A virtual filesystem exposing kernel device and driver state, used for low-level tuning.
/sys (sysfs) is procfs's newer sibling, purpose-built to expose the kernel's device and driver model in an organized, predictable structure — rather than the somewhat ad-hoc layout /proc grew into over the decades. Where /proc is mostly about processes and general kernel stats, /sys is specifically about hardware devices, buses, and the drivers bound to them.
Every device the kernel knows about gets a directory under /sys/devices/, with symlinks organizing the same devices by class (/sys/class/net/ for network interfaces, /sys/block/ for block devices) for easier navigation.
Many files under /sys are writable, not just readable — writing a new value to one directly changes live kernel/driver behavior, which is how tools configure things like disk I/O schedulers or network interface parameters without a dedicated command-line tool for every single setting.
This makes /sys genuinely dangerous compared to /proc: reading kernel state can't hurt anything, but writing the wrong value to the wrong sysfs file can change hardware behavior instantly, with no confirmation prompt.
/proc (process/kernel stats, mostly read-only) with /sys (device/driver model, often writable) — they look similar but serve different purposes.