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# 🐧 Linux Runlevels Guide
This guide provides a concise overview of traditional **runlevels** in Linux systems, particularly for **Red Hat** and **Debian**-based distributions. Runlevels define specific states of system operation, historically managed by the `init` system.
---
## 🔄 System Boot Sequence
```plaintext
BIOS → Bootloader → Kernel → init
```
* **BIOS**: Performs hardware checks via **POST** (Power-On Self Test).
* **Bootloader**: Loads the kernel.
* **Kernel**: Initializes system and mounts the root filesystem.
* **init**: Launches system processes based on the selected runlevel.
---
## 📊 Runlevels Comparison
| Runlevel | Description | Red Hat | Debian |
| -------- | ----------------------------------- | ---------------- | --------------- |
| 0 | Halt / Shutdown | ✅ Supported | ✅ Supported |
| 1 | Single-User Mode | ✅ Supported | ✅ Supported |
| 2 | Multi-User (No Network) | ❌ (Includes Net) | ✅ Supported |
| 3 | Multi-User (Network, No GUI) | ✅ Supported | ✅ Supported |
| 4 | User-Defined / Custom | ✅ Supported | ✅ Supported |
| 5 | GUI Mode / *(Halt on some systems)* | ✅ GUI Mode | ⚠️ Custom/Halt? |
| 6 | Reboot | ✅ Supported | ✅ Supported |
> 💡 **Notes**:
> • **Runlevel 5** on Red Hat typically launches a full graphical environment (GUI).
> • On Debian, runlevels 2–5 are often configured identically and can be customized.
> • Runlevel behavior is configurable via `/etc/inittab` (SysVinit systems).
---
## 🔧 Useful Commands
### ✅ Check Current Runlevel
```bash
runlevel
```
### 🔁 Change Runlevel
```bash
telinit <runlevel>
```
or
```bash
init <runlevel>
```
> ⚠️ **Caution**: Switching runlevels may stop services or terminate user sessions. Use carefully on production systems.
---
## 🚀 Modern Systems: systemd Targets
Most modern Linux distributions use **systemd**, which replaces runlevels with **targets**.
| Runlevel | systemd Target |
| -------- | ------------------- |
| 0 | `poweroff.target` |
| 1 | `rescue.target` |
| 3 | `multi-user.target` |
| 5 | `graphical.target` |
| 6 | `reboot.target` |
### 📌 Common systemd Commands
```bash
# Show default target
systemctl get-default
# Change to graphical mode
systemctl isolate graphical.target
```

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# 📁 Linux Directory Structure & Basic Commands
This document provides an overview of important Linux directories, their types, and essential file management commands.
---
## 📂 Directory Types
| Type | Description |
| ----------- | ----------------------------------------------------------------------------------- |
| **Virtual** | Do not reside on disk; dynamically generated by the system (e.g., `/proc`, `/sys`). |
| **Normal** | Standard directories that store files and data on disk. |
---
## 🗂️ Common Linux Directories
| Directory | Type | Description |
| --------- | ------- | ----------------------------------------------------------- |
| `/etc` | Normal | System configuration files |
| `/opt` | Normal | Optional or third-party software packages |
| `/bin` | Normal | Essential binary executables for all users |
| `/sbin` | Normal | System binaries, typically for administrative tasks |
| `/lib` | Normal | Shared libraries and kernel modules |
| `/home` | Normal | User home directories |
| `/proc` | Virtual | Kernel and process information (virtual files) |
| `/srv` | Normal | Data for services provided by the system (e.g., web, FTP) |
| `/sys` | Virtual | Kernel devices and system information |
| `/usr` | Normal | Secondary hierarchy: programs, libraries, and documentation |
| `/var` | Normal | Variable data: logs, mail, print spool, temporary files |
| `/dev` | Virtual | Device files (e.g., disk, USB, terminals) |
| `/mnt` | Normal | Mount point for temporary filesystems |
| `/boot` | Normal | Boot loader files, kernel images |
---
## 🛠️ Basic File & Directory Commands
| Command | Description |
| ------- | ---------------------------------------- |
| `cd` | Change directory |
| `ls` | List directory contents |
| `rm` | Remove files or directories |
| `mkdir` | Create a new directory |
| `touch` | Create an empty file or update timestamp |
| `cp` | Copy files or directories |
| `mv` | Move or rename files or directories |
### 📘 Examples
```bash
cd /etc # Navigate to /etc directory
ls -l # List files in long format
rm file.txt # Delete file.txt
mkdir new_folder # Create a directory named 'new_folder'
touch file.txt # Create a new empty file
cp file1.txt file2.txt # Copy file1.txt to file2.txt
mv file.txt /home/user/ # Move file.txt to another directory
```

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# 📦 Linux Libraries & Package Management
This guide provides a quick overview of **library types**, **package sources**, and **Debian-based package management** commands using `apt`.
---
## 📚 Library Types
Linux supports two main types of libraries:
1. **Static Libraries (`.a`)**
- Linked at **compile-time**
- Included in the final binary
- Larger file size, faster execution
2. **Shared Libraries (`.so`)**
- Linked at **run-time**
- Saved separately from the binary
- Saves space and allows updates without recompiling
---
## 📦 Package Sources
### ✅ Official Packages
Provided and maintained by the distribution (e.g., Debian, Ubuntu, Red Hat).
### 🌐 Third-Party Packages
Created by external developers or organizations. Use with caution and verify trustworthiness.
---
## 🏬 Linux Package Managers
### 📦 Debian-based Systems
- **Package Manager**: `apt`
- **Low-level Tool**: `dpkg`
- **Package Format**: `.deb`
### 📦 Red Hat-based Systems
- **Package Manager**: `yum` or `dnf` (newer)
- **Low-level Tool**: `rpm`
- **Package Format**: `.rpm`
---
## 🌍 Package Mirrors
Mirrors are alternative download sources for package repositories, often closer geographically for faster updates.

