command line, or work on standard input. Since sometimes very large files are encoded in base64, I allowed the program to be slightly larger than strictly necessary in order to optimize for speed. This version is actually noticeably faster than the utility included in GNU coreutils for 32-bit Linux.
ls.asm source
ls binary
1017 bytes
This utility was my attempt to tackle an important yet complex tool. ls is one of those programs that you really can't do without, in part because its functionality has almost no overlap with other standard tools, but also because it's tailored to provide multiple pieces of information at a glance. My version is missing a few standard features, such as sorting, but it does recognize many commonly-used command-line options, such as -l, -a, -F, and -R. The "long" output format only displays numeric user IDs, timestamps are displayed as ages instead of actual times, and the columnar output is not particularly clever. But beyond these things, it conforms pretty closely to the standard ls program we all know and love.
factor.asm source
factor binary
1020 bytes
This is an implementation of the standard Unix utility. It displays the prime factors of the integers provided on the command line, or on standard input if no arguments are given. Instead of simply trying to achieve the smallest possible size, I decided to shoot for real portability with this program, for a change. It not only conforms to the requirements of the ELF specification, it also avoids making direct system calls and instead dynamically links with libc. It should continue to work with any future version of Linux, as long as new versions of the libc ABI and the ELF specification remain backwards-compatible. It also includes online help, version information, and error messages, and therefore arguably stands as a completely functional replacement for the standard utility in GNU coreutils. Because factoring huge prime numbers can sometimes run for a long time, I attempted to optimize for speed and size both. This program makes significant use of the FPU in order to increase parallelization in the inner loop. (Back when I first wrote it, it was significantly faster than the standard GNU program. Since then, the coreutils program has been updated with some advanced mathematics, and it's now the faster program.)
puzzle.asm source
puzzle binary
995 bytes
The final program in this list is a game. And not just a game, but one that runs under X. You've probably seen the 15 sliding-tile puzzle at some point in your life. Some of you may even remember the program that came with the original 1984 Macintosh operating system. But are you aware that, in order to justify including the puzzle in the limited memory and disk space available, the author squeezed its size down to under 600 bytes? (See here for the details.) Alas, while I would have loved to be able to present a Linux binary that matched that size, the complexities of interfacing with an X server and mimicking the details of the original put it well over that target. I'm still working to reduce it further, but for now here's my hefty rendition. It uses the same graphics as the original, which unfortunately means that the window is only slightly larger that its own title bar on a modern display. But it is a faithful recreation of the original Macintosh program that runs under X Windows. And despite all of my hedging about its bloat, I'd wager that it's the only X program you've ever seen under 1k.
(Usage note: This program does not go through normal channels to find the X server; it assumes that your X server is running locally and that you are the only (or at least the first) user. If the program nevertheless fails immediately when run, you may need to give yourself permission to start X programs directly. This can be done by running xhost si:localuser:$USER. Afterwards, you can repeat the command with a prepended minus sign to remove these permissions: xhost -si:localuser:$USER.)