Chapter 10
Processes
One of the main tasks of an operating system is to run processes on behalf of its users, services, and applications. These are tracked in a process table inside the kernel, which keeps track of the current state of each process in the system, and the system scheduler decides when each process will be assigned to a CPU, and when it is taken off the CPU again. The ps command interrogates the process table. Following the Unix model of doing one thing and doing it well, ps has a set of switches to fine-tune exactly what is to be displayed from the full process tree.
The /proc pseudo-filesystem provides further insight into the running kernel; there is a directory under /proc for every process currently in the process table. In that directory can be found the state of the process — its current directory and the files it currently holds open. The Linux kernel exposes far more of the operating system than just the process table; /proc includes mechanisms for reading and writing kernel state directly, including networking settings, memory options, hardware information, and even the ability to force the machine to crash.
This chapter looks into processes, what they are, how they are managed, and how they can be manipulated. This is one of the oldest concepts of Unix and not much has changed in 40 years, but the Linux kernel in particular has brought a fresh relevance to the way in which the /proc filesystem provides real two-way interaction between the kernel and userspace. ...
