Does the Linux kernel need a file system to run?

That's rather an odd question because you don't run the kernel like you run a program. The kernel is a platform to run programs on. Of course there is setup and shutdown code but it's not possible to run the kernel on its own. There must always be a main "init" process. And the kernel will panic if it's not there. If init tries to exit the kernel will also panic.

These days the init process is something like systemd. If not otherwise specified the kernel will try to run a program from a list of locations starting with /sbin/init. See the init Param here http://man7.org/linux/man-pages/man7/bootparam.7.html in an emergency you can boot Linux with init=/bin/bash . But notice how you always specify a file on the file system to run.

So the kernel will panic if it starts up an has no file system because without one there is no way to load init.

Some confusion may arise because of an initialisation phase of the kernel. An initial ramdisk is loaded from an image on disk containing vital drivers and setup scripts. These are executed before the file system is loaded. But make no mistake the initial ramdisk is itself a file system. With an initial ramdisk /init is called (which is stored on the initial ramdisk). In many distributions it is ultimately this which calls /sbin/init. Again without a file system, this is impossible.

The answer is going to depend on whether you literally mean without a file system or if the question is intended to be interpreted a little different from how it is actually stated. The answers for slight variations in how the question is interpreted are:

• Running Linux without any block devices is entirely feasible and useful for some specialized use cases.
• Running Linux without any file system is going to require rewriting some parts of the kernel code and it's unlikely to be a useful effort.
• Running Linux without using any file descriptors is going to require a lot of effort. I'm pretty sure that's not going to be worth the effort.

The reasons you'd have to rewrite parts of the kernel code to make a working system without a file system are:

• Every thread has a root directory and a current working directory which must point to some file system.
• Programs are started by the execve system call which needs an executable from a file system.
• The kernel creates a memory based file system during the boot process.

After a program has been started using execve it is possible for it to unmap the executable from which it was started, though in order to do so it without immediately crashing it first have to create an executable memory mapping which isn't backed by a file, and it has to initialize that with some useful code before jumping to it and unmapping the executable.

Thus a running user mode program can exist in a state where it has no memory mappings backed by files and it can close all file descriptors backed by files. It cannot stop having a root directory and current working directory, but it can refrain from those.

So though in this state you could implement kernel code to rip the file system away from under the program and have it keep running, it doesn't sound like it's useful. And getting into that final state without going through an intermediate state of using a file system is going to be even more work for no useful benefit.

A useful setup for some specialized use cases

Avoiding the use of block devices can be useful. During boot the kernel creates a memory file system, and it can also populate that file system with contents from a cpio archive before executing init. That way you can run a system entirely from a memory based file system without any block device to back it.

This can be useful for systems where you don't want to preserve any state and like the system to start from a clean slate upon rebooting.

Of course the kernel and cpio archive have to get somehow exist in memory before the kernel is given control. How they got there is a job for the boot loader. The boot loader could have loaded those from a block device even though the final running system doesn't use block devices. But it's also possible for the boot loader to acquire the kernel and cpio archive without using a block device for example by booting over the network.

In Linux, nearly every device is a file, so you have to have a filesystem to run it.