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# 📦 `dpkg` – Debian Package Manager
`dpkg` is the package manager for Debian-based Linux distributions. It is used to install, remove, and manage `.deb` packages directly.
### 📘 Basic Syntax
```bash
dpkg [<option>...] <command>
```
---
## 🌐 Finding Packages
You can browse and search for Debian packages at [pkgs.org](https://pkgs.org).
---
## 📋 Listing Installed Packages
```bash
dpkg -l
dpkg --list
```
---
## 📥 Installing a Package
```bash
dpkg -i <package.deb>
dpkg --install <package.deb>
```
> ⚠️ If there are missing dependencies, run:
```bash
apt install -f
```
---
## 📂 Viewing Package Contents
```bash
dpkg -c <package.deb>
dpkg --contents <package.deb>
```
---
## ℹ️ Getting Package Info
```bash
dpkg -I <package.deb>
dpkg --info <package.deb>
```
---
## 📁 Listing Installed Files
```bash
dpkg -L <package-name>
dpkg --listfiles <package-name>
```
---
## 🧹 Removing a Package (and its config files)
```bash
dpkg -p <package-name>
dpkg --purge <package-name>
```
---
## 📝 Checking Package Status
```bash
dpkg -s <package-name>
dpkg --status <package-name>
```
---
## 🔧 Reconfiguring a Package
```bash
dpkg-reconfigure <package-name>
```

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# 📦 Managing Packages with `apt` on Debian/Ubuntu
The `apt` (Advanced Package Tool) command-line utility allows you to manage software on Debian-based Linux systems. Below are essential `apt` commands grouped by purpose, with clear explanations for each.
---
## 🔄 1. Updating and Upgrading the System
### `sudo apt update`
Fetches the latest package lists from repositories. This ensures your system is aware of the newest available versions of packages.
```bash
sudo apt update
```
---
### `apt upgrade`
Installs the newest versions of all installed packages based on the updated package lists. It does **not** remove or install any other packages.
```bash
apt upgrade
```
---
## 🔍 2. Searching and Viewing Packages
### `apt show <package>`
Displays detailed information about a specific package, including version, dependencies, description, and more.
```bash
apt show <package>
```
---
### `apt list`
Lists packages based on various filters (e.g., installed, upgradable, available). Running it without arguments shows all packages.
```bash
apt list
```
---
### `apt-cache search <pkg name>`
Searches the package cache for packages matching the given name or description. Useful for discovering packages related to a topic or function.
```bash
apt-cache search <pkg name>
```
---
## 📥 3. Installing and Reinstalling Packages
### `apt install <pkg name>`
Installs a package and its dependencies from the repositories.
```bash
apt install <pkg name>
```
---
### `apt reinstall <package>`
Reinstalls the specified package. This is useful if files from a package are accidentally deleted or corrupted.
```bash
apt reinstall <package>
```
---
## ❌ 4. Removing Packages
### `apt remove <package>`
Removes the specified package but **retains configuration files**. Useful when planning to reinstall later without losing settings.
```bash
apt remove <package>
```
---
### `apt purge <package>`
Completely removes the package **along with its configuration files**. Use when you want a clean uninstallation.
```bash
apt purge <package>
```
---
### `apt autoremove`
Automatically removes packages that were installed as dependencies but are no longer needed.
```bash
apt autoremove
```
---
## 🛠️ 5. Advanced Package Handling
### `apt install -f`
Attempts to **fix broken dependencies** by installing missing packages. Often used after a failed install.
```bash
apt install -f
```
---
### `apt install --download-only <package>`
Downloads a package without installing it. The downloaded `.deb` files are saved in:
```bash
/var/cache/apt/archives/
```
Example:
```bash
apt install --download-only <package>
```
---
## ✅ Final Notes
* Always start with `sudo apt update` before any install or upgrade.
* Use `apt-cache search` when unsure of a package’s exact name.
* Be cautious with `purge` as it deletes config files too.

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# 🍽️ Managing Packages with `yum` on RHEL/CentOS
`yum` (Yellowdog Updater, Modified) is the traditional package manager for RPM-based Linux distributions such as **Red Hat Enterprise Linux (RHEL)** and **CentOS**. Below is a breakdown of commonly used `yum` commands, categorized and explained in detail.
---
## 🔄 1. Updating the System
### `yum update`
Fetches the latest package metadata and installs available updates for all packages currently installed on the system.
```bash
yum update
```
---
### `yum upgrade`
Performs the same function as `yum update`. In older versions of `yum`, `upgrade` was used for more aggressive upgrades, but in recent versions, they are functionally equivalent.
```bash
yum upgrade
```
---
## 📥 2. Installing and Reinstalling Packages
### `yum install <pkg-name>`
Installs a new package along with any required dependencies.
```bash
yum install <pkg-name>
```
---
### `yum reinstall <pkg-name>`
Reinstalls an already installed package, which is useful if system files have been corrupted or modified.
```bash
yum reinstall <pkg-name>
```
---
### `yum localinstall <file-dir>`
Installs a `.rpm` package from a local file path while resolving dependencies using remote repositories.
```bash
yum localinstall <file-dir>
```
---
## ❌ 3. Removing Packages
### `yum remove <pkg-name>`
Uninstalls the specified package from the system, including any dependencies no longer required.
```bash
yum remove <pkg-name>
```
---
## 🔍 4. Querying and Searching
### `yum list`
Lists all available, installed, and updatable packages.
```bash
yum list
```
---
### `yum info <pkg>`
Displays detailed information about the specified package, including version, repository, summary, and description.
```bash
yum info <pkg>
```
---
### `yum search`
Searches for a keyword in package names and descriptions. Add the search term to complete the command.
```bash
yum search <keyword>
```
Example:
```bash
yum search python
```
---
### `yum deplist <pkg>`
Shows the list of dependencies for a given package.
```bash
yum deplist <pkg>
```
---
## 🕓 5. Package History and Transactions
### `yum history`
Displays a history of all yum transactions (installs, updates, removals), which can be reviewed or undone if needed.
```bash
yum history
```
---
## 💾 6. Downloading Packages (Without Installing)
### `yum download <pkg>`
Downloads an RPM package file without installing it. Useful for transferring or offline installations. Requires the `yum-utils` package.
```bash
yum download <pkg>
```
> 💡 **Note:** If you get an error, try installing `yum-utils`:
>
> ```bash
> yum install yum-utils
> ```
---
## ✅ Final Notes
* `yum` has been replaced by `dnf` in newer distributions (e.g., RHEL 8+), but still widely used in older systems.
* Always run `yum update` before installing new packages.
* Combine with `yum history` to review or undo changes when troubleshooting.

