eolas/zk/Processes.md

---
id: t70u
title: Processes
tags: [operating-systems]
created: Friday, June 21, 2024
---

# Processes and threads

Programs are sequences of machine instructions stored in a file. However they do
not work by themselves. Something needs to load the file's intructions into
memory and direct the CPU to run the program. The OS does this via processes.

A process **is a running instance of a given program**. It can be thought of as
a container in which a program runs. This container includes:

- a copy of the program code loaded into memory
- a memory address
- other information about the state of the process

Other than the `init` process started by the kernel (PID1), every process has a
parent process that started it. This parent-child relationship creates a tree of
processes.

It is possible that a parent process will terminate before one of its child
processes. In this instance the child becomes an orphan. When this occurs in
#Linux, the orphan process is adopted by `init`.

Below, I have used the `pstree` utility to list all the running processes on my
machine hierarchically.

![diagram of `pstree` output](../img/ps-tree.png)

Children are represented vertically and horizontally.

```
├─terminator─┬─zsh───tmux: client
│            ├─zsh───pstree
│            └─6*[{terminator}]

```

For instance here, `terminator` is a child of `init`, as are `zsh` and `tmux`
but they are also children of `terminator`.`pstree` is a child of `zsh` and
therefore also a child (grandchild) of `terminator`.

## Related notes

![systemd](./systemd.md)

![ps](./ps.md)