Processes

Every program, whether it’s a command, application, or script, that runs on your system is a process. Your shell is a process, and every command you execute from the shell starts one or more processes of its own (referred to as child processes). Attributes and concepts associated with these processes include:

Lifetime

A process lifetime is defined by the length of time it takes to execute (while it “lives”). Commands with a short lifetime such as ls will execute for a very short time, generate results, and terminate when complete. User programs such as web browsers have a longer lifetime, running for unlimited periods of time until terminated manually. Long-lifetime processes include server daemons that run continuously from system boot to shutdown. When a process terminates, it is said to die (which is why the program used to manually signal a process to stop execution is called kill; succinct, though admittedly morbid).

Process ID (PID)

Every process has a number assigned to it when it starts. PIDs are integer numbers unique among all running processes.

User ID (UID) and Group ID (GID)

Processes must have associated privileges, and a process’s UID and GID are associated with the user who started the process. This limits the process’s access to objects in the filesystem.

Parent process

The first process started by the kernel at system start time is a program called init. This process has PID 1 and is the ultimate parent of all other processes on the system. Your shell is a descendant ...

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