Flutterby™! : Random notes on lightweight Linux/Un*x

This morning on Twitter, Columbine was whining about having to compile Emacs for a machine, wondering why Linux distributions even bothered with X (let alone those bloated overblown things like KDE and Gnome), and pondering that it was ludicrous to learn vi because it was better to copy the file off, edit it, then copy it back on rather than resort to that bit of antiquity.

(I'm paraphrasing.)

So since I'm actually starting to get some new readers, I figure we'd scare 'em off with an intro to "how to build a lightweight Un*x system". I've done this with Linux on a bunch of different platforms, but I'm gonna try to stay conceptual.

First thing you need is a boot loader. Intel based people have seen "lilo" and "grub", Atmel based systems use U-boot. The point of the boot loader is to locate a kernel and read it into memory, then call the kernel. Sometimes the kernel is stored on disk, sometimes in flash or other read-only memory, sometimes on a network to be accessed via "tftp" or a similar protocol.

Since the boot loader loads the kernel, that'd be the next stage. In my case this has been Linux, but I'm staying high level and the various BSD kernels and what-have-you are similar. So the boot loader gets the kernel into memory, and calls it.

The kernel then initializes whatever hardware's been compiled in to it (those of you looking for fast boot times, this is a reason to use modules: they haven't been loaded yet), and then mounts "the root filesystem" read-only, and executes "/sbin/init" as user "0" (abstractly, the kernel itself knows nothing of user names, just numbers).

If you just want the kernel and the root filesystem read-only, "/sbin/init" can be any application you want it to be. You could, at this point, link it to emacs. Of course that binary needs to only use shared libraries on the root filesystem, but it can be anything.

But usually this is a program that sets up additional filesystems, possibly remounts the root filesystem read-write, sets up network interfaces (this gets tricky if your root filesystem is on a network...), and then offers the user the chance to login.

If you're building a lightweight Un*x like system, I highly recommend BusyBox rather than the full-on GNU POSIX environment. BusyBox is just enough "init" and the standard POSIX environment to be useful, with various options to pare it down or build it up a bit, without the overhead of knowing too much about things like locales and character sets and languages and all of that crap.

And if you're building this environment from scratch for an unfamiliar platform (ie: you're cross-compiling and don't yet have a compiler that can run on a machine you've got and build for the new machine), I also recommend buildroot, a set of tools to download and compile lightweight Linux environments that can range from "give me a boot loader, a kernel, and, optionally, a login prompt" to X and all sorts of additional stuff.

But if you're a geek who likes to play around, I also recommend starting with something like my now 7 year old instructions on building a CompactFlash based embedded Linux install to bring a machine up from scratch, without all those funky package managers and what-have-you. There's a lot of beauty and elegance in the design of the Un*x like kernels and the way the layers get added as the system comes up that's worth stopping and appreciating, if you've the background to do so.