What is the most commonly used file system in Linux?

Currently, ext4 is likely the most common file system for standard distribution roots, although there isn’t technically a 'main' file system. Debian-based systems usually favor ext4, while Red Hat-based ones often use xfs.

Ext4 has been the standard for quite some time. It's solid and significantly more reliable than Windows NTFS. These days though, some distributions are starting to use btrfs and zfs as well. I stick with ext4 for my two systems, my PC and laptop.

For newcomers to Linux, I'd recommend just going with ext4 since most distributions default to it. It's simple and works well.

The Linux kernel itself doesn’t have a single main or preferred file system; each distribution usually chooses one, and they're not all the same.

Most regular desktop users tend to lean towards ext4 as their go-to file system. If you’re managing a big NAS, you might see different systems in play, but for everyday use, ext4 is where it’s at.