Some Important Terms to Know in Linux

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This post was originally published on my personal blog mariadcampbell.com.

In order to be able to understand what Linux is all about, there are certain
Linux terms you should become familiar with from the get go.

The Linux kernel

First an foremost, you should know what is meant by the
Linux Kernel.

The Linux® kernel is the main component of a Linux operating system (OS)
and is the core interface between a computer’s hardware and its
processes. It communicates between the 2, managing resources as efficiently
as possible.

The kernel is so named because—like a seed inside a hard shell—it exists
within the OS and controls all the major functions of the hardware, whether
it’s a phone, laptop, server, or any other kind of computer. -- What is the
Linux kernel?,
RedHat

The Linux kernel accomplishes 4 tasks:

• Memory management: keeps track of how much memory is used to store what and where
• Process management: determines which processes can use the central processing unit (CPU), and for how long.
• Device drivers: acts as a mediator/interpreter between the hardware and the processes.
• System calls and security: receives requests for service from processes.

The kernel works invisibly in the kernel space, where it
allocates memory and keeps track of where everything is stored. The
user sees things like web browsers and files, and these live in the
user space. These applications interact with the kernel via a
system call interface (SCI).

GNU

GNU, or "GNU's Not Unix", is a collection of open source tools that
anyone can use for free to develop their own apps and
operating systems.

The GNU Project, founded by Richard Stallman, develops and
distributes software for free and provides every developer with
open-source tools to do the same.

The Linux kernel is licensed under GNU's GPL (General Public License)
and is known as GNU/Linux. Any operating system developed using
open-source GNU tools and the Linux kernel falls under the category
of Linux distributions.

Linux distribution

A Linux distribution (or distro) is an operating system (OS) compiled
from components developed by open source projects/programmers.

Each distribution includes:

• the Linux kernel (the core of the OS),
• GNU (GNU's Not Unix) shell utilities (terminal interface and commands),
• the X server (a graphical desktop),
• the desktop environment,
• package management system,
• an installer,
• other services

Distributions also include an internet browser, management tools, and
other software such as the KVM (Kernel-based Virtual Machine) hypervisor
[^1]. A single Linux distribution may contain thousands of
software packages, utilities, and applications.

Linux package managers

Package managers are programs that manage packages on a Linux system. It
allows us to add or delete packages from sources such as our distribution's
repositories or third party repositories.

APT, RPM, and pacman are the three most popular package managers
used by Linux distributions. Debian and Ubuntu-based distributions
use the APT package manager. Fedora, CentOS, and RHEL have the RPM
package manager. Arch Linux and its derivatives ship with pacman.

Source and binary packages

A Linux package is a compressed software archive file containing all the
files comprising a software application that provides a
particular functionality.

Linux software ere usually distributed as packages, and there are
two types of packages available: source and binary.

Source packages contain program source code that a user has to manually
compile and install in order to run the software.

Binary packages contain pre-built and pre-compiled executables for the
software.

Linux repositories

Software repositories are remote servers that store a collection of
packages along with related metadata [^3]. Every
Linux distribution either has its own repositories or uses
repositories of its parent distribution to provide software to the
users.

Linux Boot loader

A boot loader is is program which loads the OS into RAM during the
boot process [^2]. The boot loader helps load the correct
operating system during boot time and adds initial processes into
memory (RAM).

Linux services

A Linux service
(cron daemon is a
service) is a process or application that runs in the background, either
doing some predefined task or waiting for some event. systemd, which is
a system and service manager for Linux, is the first process to run in
the system.

Linux filesystem

At a high level, the Linux filesystem follows the
Filesystem Hierarchy Standard (FHS). This standard describes the
common layout conventions used by most UNIX and UNIX-like systems. It
consists of a single primary or root directory with multiple branching
sub-directories.

/ represents the root of the Linux filesystem. It is under this
root directory that all other files and directories reside. The
Filesystem Hierarchy Standard
(FHS) describes the different directories and what files are located
in which directories.

If you have Ubuntu or a Ubuntu derivative installed (like Linux Mint for
example), you can install a package called tree by running the
following command in Terminal:

# to update current packages. updates the list of available packages and their versions, but it does not install or upgrade any packages.
sudo apt update
# actually installs newer versions of the packages you have. After updating the lists, the package manager knows about available updates for the software you have installed. This is why you first want to update.
sudo apt upgrade
# now you are ready to install tree
sudo apt install tree

Now you can run a command like the following:

tree /

The tree command lists contents of directories in a tree-like format in
Linux. With no args, it lists the files and sub-directories in
the current directory. With directory args (such as / or ~), tree
lists all the files and/or directories found in the
given directories. When completing the list of all files and
directories found, tree returns the total number of files and/or
directories listed.

If you want a short list of files and directories in root (/), for
example, you can run something like the following instead:

# run from root directory `/`
tree -L 1 /

What this command essentially is doing is it shows only the first level
of the directory tree starting at root (/). The -L option tells
tree how many levels down we want to see. Here, we indicate
1 level.

Types of files in Linux

In Linux and Unix systems, everything is considered a file. If it
isn't, it's a running process. Basically, files fall under three different
categories in Linux:

• Regular files: these include text files, photos, videos, programs, and executable files.
• Directories: in Linux, directories are also considered files since they provide storage for other files and sub-directories.
• Special files: device files that are made up of symbolic links, block files, socket files, and named pipe files.

I will discuss the Linux Filesystem Tree in greater detail in an upcoming
article. It's too lengthy to include here.

root directory vs root user

The root (/) directory, mentioned previously, represents the root of
the Linux filesystem. It is under this root directory that all other
files and directories reside.

