Pseudo Filesystems

The proc virtual filesystem exists since the beginning of Linux
It allows
- The kernel to expose statistics about running processes in the system
- The user to adjust at runtime various system parameters about process management, memory management, etc.
The proc filesystem is used by many standard user space applications, and they expect it to be mounted in /proc
Applications such as ps or top would not work without the proc filesystem
Command to mount /proc: mount -t proc nodev /proc
Documentation/filesystems/proc.txt in the kernel sources
man proc

proc contents

One directory for each running process in the system
- /proc/
- cat /proc/3840/cmdline
- It contains details about the files opened by the process, the CPU and memory usage, etc.
/proc/interrupts, /proc/devices, /proc/iomem, /proc/ioports contain general device-related information
/proc/cmdline contains the kernel command line
/proc/sys contains many files that can be written to to adjust kernel parameters
- They are called sysctl. See Documentation/sysctl/ in kernel sources.
- Example echo 3 > /proc/sys/vm/drop_caches

The sysfs filesystem is a feature integrated in the 2.6 Linux kernel
It allows to represent in user space the vision that the kernel has of the buses, devices and drivers in the system
It is useful for various user space applications that need to list and query the available hardware, for example udev or mdev.
All applications using sysfs expect it to be mounted in the /sys directory
Command to mount /sys: mount -t sysfs nodev /sys
$ ls /sys/ block bus class dev devices firmware fs kernel module power