An instance of a running program is called a process.
Every time you run a shell command, a program is run and a process is created for it.
In any modern operating system there can be more than one instance of a program loaded in memory at the same time.
== Linux process management
Operating systems need some ways to create processes.
* In a very simple system designed for running only a single application (e.g., the controller in a microwave oven), it may be possible to have all the processes that will ever be needed be present when the system comes up.
* In general-purpose systems, however, some way is needed to create and terminate processes as needed during operation.
[NOTE]
====
*There are four principal events that cause a process to be created:*
* System initialization.
* Execution of process creation system call by a running process.
* A user request to create a new process.
* Initiation of a batch job.
====
Process creation in UNIX and Linux are done through *fork()* or *clone()* system calls.
There are several steps involved in process creation.
An operating system kernel that allows multitasking needs processes to have certain states. Names for these states are not standardised, but they have similar functionality.
[.float-group]
--
[.right]
image:./Process_states.png[350,350,role="related thumb right"]
* the process is *"created"*.
* After that the process scheduler assigns it the *"waiting"* state.
* While the process is *"waiting"*, it waits for the scheduler to do a so-called context switch.
* The context switch loads the process into the processor and changes the state to *"running"* while the previously *"running"* process is stored in a *"waiting"* state.
* If a process in the *"running"* state needs to wait for a resource, it is assigned the *"blocked"* state.
* The process state is changed back to *"waiting"* when the process no longer needs to wait (in a blocked state).
* Once the process *finishes execution*, or is *terminated* by the operating system, it is no longer needed.
* The process is *removed instantly* or is moved to the *"terminated"* state.
* When removed, it just waits to be *removed from main memory*.
--
=== Inter-process communication
When processes need to communicate with each other they must share parts of their address spaces or use other forms of inter-process communication (IPC).
* For instance in a shell pipeline, the output of the first process need to pass to the second one.
* another example is a task that can be decomposed into cooperating but partially independent processes which can run at once
** (i.e., using concurrency, or true parallelism - the latter model is a particular case of concurrent execution and is feasible whenever enough CPU cores are available for all the processes that are ready to run).
NOTE: It is even possible for two or more processes to be running on different machines that may run different operating system (OS), therefore some mechanisms for communication and synchronization (called communications protocols for distributed computing) are needed (e.g., the *Message Passing Interface, often simply called MPI*).
==== communication between processes
Processes can communicate with each other through:
* Shared Memory
* Message passing
[.float-group]
--
[.right]
image:./1-76.png[350,350,role="related thumb right"]
[.left]
image:./2-50.png[350,350,role="related thumb left"]
|ls| Displays a list of files in the current working directory.
|cd directory| change directories
|passwd |change the password for the current user
|file filename| display file type of file with name filename
|cat textfile| throws content of textfile on the screen
|pwd| display present working directory
|exit or logout| leave this session
|man command| read man pages on command
|=======================
=== Key combinations
.Key combinations
[options="header,footer"]
|=======================
|Key or key combination| Function
|Ctrl+C| End a running program and return the prompt
|Ctrl+Z| Suspend a program
|ArrowUp and ArrowDown| Browse history. Go to the line that you want to repeat, edit details if necessary, and press Enter to save time.
|Tab| Command or filename completion; when multiple choices are possible, the system will either signal with an audio or visual bell, or, if too many choices are possible, ask you if you want to see them all.
|Tab Tab| Shows file or command completion possibilities.
|Shift+PageUp and Shift+PageDown| Browse terminal buffer (to see text that has "scrolled off" the screen).
zeus
3 years ago