/proc filesystem

Remember everything in Linux is a file, even processes. Process information is stored in a special filesystem known as the /proc filesystem.

$ ls /proc

You should see multiple values in here, there are sub-directories for every PID. If you looked at a PID in the ps output, you would be able to find it in the /proc directory.

Go ahead and enter one of the processes and look at that file:

$ cat /proc/12345/status

You should see process state information and well as more detailed information. The /proc directory is how the kernel is views the system, so there is a lot more information here than what you would see in ps.