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# 📦 Managing Packages with `rpm` on RHEL/CentOS
The `rpm` command (RPM Package Manager) is the **low-level package management tool** used on **Red Hat-based** systems such as RHEL, CentOS, Fedora, and others. Unlike `yum`, `rpm` does not resolve dependencies automatically and is best used for advanced or manual package management.
Below are the core `rpm` commands categorized and explained.
---
## 📥 1. Installing Packages
### `rpm -i <file.rpm>`
### `rpm --install <file.rpm>`
Installs a new `.rpm` package file. This command will **fail if the package's dependencies are not already installed**.
```bash
rpm -i <file.rpm>
```
or
```bash
rpm --install <file.rpm>
```
> 💡 Tip: Use `yum localinstall` or `dnf install` instead for dependency resolution.
---
## 🔄 2. Upgrading Packages
### `rpm -U <file.rpm>`
### `rpm --upgrade <file.rpm>`
Upgrades an existing package if it is already installed. If the package is not already present, this command will install it.
```bash
rpm -U <file.rpm>
```
or
```bash
rpm --upgrade <file.rpm>
```
---
## ❌ 3. Removing Packages
### `rpm -e <package>`
### `rpm --erase <package>`
Removes a package from the system by name (not the filename). Note that this command also does **not resolve dependencies**, so it may break things if you're not careful.
```bash
rpm -e <package>
```
or
```bash
rpm --erase <package>
```
> ⚠️ Caution: Always check what depends on a package before removing it.
---
## 🔍 4. Querying Packages
### `rpm -q <package>`
Query whether a package is installed, and show its version.
```bash
rpm -q <package>
```
---
### `rpm -qa`
List **all installed packages**.
```bash
rpm -qa
```
---
### `rpm -qi <package>`
Displays detailed **information** about a specific installed package.
```bash
rpm -qi <package>
```
---
### `rpm -ql <package>`
Lists all files installed by a package.
```bash
rpm -ql <package>
```
---
### `rpm -qf <file>`
Finds out **which package** owns a specific file.
```bash
rpm -qf /usr/bin/wget
```
---
### `rpm -qp <file.rpm>`
Query a package file (without installing it) to see its metadata.
```bash
rpm -qp <file.rpm>
```
---
### `rpm -K <file.rpm>`
Verifies the **signature and integrity** of an RPM package file.
```bash
rpm -K <file.rpm>
```
---
## 🛠️ 5. Verifying and Checking
### `rpm -V <package>`
Verifies installed package files against their original metadata (modifications, corruption, etc.).
```bash
rpm -V <package>
```
---
### `rpm --test -i <file.rpm>`
Simulates an install without actually installing the package — useful for testing.
```bash
rpm --test -i <file.rpm>
```
---
## ✅ Final Notes
* `rpm` is a **powerful low-level tool**, but not recommended for resolving dependencies.
* For **automated dependency handling**, prefer using `yum` or `dnf`.
* Use `rpm` when you need precise control over the package management process or are working with standalone `.rpm` files.

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# 🔐 Secure Shell (SSH) – Remote Access & Management
**SSH (Secure Shell)** is a cryptographic network protocol that allows users to **securely access and control remote machines** over a network. It's a foundational tool for system administrators, developers, and anyone managing remote systems.
---
## 🧠 What SSH Does
* **🔒 Encrypts Communication**
All data transmitted between the client and the remote server is encrypted, protecting it from eavesdropping and man-in-the-middle attacks.
* **🧑‍💻 Authenticates Users**
Supports both password-based and public key-based authentication to ensure that only authorized users gain access.
* **💻 Enables Remote Command Execution**
Run commands on a remote machine as if you're using its terminal directly.
* **📁 Supports Secure File Transfers**
Tools like `scp` (Secure Copy) and `sftp` (SSH File Transfer Protocol) allow encrypted file transfers between machines.
---
## 🚀 Installing the SSH Server
To accept SSH connections on a machine, you must install and start the **OpenSSH server**.
### On Debian/Ubuntu systems
```bash
sudo apt install openssh-server
```
* This installs the OpenSSH daemon (`sshd`), which listens for incoming SSH connections.
* To enable and start the service:
```bash
sudo systemctl enable ssh
sudo systemctl start ssh
```
---
### On RHEL/CentOS systems
```bash
sudo yum install openssh-server
```
* Start and enable the SSH service:
```bash
sudo systemctl enable sshd
sudo systemctl start sshd
```
---
## 🛠️ Common SSH Usage Examples
### 🔗 Connect to a Remote Server
```bash
ssh username@remote_host
```
### 📁 Copy a File to a Remote Server Using `scp`
```bash
scp file.txt user@remote_host:/path/to/destination/
```
### 📥 Copy a File from a Remote Server
```bash
scp user@remote_host:/path/to/file.txt .
```
### 🗂️ Use Interactive File Transfer with `sftp`
```bash
sftp user@remote_host
```
---
## 🔐 Key-Based Authentication (Optional but Recommended)
To use SSH without typing a password every time, you can set up key-based authentication:
1. **Generate SSH key pair on your local machine:**
```bash
ssh-keygen
```
2. **Copy the public key to the remote server:**
```bash
ssh-copy-id user@remote_host
```
3. Now you can SSH without a password prompt:
```bash
ssh user@remote_host
```
---
## ✅ Final Notes
* SSH is a vital tool for managing servers securely.
* Always **disable root login** and **use key authentication** for better security.
* You can configure SSH behavior in `/etc/ssh/sshd_config`.

