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This chapter describes what a process is and how the Linux kernel creates, manages and deletes the processes in the system.

Processes carry out tasks within the operating system. A program is a set of machine code instructions and data stored in an executable image on disk and is, as such, a passive entity; a process can be thought of as a computer program in action.

It is a dynamic entity, constantly changing as the machine code instructions are executed by the processor. As well as the program's instructions and data, the process also includes the program counter and all of the CPU's registers as well as the process stacks containing temporary data such as routine parameters, return addresses and saved variables. The current executing program, or process, includes all of the current activity in the microprocessor. Linux is a multiprocessing operating system. Processes are separate tasks each with their own rights and responsibilities. If one process crashes it will not cause another process in the system to crash. Each individual process runs in its own virtual address space and is not capable of interacting with another process except through secure, kernel managed mechanisms.

During the lifetime of a process it will use many system resources. It will use the CPUs in the system to run its instructions and the system's physical memory to hold it and its data. It will open and use files within the filesystems and may directly or indirectly use the physical devices in the system. Linux must keep track of the process itself and of the system resources that it has so that it can manage it and the other processes in the system fairly. It would not be fair to the other processes in the system if one process monopolized most of the system's physical memory or its CPUs.

Shared Memory Basics

Basic Concepts

Shared memory can best be described as the mapping of an area (segment) of memory that will be mapped and shared by more than one process. This is by far the fastest form of IPC, because there is no intermediation (i.e. a pipe, a message queue, etc). Instead, information is mapped directly from a memory segment, and into the addressing space of the calling process. A segment can be created by one process, and subsequently written to and read from by any number of processes.

Interprocess Communication (IPC), Pipes

There is no form of IPC that is simpler than pipes. Implemented on every flavor of Unix, pipe() and fork() make up the functionality behind the "|" in "ls | more". They are marginally useful for cool things, but are a good way to learn about basic methods of IPC. We have now began to see how multiple processes may be running on a machine and maybe be controlled (spawned by fork() by one of our programs.

FIND command example

Some examples of FIND command in linux/ unix environment

SED: A powerfull Text Manipulator in linux/unix

 

                    You need to use find command which is used to search files and directories under UNIX and Linux like operating systems. You can specify criteria while search files. If no criteria is set, it will returns all files below the current working directory. find also supports regex matching and other advanced options.