is an operating system developed at Bell Labs in the late 1980s and early 1990s by the creators of Unix. The idea was to create a new operating system based on the principles used in creating Unix, but avoid the mistakes that went into the first iteration.
The result was a small, modern, distributed operating system in which everything from network access to window management is represented by standard read/write operations on files (no ioctl like in Unix) over the network-transparent 9p filesystem protocol. By breaking everything down into that one abstraction, it becomes trivial to control any device anywhere, even on another computer, as if it were attached to your own. Just mount a remote system's /dev to gain access to its devices, or /net to gain access to its network.
The distributed nature of Plan 9 also means that it is designed so that one operating system can be spread across multiple different machines. For instance, you can run a file server on a system with lots of disks, and boot using that filesystem from a laptop with a small SSD. Because 9p is network-transparent, there is no difference to the OS between using a local filesystem and a remote one.
In recent times, modern Unix systems have gained some of the capabilities of Plan 9 (in large part due to its inspiration), but none of them has the same transparency that 9p provides for Plan 9.9front
is a fork of the original Plan 9 created in 2011, after it was abandoned by its original maintainers. It has a focus on modernisation, such as allowing 9p network mounts to be encrypted with TLS, rewriting the filesystem to better suit modern disks, and supporting WiFi on laptops.
I installed 9front some time ago in a VM, which was interesting, but not really a good way to get to know the OS. Over the past few days, I've set up 9front on an old laptop, and I'm still getting used to the system, but so far I'm quite enjoying the experience. Even if I don't end up using it regularly, it's quite interesting from a theoretical perspective because it's so different from any other OS in existence.
This post was made on a ThinkPad X201 running 9front, using the mothra web browser.