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# Getting Started with Vim 📝
## 1. Installing Vim
To install Vim on a Debian‑based system (like Ubuntu), run:
```bash
sudo apt update
sudo apt install vim
```
---
## 2. Opening Files or Directories
* **Open a file**
```bash
vim <filename>
```
* **Open a directory**
```bash
vim <directory_name>
```
(This opens Vim’s file browser—helpful for navigating and editing multiple files.)
---
## 3. Vim Modes
Vim works in multiple modes—here are the core ones:
1. **Normal mode** – The default mode; used for navigation and commands.
2. **Insert mode** – For editing text; enter it by pressing `i`, `a`, or `o`.
3. **Visual mode** – For selecting text; enter it with `v`, `V`, or `Ctrl+v`.
---
## 4. Essential Commands
(Type these in **Normal mode** and then press Enter when needed)
| Command | Description |
| ----------- | ----------------------------------------- |
| `:w` | Save (write changes to the current file) |
| `:w <name>` | Save changes to a specified file `<name>` |
| `:wq` | Save and quit Vim |
| `:q` | Quit (only if no unsaved changes exist) |
| `:q!` | Quit without saving (discard all changes) |
---
## 5. Handy Shortcuts
These are core shortcuts used in **Normal mode**:
| Shortcut | Action |
| -------- | -------------------------------- |
| `dd` | Delete (cut) the current line |
| `yy` | Yank (copy) the current line |
| `p` | Paste after the cursor |
| `u` | Undo the last change |
| `gg` | Go to the first line of the file |
---
## 6. Quick Usage Flow
1. **Start Vim**:
```bash
vim example.txt
```
2. **Insert text**:
Press `i` → type your content → press `Esc` to return to **Normal mode**.
3. **Save your work**:
Type `:w` in Normal mode and press Enter.
4. **Make edits**:
* Use `dd` to delete a line
* Use `yy` to copy (yank) a line
* Move the cursor to a new location, then hit `p` to paste
* Press `u` to undo any mistake
5. **Navigate quickly**:
* `gg` to jump to the beginning
* Use arrow keys or `h`, `j`, `k`, `l` for navigation
6. **Finish editing**:
* `:wq` to save and exit
* `:q!` to exit without saving
---
## 7. Tips & Tricks
* **Visual mode**:
Press `v` to start selecting character by character, `V` for line selection, or `Ctrl+v` for block-wise selection.
* **Other navigation**:
* Use `G` to go to the end of the file
* Use a number before a command, e.g., `5dd` deletes 5 lines
---
### Summary
Vim may feel different at first, but once you get comfortable switching between **Insert**, **Normal**, and **Visual** modes, you'll find it’s a powerful and efficient editor.
Happy Vimming! 😊

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# 🗜️ ZIP Compression Cheatsheet
The `zip` command is a widely used tool for compressing files and folders into a `.zip` archive. It is cross-platform and supports features like encryption and customizable compression levels.
---
## 📦 Basic Syntax
```bash
zip [options] archive.zip files
```
---
## 🔧 Create a ZIP Archive
```bash
zip archive.zip file1 file2
```
* Compresses `file1` and `file2` into `archive.zip`
---
## 📈 Maximum Compression
```bash
zip -9 archive.zip file1 file2
```
* `-9`: Use maximum compression (slower, but smaller file size)
---
## 🔐 Create a Password-Protected ZIP
```bash
zip -e archive.zip file1 file2
```
* `-e`: Prompts for password encryption on the archive
---
## 📋 List Files in a ZIP Archive
```bash
unzip -l archive.zip
```
* `-l`: Lists contents of the ZIP without extracting
---
## 📂 Extract ZIP Archive
```bash
unzip archive.zip
```
* Extracts contents into the current directory
### 📁 Extract to Custom Directory
```bash
unzip archive.zip -d <custom-dir>
```
* `-d <custom-dir>`: Extracts files into the specified folder

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# 📦 Linux Compression & Archiving Cheatsheet
## 🔧 Tar Command Basics
The `tar` command is used to create, extract, and manage archive files.
**Syntax:**
```bash
tar [options] [archive_name.tar] [file(s)/directory]
```
### 🛠 Create a `.tar` Archive
```bash
tar -cf archive_file.tar file1 file2 dir1
```
* `-c`: Create a new archive
* `-f`: Specify the archive file name
### 🛠 Create with Verbose Output
```bash
tar -cvf archive_file.tar file1 file2 dir1
```
* `-v`: Verbose mode (shows progress)
### 📂 Extract a `.tar` Archive
```bash
tar -xf archive_file.tar
```
```bash
tar -xvf archive_file.tar
```
* `-x`: Extract files
---
## 🗜 Gzip Compression
**Gzip** (GNU zip) is a fast, commonly used compression tool.
### 🔧 Create `.tar.gz` Archive
```bash
tar -czf archive_file.tar.gz file1 file2 dir1
```
* `-z`: Compress using gzip
### 🔧 Verbose Creation
```bash
tar -czvf archive_file.tar.gz file1 file2 dir1
```
### 📂 Extract `.tar.gz`
```bash
tar -xzf archive_file.tar.gz
```
```bash
tar -xzvf archive_file.tar.gz
```
---
## 🗜 Bzip2 Compression
**Bzip2** offers better compression than gzip but is slower.
### 🔧 Create `.tar.bz2` Archive
```bash
tar -cjf archive_file.tar.bz2 file1 file2 dir1
```
* `-j`: Compress using bzip2
### 🔧 Verbose Creation
```bash
tar -cjvf archive_file.tar.bz2 file1 file2 dir1
```
### 📂 Extract `.tar.bz2`
```bash
tar -xjf archive_file.tar.bz2
```
```bash
tar -xjvf archive_file.tar.bz2
```
---
## 🗜 XZ Compression
**XZ** provides the best compression ratios but is the slowest.
### 🔧 Create `.tar.xz` Archive
```bash
tar -cJf archive_file.tar.xz file1 file2 dir1
```
* `-J`: Compress using xz
### 🔧 Verbose Creation
```bash
tar -cJvf archive_file.tar.xz file1 file2 dir1
```
### 📂 Extract `.tar.xz`
```bash
tar -xJf archive_file.tar.xz
```
```bash
tar -xJvf archive_file.tar.xz
```
---
## 📊 Compression Format Comparison
| Feature | `gzip` | `bzip2` | `xz` |
| ------------------- | ------------ | ---------- | --------- |
| Compression Speed | Fast | Slow | Slowest |
| Decompression Speed | Fast | Slower | Moderate |
| Compression Ratio | Good | Better | **Best** |
| File Extension | `.tar.gz` | `.tar.bz2` | `.tar.xz` |
| Common Use | Web, general | Backups | Archival |

