Introduction to FreeBSD for Linux Users

I'd really like to see one of these get written one day from a couple of deeper perspectives... The difference between Linux and FreeBSD is bigger than just "they are variants on a theme", even if this is true at the sysadmin level, and even from the application programming point of view.

At a deeper level - things like "non-posix" syscalls vary a lot, and the subsystems they interact with are wildly different. Some of it is again variations on a theme - a special syscall vs an ioctl, or what not, but the way those subsystems interact starts becoming very interesting at a systems architecture level. For instance, I might want to write a specialized networking tool mostly in userland in linux, via nf_queue, whereas in FreeBSD I might want to split the operations between the kernel (via netgraph) and userland (via the pf equivalent to nf_queue).

It would be nice to see a nice write-up, or survey of writeups of the differences at that level.

What I like about this article is that it seems very unbiased. Most articles highlight a lot of key features from (GNU/)Linux or FreeBSD, depending on whom they're written by. This reads like a basic overview of how the systems came to be and why they work the way they work.

I would be interested to know the reasoning behind using different shells for interactive sessions vs. scripting on FreeBSD. Wouldn't it be easier to pick one for both? Is this just historical legacy?