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Linux – Basics
1. What Is Linux?
Linux is a family of open-source operating systems built around the Linux kernel. The kernel was created in 1991 by Linus Torvalds and is now maintained by a large community.
In practice, "Linux" usually refers to a complete system that combines:
- The Linux kernel (manages hardware, memory, processes).
- User space tools and libraries (often from the GNU project).
- Additional software such as shells, utilities, and applications.
These complete systems are distributed as Linux distributions.
2. Linux Distributions
A Linux distribution ("distro") bundles the kernel with a selection of software, configuration defaults, and a package manager.
Common examples include:
- Debian / Ubuntu – widely used on servers and desktops; focus on stability and large software repositories.
- Fedora / RHEL / CentOS / AlmaLinux – often used in enterprise environments.
- Arch Linux – rolling release, minimal by default, focused on simplicity and control.
- OpenSUSE, Gentoo, and many others.
Distributions differ mainly by:
- Package manager (
apt,dnf,pacman, etc.). - Release model (stable vs rolling release).
- Default configuration and tooling.
Despite these differences, the core concepts (filesystem layout, permissions, processes, services) are very similar across distributions.
3. Kernel, User Space, and Shell
3.1 Kernel
The kernel is the central part of the operating system. It is responsible for:
- Managing memory and CPU time.
- Talking to hardware devices (disks, network cards, etc.).
- Creating and scheduling processes.
- Enforcing basic security boundaries between processes.
Users and applications normally do not talk directly to the kernel; they use system calls via libraries and tools.
3.2 User Space
User space contains all programs that run "on top" of the kernel, such as:
- Command-line tools (
ls,cp,mv,ps, etc.). - System utilities (
ip,systemctl,journalctl, etc.). - Libraries (for example the C standard library
glibc).
These tools are what administrators and scripts interact with in daily work.
3.3 Shell
The shell is the command-line interpreter used to run commands and scripts. Common shells include:
- Bash (Bourne Again SHell).
- Zsh.
- Fish.
The shell provides features such as:
- Command history and completion.
- Variables and environment management.
- Scripting (loops, conditions, functions).
For concrete shell usage and commands, see:
4. Filesystem and Paths
Linux uses a single hierarchical filesystem tree that starts at the root directory /.
Typical top-level directories include:
/bin,/usr/bin– essential user commands./sbin,/usr/sbin– system administration commands./lib,/usr/lib– shared libraries./etc– system-wide configuration files./var– variable data (logs, spool, caches)./home– home directories for regular users./root– home directory for therootuser./tmp– temporary files.
Additional mount points (such as /mnt, /srv, or custom locations) are used for disks, network shares, or application data.
Paths can be:
- Absolute – start with
/(for example/etc/hosts). - Relative – based on the current working directory (for example
../logs).
5. Users, Groups, and Permissions
Linux is a multi-user system. Access control is based on users, groups, and file permissions.
5.1 Users and Groups
- Each user has a numeric user ID (UID) and belongs to one or more groups (GID).
- The special user
root(UID 0) has full administrative privileges. - Regular users have limited permissions and may use tools like
sudoto perform administrative tasks.
5.2 File Permissions
Every file and directory has:
- An owner user.
- An owner group.
- Permission bits for user, group, and others:
- Read (
r) - Write (
w) - Execute (
x)
- Read (
Permissions control who can read, modify, or execute a file or enter a directory.
6. Processes and Services
6.1 Processes
A process is an instance of a running program. Each process has:
- A process ID (PID).
- A parent process.
- An associated user.
Basic operations include listing processes, checking resource usage, and sending signals to stop or reload them.
Concrete commands for process inspection are listed in:
6.2 Services and systemd
Many modern distributions use systemd as the init system and service manager.
systemd is responsible for:
- Starting services at boot.
- Managing service dependencies.
- Monitoring and restarting services on failure.
Service definitions are stored as units (for example ssh.service, docker.service).
For more detailed administration topics (services, logs, units), see:
7. Networking Overview
Linux provides a full networking stack, including:
- IPv4 and IPv6 addressing.
- Routing between interfaces.
- Firewalling and packet filtering.
Network configuration usually involves:
- Assigning IP addresses to interfaces.
- Setting gateways and routes.
- Configuring DNS resolvers.
For networking concepts specific to this knowledge base, see:
8. Package Management
Each distribution uses a package manager to install, update, and remove software:
- Debian/Ubuntu:
apt,apt-get,dpkg. - RHEL/Fedora:
dnf,yum,rpm. - Arch Linux:
pacman.
Package managers handle:
- Downloading software from repositories.
- Dependency resolution.
- Keeping a local database of installed packages.
Concrete command examples for package management can be added to the Cheat Sheet or distribution-specific notes.
9. Where to Go Next
- For hands-on commands and quick reminders, see Linux / Cheat Sheet.
- For administration tasks (services, logs, users, updates), see Linux / Administration.
- For networking details, see Network / Basics.
This basics document is intended as a neutral foundation that other, more specific guides can reference.