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# 📘 **Using `grep` in Linux/Unix**
`grep` (Global Regular Expression Print) is a powerful command-line utility used to search for text patterns in files. Below are common variations of the `grep` command with examples and explanations.
---
## 🔍 Basic Search
```bash
grep "hello" file1
```
**Description**:
Searches for lines containing the word `hello` in `file1`. The search is **case-sensitive**.
---
## 🔍 Case-Insensitive Search
```bash
grep -i "hello" file1
```
**Description**:
Performs a **case-insensitive** search for `hello` in `file1`. Matches `hello`, `Hello`, `HELLO`, etc.
---
## 🔢 Show Line Numbers
```bash
grep -n "hello" file1
```
**Description**:
Displays matching lines **with their line numbers**.
---
## 🔢 Case-Insensitive with Line Numbers
```bash
grep -in "hello" file1
```
**Description**:
Combines `-i` and `-n` to show line numbers and ignore case.
---
## 🚫 Invert Match
```bash
grep -v "hello" file1
```
**Description**:
Shows lines that **do NOT** contain the word `hello`.
---
## 🚫 Invert, Ignore Case, and Show Line Numbers
```bash
grep -ivn "hello" file1
```
**Description**:
Combines all the above:
* `-i`: Ignore case
* `-v`: Invert match
* `-n`: Show line numbers
Shows all lines that **don’t contain** `hello`, regardless of case, and includes line numbers.
---
## ✅ Summary of Flags
| Flag | Description |
| ---- | -------------------------- |
| `-i` | Ignore case |
| `-n` | Show line numbers |
| `-v` | Invert the match (exclude) |

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# 📘 **Using `less` in Linux/Unix**
`less` is a terminal pager program used to view the content of files one screen at a time. It's especially useful for viewing large files or logs.
---
## 📂 Basic Usage
```bash
less file
```
**Description**:
Opens `file` for viewing.
```bash
less /var/log/syslog
```
**Description**:
Opens the system log file for reading. Useful for examining system logs line by line.
---
## ⌨️ Common Keyboard Shortcuts
Here are the most frequently used keyboard shortcuts when using `less`:
| Key/Command | Action |
| ----------- | ------------------------------------ |
| `space` | Scroll **down** one screen |
| `b` | Scroll **up** one screen |
| `q` | **Quit** `less` |
| `/pattern` | **Search** forward for `pattern` |
| `n` | Go to the **next** match of pattern |
| `N` | Go to the **previous** match |
| `g` | Go to the **first line** of the file |
| `G` | Go to the **last line** of the file |
---
## ✅ Summary
`less` is a vital tool for system administrators, developers, and anyone dealing with large text files. With simple navigation and powerful search capabilities, it makes file reading efficient and user-friendly.

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# 📘 **Using `head` and `tail` Commands in Linux/Unix**
Both `head` and `tail` are essential commands for viewing specific portions of a file quickly, without opening the entire file.
---
## 🔝 `head` — Show the Top of a File
The `head` command displays the beginning part of a file.
### Syntax
```bash
head [options] file
```
### Examples
```bash
head file1
```
**Description**:
Displays the first 10 lines of `file1` (default behavior).
```bash
head -n 5 file1
```
**Description**:
Shows the first 5 lines of `file1`.
---
## 🔚 `tail` — Show the Bottom of a File
The `tail` command displays the end part of a file.
### Syntax
```bash
tail [options] file
```
### Examples
```bash
tail file1
```
**Description**:
Displays the last 10 lines of `file1` (default behavior).
```bash
tail -n 20 file1
```
**Description**:
Shows the last 20 lines of `file1`.
```bash
tail -f file1
```
**Description**:
Follows the file as it grows — useful for watching logs in real-time.
---
## ✅ Summary of Options
| Command | Option | Description |
| ------- | ------------- | ------------------------------------------- |
| `head` | `-n <number>` | Show the first `<number>` lines |
| `tail` | `-n <number>` | Show the last `<number>` lines |
| `tail` | `-f` | Follow the file in real-time (live updates) |

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# 📘 **Using the `wc` Command in Linux/Unix**
`wc` (word count) is a utility that counts lines, words, and bytes or characters in files. It’s useful for quickly getting file size details in text terms.
---
## ⚙️ Syntax
```bash
wc [option] file
```
---
## 🔎 Basic Usage
```bash
wc file
```
**Example output:**
```
5 6 43 file1
```
This output means:
| Number | Meaning |
| ------ | ------------------- |
| `5` | Number of **lines** |
| `6` | Number of **words** |
| `43` | Number of **bytes** |
---
## 📋 Common Options
| Option | Description | Example |
| ------ | ------------------------- | ------------ |
| `-l` | Count **lines** only | `wc -l file` |
| `-w` | Count **words** only | `wc -w file` |
| `-c` | Count **bytes** only | `wc -c file` |
| `-m` | Count **characters** only | `wc -m file` |
---
## 📌 Notes
* `bytes (-c)` counts raw bytes, which might differ from characters (`-m`) in multibyte encodings like UTF-8.
* Without options, `wc` outputs lines, words, and bytes by default.

