What does it mean to "just add /dev, /etc, and a Linux kernel" - BusyBox

Question

I have been reading the documentation of BusyBox with the intention of using its small filesize to make a bootable cd.

The documentation states

To create a working system, just add /dev, /etc, and a Linux kernel.

I have used some linux Distro's such as Ubuntu and CentOS, when running cmds such as fdisk a list of attached devices starting with /dev is usually returned, when running ls on /etc we get some files show up.

I am confused at how/what I am supposed to add a /dev and /etc to BusyBox before booting up... Also I thought these would be added whilst in a running enviroment.

Are /dev and /etc to be perceived from a different view when spoken about in this manner, or have I just created a false view when using pre built Distros without understanding much of the underlying mechanics?

Answer 1

The point the documentation is trying to make is that this is a pretty complete userspace, but you (as someone who is looking to roll a firmware for an embedded device) still have a role to play.

Busybox has some assumptions about the environment it will be run within that this statement is documenting here:

1. You will somehow take care of the early boot process. That includes getting a bootloader to hand over to a kernel. The kernel needs to be appropriate to the hardware and sufficiently "sane" generally.
2. Busybox is not in and of itself sufficient to boot a usable device without some additional work. You can't simply compile busybox, dump it on a filesystem and hand over control as soon as the kernel is booted without doing a little more work somewhere first. You'll need to make sure /dev exists in the filesystem and is populated with device inodes to match your hardware. The exact mechanism for doing that depends on the kernel version and the hardware/platform you're targeting, it has changed a lot over the evolution of Linux kernel versions and there's more than one way to do it.
3. Finally lots of the pieces busybox does supply (and ones that you'll probably want from elsewhere) need some kind of configuration. How you manage that is up to you, but the bottom line is that many of the commands you'll run from busybox read config files from /etc and simply won't work if you haven't provided a mechanism to populate and maintain those.

Potentially on your embedded device you will have just one filesystem, using something like squashfs and everything will be read-only inside that. But the point is that your image/installer needs to do all the work to get that in the right place by the time you've handed over control.