The root user, or superuser, or just root, is the user that has all
administrative privileges. The root user can read, write/edit, and
delete any file, make changes to the system or other users, and even
delete the whole directory tree. root is the Linux user with the
highest level of control. To learn a bit more about root and related
sudo, sudo su, and su commands, please visit my article
The su command in Linux and Unix (macOS).

X Window System

The X Window System
provides the base technology for developing graphical user interfaces.

Linux desktop environment

A Linux desktop environment bundles together various components to provide
common graphical user interface elements such as icons, toolbars,
wallpapers, and desktop widgets. And most desktop environments include a
set of integrated applications and utilities. It is the
desktop environment that permits us to use Linux graphically using a
mouse and keyboard like we do in other OS like Windows or macOS.

Linux Command Line Interface

The Linux Command Line Interface (also available in other OS like Windows
and macOS) is what we use to interact with a computer's OS using the
computer keyboard. We can enter text commands to navigate,
run programs, or configure our computer system. All operating systems
(OS), including Windows, Linux and macOS, provide a CLI in
addition to a graphical user interface (GUI).

Oftentimes Terminal, Command Line, Console, or Shell are used
interchangeably. However, they are not all exactly the same.

A "Terminal" is a text input/output (I/O) environment.

A Terminal window, aka Terminal emulator, is a text only window that
emulates a user's Console in a Graphical User Interface (GUI).

A Terminal is a program that allows us to run a shell. It is a
device with a human readable display that accepts a stream of
characters and displays them. They are used frequently in networking and
device debugging interfaces.

A Console is a physical Terminal. It is an instrument panel with
computer controls. It is a type of terminal, and is a window which
contains active text-mode programs. The Console recognizes
key presses (stdin) and is aware of its width and height.
Full screen mode is available.

A Shell is a command line interpreter. It processes commands (stdin) and
outputs the results (stdout). A Shell is a layer that sits on top
of the kernel:

• It interprets and processes the commands entered by the user. Users don't have access to the kernel, but the shell does. Users can only gain access to the kernel by using a shell and entering commands or running programs.
• System calls are used by programs to gain access to the kernel's functionality.

The Command Line is an area to the right of the command prompt in the
Terminal window where a user enters commands and data. This is
the line where we type our commands.

Linux Shells

Linux shells can be either be a command line interpreter like Terminal
or a graphical interpreter like GNOME Shell. We can also install
different shells in Linux. Examples include Bash, Zsh,
Fish, sh, and Ksh.

The following are two ways one can list the shells currently available
in one's Linux distribution:

Using the cat command:

cat /etc/shells
# which returns:
# /etc/shells: valid login shells
/bin/sh
/bin/Bash
/usr/bin/bash
/bin/rbash
/usr/bin/rbash
/usr/bin/sh
/bin/dash
/usr/bin/dash

Using grep:

grep -v '^#' /etc/shells
# which returns the same as with cat command except for the message:
/bin/sh
/bin/Bash
/usr/bin/bash
/bin/rbash
/usr/bin/rbash
/usr/bin/sh
/bin/dash
/usr/bin/dash

cat /etc/shells displays the contents of /etx/shells.

grep -v '^#' /etc/shells allows to filter and display only the lines
that contain valid shell entries. The regular expression ^ character
means beginning of the line, and # means to search for lines
that begin with #. But the -v flag, or --invert-match, means to select
non-matching lines, so lines that don't begin with #, which
would represent a comment. That is why the comment
# /etc/shells: valid login shells does not appear here.

Linux shell scripting

Shell scripting (not just in Linux) refers to the process of
writing scripts using (Linux) commands, which are then interpreted
by the shell installed in Linux. Batch scripts in Windows are the
equivalent to shell scripts in Linux.

The most popular form of shell scripting in Linux is Bash scripting,
which means writing and executing scripts using the Bash shell.
Shell scripts are used to automate tasks and are added to
crontab files as cron jobs. To learn how to set up a basic cron job
using the Cron daemon, please visit my article entitled
The Cron daemon in Linux and how to create a cron job.
I will be adding more articles regarding creating different cron jobs in
the near future.

Footnotes

[^1]: a
hypervisor is a
program that separates a computer's OS and applications from the underlying
physical hardware. This is what enables virtualization and cloud computing
solutions for modern IT infrastructures.

A hypervisor allows the physical host machine to operate multiple virtual
machines (VMs like VirtualBox, for example) as guests. This increases the
effectiveness of computing resources such as memory, network bandwidth, disk
space and CPU cycles.

VMs can run on the same physical hardware, but they are logically separated from
each other, preventing a failure in one VM from affecting other VMs on the same
physical machine.

A hypervisor creates, runs, and manages virtual machines.

[^2]: The boot process is the process of loading an OS. It starts when we
turn on a computer and ends when the OS is loaded into memory.

[^3]: Linux package metadata describes a package for its consumers: who
wrote it, where its repository is, and what versions of it have been published.
It also contains a description of each version of a package present in the
registry, listing its dependencies, giving the url of its tarball, etc.

Related Resources

• How to Install and Use Popular Linux Bootloaders:
  baeldung.com

• Computer Boot Process Explained:
  baeldung.com

• The Linux Filesystem:
  by perfectogo, medium.com

• Classic SysAdmin: The Linux Filesystem Explained:
  The Linux Foundation

• Linux command line for you and me!¶:
  LYM

• Difference between Terminal, Console, Shell, and Command Line:
  Geeks for Geeks

• 15 Linux Terms, Jargons, and Lingo You Should Know About:_
  makeuseof_