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# ⚙️ PS Command
The `ps` (process status) command is used to **view running processes** on a Linux system. It’s useful for monitoring and troubleshooting tasks.
---
## 🧾 Basic Usage
### 🔍 Show tasks in the current shell
```bash
ps
```
### 🔍 Show tasks in the current shell with **full info**
```bash
ps -f
```
---
## 🌍 View System-Wide Processes
### 📋 Show **all** processes
```bash
ps -A
# or
ps -e
```
---
### 👤 Show tasks by **specific user**
```bash
ps -u <username>
```
📌 Replace `<username>` with the actual user name.
---
### 📊 Show **detailed info for all** tasks
```bash
ps aux
```
---
## 📘 Output Fields Explained
| Column | Description |
| --------- | -------------------------------------------------- |
| `USER` | Owner of the process (often `root` or your user) |
| `PID` | Process ID |
| `%CPU` | CPU usage percentage |
| `%MEM` | Memory usage percentage |
| `STAT` | Process state: `R` (running), `S` (sleeping), etc. |
| `START` | Time when the process started |
| `TIME` | Total CPU time used |
| `COMMAND` | Command that started the process |
### 📑 Show List Jobs
```bash
jobs
```
### 🔄Move Process From Background To Forground
```bash
fg
```

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# 🗡️ Kill Command
The `kill` command is used to **send signals to processes**, typically to terminate or control them.
```bash
kill [options] <PID>
```
📌 Replace `<PID>` with the Process ID of the target process.
---
## 🚦 Common `kill` Usage Examples
### 💤 Graceful Termination — `SIGTERM` (Signal 15)
```bash
kill 123
```
✅ Politely asks the process to terminate.
*Allows the process to clean up before exiting.*
---
### 🪓 Force Kill — `SIGKILL` (Signal 9)
```bash
kill -9 123
```
💥 **Immediate termination**.
*Doesn’t allow cleanup — use only when necessary.*
🗡️ *Think of it as the "katana" of kill commands.*
### 📑 Multi Process Kill
```bash
pidof <ps-name> | xargs kill -9
```
---
### ⌨️ Interrupt — `SIGINT` (Signal 2)
```bash
kill -2 123
```
🛑 Mimics pressing `Ctrl + C`.
*Often used to stop processes gracefully from the terminal.*
---
### 🔄 Hangup — `SIGHUP` (Signal 1)
```bash
kill -1 123
```
♻️ Requests the process to **reload or restart**.
*Commonly used for daemons or services to reload configs.*
---
## 📋 Summary of Signals
| Signal | Name | Description |
| ------ | --------- | ------------------------------ |
| `1` | `SIGHUP` | Reload configuration / restart |
| `2` | `SIGINT` | Interrupt (like Ctrl + C) |
| `9` | `SIGKILL` | Force kill (cannot be ignored) |
| `15` | `SIGTERM` | Graceful termination (default) |

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# 🧑‍💻 Linux User & Group Management Cheat Sheet
---
## 🔍 Viewing User Info from `/etc/shadow`
```bash
cat /etc/shadow
```
Example entry:
```bash
radin:$y$j9T$gxn.Bgl/nVnzPEeWn0Hrz.$CEeQGtg1TlZ/jwu9Zsz2kkIq3dBtbhJ/bzhVT7cJ1.9:20270:0:99999:7:::
```
| Field | Description | Example |
| ----------------- | -------------------------------------------- | ----------------------------------------------- |
| 👤 Username | User’s login name | `radin` |
| 🔒 Password Hash | Encrypted password | `$y$j9T$gxn.Bgl/nVnzPEeWn0Hrz.$CEeQGtg1TlZ/...` |
| 🗓️ Last Changed | Days since Jan 1, 1970 when password changed | `20270` (which is 20270 + 1970 = year) |
| ⏳ Min Age | Minimum days between password changes | `0` |
| ⏰ Max Age | Maximum days before password must be changed | `99999` |
| ⚠️ Warning Period | Days before expiration user is warned | `7` |
---
## 🛡️ Viewing Groups from `/etc/group`
```bash
cat /etc/group
```
Example:
```bash
radin:x:1000:
sudo:x:27:radin
```
| Field | Description | Example |
| ------------- | ------------------------------------ | --------------- |
| 👥 Group Name | Name of the group | `radin`, `sudo` |
| 🔑 Password | Password (usually `x` means none) | `x` |
| 🆔 GID | Group ID | `1000`, `27` |
| 👤 Users | Users in the group (comma-separated) | `radin` |
---
## 👥 Check Groups for a User
```bash
groups <username>
```
---
## 🆔 View User and Group IDs
```bash
id <username>
```
Example output:
```bash
uid=1000(radin) gid=1000(radin) groups=1000(radin),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),100(users),114(lpadmin)
```
---
## ➕ Adding Users
| Command | Description |
| -------------------------------------- | ----------------------------------------- |
| `useradd <username>` | Add user without home directory |
| `useradd -m <username>` | Add user **with** home directory |
| `useradd -s /bin/bash -m <username>` | Add user with bash shell & home directory |
| `useradd -G group1,root -m <username>` | Add user with home and extra groups |
---
## 🔑 Changing Password
```bash
passwd <username>
```
---
## ❌ Deleting Users
| Command | Description |
| ----------------------- | ------------------------------------ |
| `userdel <username>` | Delete user |
| `userdel -r <username>` | Delete user and their home directory |
| `userdel -f <username>` | Force delete user |
---
## 🔧 Modifying Users (`usermod`)
| Command | Description |
| ---------------------------------------- | ----------------------------------------- |
| `usermod [options] username` | Modify user with various options |
| `usermod -l <newusername> <oldusername>` | Rename user |
| `usermod -d <dir> -m <username>` | Change user home directory and move files |
| `usermod -G <groups> <username>` | Set new groups (replaces old groups) |
| `usermod -aG <group> <username>` | Add user to a supplementary group |
| `usermod -s <shell> <username>` | Change user login shell |

