# Linux Basics

## Introduction to Linux/Unix

**Linux** and **Unix** are powerful, multi-user, and multitasking operating systems widely used for servers, desktops, and embedded systems. While Unix originated in the 1970s as a proprietary system, Linux was developed in 1991 as an open-source alternative.

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### Key Characteristics:

1. **Open-Source**: Linux is freely available, and its source code can be modified by anyone.
    
2. **Command-Line Oriented**: Offers robust CLI tools for advanced users.
    
3. **Highly Customizable**: Allows tailoring of the system to specific needs.
    
4. **Stable and Secure**: Frequently used in enterprise environments due to its reliability.
    
5. **Unix-like**: Linux shares many features and commands with Unix, making it a versatile system for developers and system administrators.
    

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## 1\. Kernel

The core part of the Linux operating system that interacts directly with the hardware. It manages system resources, such as CPU, memory, and devices.

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## 2\. Shell

An interface that allows users to interact with the operating system by typing commands. Popular shells include:

* **Bash** (Bourne Again Shell→ recommended)
    
* **Zsh**
    
* **Fish**
    

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## 3\. Command Line Interface (CLI)

A text-based interface where users enter commands to perform tasks.

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## 4\. File System

The hierarchical structure used to store and organize files.

* Root directory: `/` (base of the filesystem)
    
* Home directory: `/home/username`
    

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## 5\. Processes

Programs running on your system. Each process has a unique identifier called a **PID** (Process ID).

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## 6\. Package Manager

Tools used to install, update, and remove software. Common ones include:

* **yum/dnf** (Red Hat/CentOS/Fedora→ recommended)
    
* **apt** (Debian/Ubuntu)
    
* **pacman** (Arch Linux)
    

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## 7\. Permissions

Define who can read, write, or execute a file. Represented by:

* **Owner**
    
* **Group**
    
* **Others**
    

Example: `-rw-r--r--`

* **r**: Read
    
* **w**: Write
    
* **x**: Execute
    

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## 8\. Users and Groups

* **Users**: Individuals with accounts on the system.
    
* **Groups**: Collections of users with shared permissions.
    

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## 9\. Root User

The superuser with full administrative privileges. Be cautious while using root permissions.

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## 10\. Directory

A folder that can contain files or other directories. Common ones include:

* `/home`: User home directories.
    
* `/etc`: Configuration files.
    
* `/var`: Log files and variable data.
    
* `/usr`: User-installed software and libraries.
    

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## 11\. Shell Commands

Examples:

* `ls`: List files in the current directory.  
    Example: `ls -l` (shows detailed information about files and directories).
    
* `cd`: Change to a specific directory.  
    Example: `cd /home` (navigates to the `/home` directory).
    
* `pwd`: Print the current working directory.  
    Example: `pwd` (displays the full path of the current directory).
    
* `cp`: Copy a file from one location to another.  
    Example: `cp file1.txt /home/user/` (copies `file1.txt` to the `/home/user/` directory).
    
* `mv`: Move or rename a file.  
    Example: `mv oldname.txt newname.txt` (renames `oldname.txt` to `newname.txt`).
    
* `rm`: Remove a file or directory.  
    Example: `rm file1.txt` (deletes `file1.txt`).
    

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## 12\. Environment Variables

Settings that affect system or application behavior. Example: `$PATH`, which defines directories to search for executable files.

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## 13\. Text Editors

Used to create or edit text files. Popular ones include:

* **vim** (recommended)
    
* **nano**
    
* **emacs**
    

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## 14\. Scripts

Files containing a series of commands to automate tasks, often written in Bash or other scripting languages.

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## 15\. Log Files

Files that record system and application activity, usually stored in `/var/log`.

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## 16\. Services and Daemons

* **Service**: A background process running continuously (e.g., web server).
    
* **Daemon**: A type of service that starts at boot.
    

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## 17\. Mounting

Connecting storage devices (like USB drives) to the system so you can access their contents.

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## 18\. Networking Commands

* `ping`: Check connectivity to a server.  
    Example: `ping` [`google.com`](http://google.com) (checks if [`google.com`](http://google.com) is reachable).
    
* `ifconfig`/`ip`: Configure network interfaces.  
    Example: `ip addr show` (displays IP addresses of network interfaces).
    
* `ssh`: Connect to a remote machine.  
    Example: `ssh user@192.168.1.1` (connects to a remote machine with the specified IP).
    

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