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# 📁 Linux File Permissions: A Quick & Clear Guide
---
## 🔍 Example: Output of `ls -lh`
Run this command in your terminal:
```bash
ls -lh
```
Example output:
```bash
drwxr-xr-x 3 root root 4.0K Jul 1 22:22 dir1
-rw-r--r-- 1 root root 2.0G Jul 1 21:59 test2.tar
```
---
## 🧠 Understanding the Output Columns
| Column | Description |
| -------------------- | --------------------------------------- |
| `d` / `-` | File type (`d` = directory, `-` = file) |
| `rwxr-xr-x` | Permissions (user, group, others) |
| `3` | Number of hard links |
| `root` | Owner (user) |
| `root` | Group |
| `4.0K` / `2.0G` | File size |
| `Jul 1 22:22` | Last modification date |
| `dir1` / `test2.tar` | File or directory name |
---
## 🔤 First Character: File Type Indicator
* `d` → Directory 📂
* `-` → Regular file 📄
---
## 🔠 File Permissions Breakdown
```
drwxr-xr-x
│││ │ │ │
│││ │ │ └─ Permissions for Others (o)
│││ │ └── Permissions for Group (g)
│││ └──── Permissions for User (u)
│└──────── File type (d = directory, - = file)
└───────── Read (r), Write (w), Execute (x)
```
---
## 🔐 Permission Symbols Explained
| Symbol | Meaning |
| ------ | ------- |
| `r` | Read |
| `w` | Write |
| `x` | Execute |
---
## 👥 Permission Entities
| Symbol | Meaning |
| ------ | ------------ |
| `u` | User (owner) |
| `g` | Group |
| `o` | Others |
| `a` | All |
---
## 🔢 Numeric Permission Values (Octal)
| Value | Binary | Permissions | Meaning |
| ----- | ------ | ----------- | ---------------------- |
| 0 | 000 | --- | No permissions |
| 1 | 001 | --x | Execute only |
| 2 | 010 | -w- | Write only |
| 3 | 011 | -wx | Write + Execute |
| 4 | 100 | r-- | Read only |
| 5 | 101 | r-x | Read + Execute |
| 6 | 110 | rw- | Read + Write |
| 7 | 111 | rwx | Read + Write + Execute |
---
## 🛠️ Changing Permissions with `chmod`
### Syntax:
```bash
chmod [permissions] [filename]
```
### Example (numeric):
```bash
chmod 755 myscript.sh
```
### What does `755` mean?
| Entity | Value | Permission |
| ------ | ----- | -------------------------- |
| User | 7 | rwx (read, write, execute) |
| Group | 5 | r-x (read, execute) |
| Others | 5 | r-x (read, execute) |
### Recursive permission change:
```bash
chmod -R <permission> <directory>
```
---
## 👑 Changing Ownership with `chown`
### Syntax:
```bash
chown [options] owner[:group] file
```
### Examples:
Change owner only:
```bash
sudo chown radin file.txt
```
Change owner and group:
```bash
sudo chown radin:dev file.txt
```
Change owner and group recursively (for directories):
```bash
sudo chown -R radin:dev files/
```

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# 🌐 IP Command Cheat Sheet
The `ip` command is a powerful tool for network configuration and management in Linux. It replaces many of the older `ifconfig` functionalities.
---
## 📌 General Syntax
```bash
ip [ OPTIONS ] OBJECT { COMMAND | help }
```
**Common OBJECTS:**
* `addr` – IP addresses
* `link` – Network interfaces
* `route` – Routing tables
* `neigh` – ARP table
* `rule`, `netns`, etc.
---
## 🔍 View Network Information
### Show All IP Addresses
```bash
ip addr show
# or shorthand:
ip a
```
### Show IP for Specific Interface
```bash
ip addr show dev eth0
```
### Show Link Layer Information
```bash
ip link show
```
---
## 🔧 Interface Management
### Bring Interface Down
```bash
ip link set eth0 down
```
### Bring Interface Up
```bash
ip link set eth0 up
```
---
## 🌐 IP Address Configuration
### Add an IP Address
```bash
ip addr add 192.168.1.10/24 dev eth0
```
### Delete an IP Address
```bash
ip addr del 192.168.1.10/24 dev eth0
```
---
## 🛣️ Routing
### Show Routing Table
```bash
ip route show
```
### Add a Default Gateway
```bash
ip route add default via 192.168.1.1
```
### Add a Specific Route
```bash
ip route add 192.168.10.0/24 via 192.168.5.2
```
### Delete a Specific Route
```bash
ip route del 192.168.10.0/24 via 192.168.5.2
```
---
## 🧱 Bridge Interface Management
### Create a Bridge
```bash
ip link add name br0 type bridge
```
### Delete a Bridge
```bash
ip link delete br0
```
### Bring Bridge Up
```bash
ip link set br0 up
```
---
## 🛠️ Example: Set a Static IP Address
```bash
ip link set ens33 up
ip addr add 10.10.40.12/24 dev ens33
ip route add default via 10.10.40.1
```
---
### ✅ Tip:
Always verify with:
```bash
ip a
ip r
```

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# 🛣️ `route` Command Documentation
The `route` command is used to view and manipulate the IP routing table in Linux systems. Below is a concise guide to listing, adding, and deleting routes using `route`.
---
## 📋 View Routing Table
```bash
route -n
```
* **Description**: Displays the kernel routing table.
* **`-n`**: Shows numerical addresses instead of resolving hostnames (faster and cleaner).
---
## ➕ Add Routes
### Add a Network Route
```bash
route add -net 10.10.10.0 netmask 255.255.255.0 gw 192.168.1.1
```
* **`-net 10.10.10.0`**: Specifies the network address.
* **`netmask 255.255.255.0`**: Defines the subnet mask for the network.
* **`gw 192.168.1.1`**: Sets the gateway through which packets will be routed.
### Add a Default Gateway
```bash
route add default gw 192.168.1.1
```
* **default**: Indicates this is the default route.
* **`gw 192.168.1.1`**: The default gateway IP address for all traffic not destined for a known network.
---
## ❌ Delete Routes
### Delete a Network Route
```bash
route del -net 10.10.10.0 netmask 255.255.255.0
```
* Removes the specified network route.
### Delete the Default Route
```bash
route del default
```
* Removes the current default gateway.
---
## 📎 Notes
* These commands typically require **superuser (root)** privileges. Use `sudo` if needed:
```bash
sudo route add ...
```
* Consider using `ip route` instead of `route`, as `route` is deprecated on some modern distributions:
```bash
ip route show
```

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# 🌐 `netstat` and `ss` Command Documentation
This guide provides essential usage examples for the `netstat` and `ss` commands to monitor network connections, listening ports, and socket statistics in Linux.
---
## 📡 `netstat` – Network Statistics
`netstat` is a legacy tool used for displaying network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.
> ⚠️ `netstat` may be deprecated on some systems. Consider using `ss` as a modern replacement.
### 🔍 Show All Active Network Connections
```bash
netstat
```
* Displays all active sockets (both listening and non-listening).
* Includes TCP, UDP, UNIX domain sockets, etc.
---
### 🎧 Show Listening Ports
```bash
netstat -l
```
* Lists all **listening** ports (TCP and UDP).
* Useful for checking which services are waiting for incoming connections.
---
### 🔒 Show Listening TCP Ports
```bash
netstat -lt
```
* Lists only **TCP** ports in the **listening** state.
---
### 📡 Show Listening UDP Ports
```bash
netstat -lu
```
* Lists only **UDP** ports in the **listening** state.
---
### 🧠 Show Listening TCP/UDP Ports with Process Info
```bash
netstat -tulpn
```
* Shows all listening **TCP/UDP** ports.
* Includes **process ID (PID)** and **program name**.
* Useful for identifying which service is using a specific port.
---
## ⚡ `ss` – Socket Statistics (Modern Alternative)
`ss` is a faster and more powerful alternative to `netstat` for displaying socket statistics.
### Common `ss` Options
| Option | Description |
| ------ | -------------------------------------------- |
| `-t` | Show TCP sockets |
| `-u` | Show UDP sockets |
| `-l` | Show only listening sockets |
| `-n` | Show numerical addresses (no DNS resolution) |
| `-p` | Show process using the socket |
| `-a` | Show all sockets |
### Example – Show Listening TCP/UDP with Process Info
```bash
ss -tulpn
```
* Equivalent to `netstat -tulpn`
* Recommended for modern Linux distributions.
---
## ✅ Summary
| Task | `netstat` Command | `ss` Equivalent |
| ----------------------------------- | ----------------- | --------------- |
| Show all connections | `netstat` | `ss -a` |
| Show listening ports | `netstat -l` | `ss -l` |
| Show listening TCP ports | `netstat -lt` | `ss -lt` |
| Show listening UDP ports | `netstat -lu` | `ss -lu` |
| Show listening TCP/UDP with process | `netstat -tulpn` | `ss -tulpn` |

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# 🔗 **Linux Links: Soft Link vs Hard Link**
## 📝 **Types of Links**
In Linux, there are **two types** of links:
1. 🪶 **Soft Link** (Symbolic Link)
2. 🪨 **Hard Link**
---
## ⚙️ **Commands**
### 🪶 **Soft Link (Symbolic Link)**
Acts like a **shortcut** pointing to the original file.
```bash
ln -s <base-file> <link-file>
```
💡 **Example:**
```bash
ln -s file.txt file_link.txt
```
---
### 🪨 **Hard Link**
Points directly to the file's **inode** (physical data on disk).
```bash
ln <base-file> <link-file>
```
💡 **Example:**
```bash
ln file.txt file_hard.txt
```
---
## 📊 **Soft Link vs Hard Link**
| 🏷️ Feature | 🪶 Soft Link (Symbolic) | 🪨 Hard Link |
| ------------------------- | --------------------------------- | ----------------------------- |
| 🔢 **Inode Number** | Different from the original file | Same as original file |
| 🗂 **Cross Filesystem** | ✅ Yes | ❌ No |
| ❌ **If Original Deleted** | Link breaks (becomes invalid) | File still exists |
| 📦 **Storage** | Stores path to original file | Stores actual data reference |
| 🔄 **Update** | Reflects changes in original file | Reflects changes (same inode) |
---
## 🧠 **Quick Notes**
* **Soft Link** → Think *shortcut* 📎
* **Hard Link** → Think *clone reference* 📀
* If you delete the **base file**:
* 🪶 Soft Link → ❌ Broken link
* 🪨 Hard Link → ✅ Still works (data intact)

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# 💽 **Disk Partition Types & Commands Guide**
## 🗂 **Partition Types**
### 1️⃣ **MBR (Master Boot Record)**
* 📏 **Max Disk Size:** 2 TB
* 📦 **Partition Entries:** 4
---
### 2️⃣ **GPT (GUID Partition Table)**
* 📏 **Max Disk Size:** 9 ZB (zettabytes)
* 📦 **Partition Entries:** 128
---
## 📊 **Disk Space & Usage Commands**
### 🖥 **`df` – Show Disk Space Usage**
The `df` command displays information about available and used disk space.
| Command | Description |
| --------------------- | ---------------------------------------- |
| `df` | 📄 Show disk space in default format |
| `df -h` | 🧍 Human-readable sizes (e.g., GB, MB) |
| `df -T` | 📂 Show file system type |
| `df -i` | 🔢 Show inode usage |
| `df -h /home` | 🏠 Show usage for `/home` directory only |
---
### 📦 **`du` – Show Disk Usage**
The `du` command shows how much space files and directories take up.
| Command | Description |
| ----------------------------- | ---------------------------------------- |
| `du` | 📄 Show disk usage in blocks |
| `du -h` | 🧍 Human-readable sizes |
| `du -sh` | 📊 Summary of current directory |
| `du -h --max-depth=1` | 📂 Usage per subdirectory (1 level deep) |
---
## 🔍 **Disk & Partition Information Commands**
### 💿 **`lsblk`**
Shows partitions, disks, sizes, and mount points.
```bash
lsblk
```
---
### 🆔 **`blkid`**
Displays partition UUIDs and types.
```bash
blkid
```