# Linux cheatsheet

Here is an **extended and fully annotated version** of the **Linux Cheatsheet** blog in Markdown, with:

* **Brief descriptions** for each command
    
* An **Advanced Topics** section covering `cron`, `systemd`, `firewalld`, `SELinux`, and more
    

---

# Linux Cheat sheet

## System Information

| Command | Description |
| --- | --- |
| `uname -a` | Show kernel version and system architecture |
| `hostname` | Display system hostname |
| `uptime` | Show how long the system has been running |
| `whoami` | Show the currently logged-in user |
| `id` | Display user ID and group information |
| `lsb_release -a` | Display Linux distribution info (Debian-based) |
| `cat /etc/os-release` | Show OS information (universal method) |

---

## File and Directory Management

| Command | Description |
| --- | --- |
| `ls -l` | List files with details |
| `ls -a` | Include hidden files |
| `pwd` | Show current directory |
| `cd /path/to/dir` | Change to target directory |
| `mkdir newdir` | Create a new directory |
| `touch file.txt` | Create an empty file |
| `cp source dest` | Copy file or directory |
| `mv old new` | Rename or move a file/directory |
| `rm file.txt` | Delete a file |
| `rm -r dir` | Recursively delete a directory and its contents |

---

## File Permissions

| Command | Description |
| --- | --- |
| `chmod 755 file` | Set permissions (owner: rwx, group/others: rx) |
| `chown user:group file` | Change file ownership |
| `ls -l file` | Show file permissions |

Shorthand permission examples:

```bash
chmod u+x script.sh     # Add execute to user
chmod go-rw file.txt    # Remove read/write from group and others
```

---

## User Management

| Command | Description |
| --- | --- |
| `adduser arpit` | Add new user |
| `passwd arpit` | Set/change password for a user |
| `deluser arpit` | Delete a user |
| `usermod -aG sudo arpit` | Add user to the sudo group |
| `groups` | Show current user's groups |

---

## Package Management

### Debian/Ubuntu

```bash
sudo apt update               # Update package index
sudo apt upgrade              # Upgrade installed packages
sudo apt install <pkg>        # Install package
sudo apt remove <pkg>         # Remove package
sudo apt purge <pkg>          # Remove including config files
```

### Amazon Linux / RHEL / CentOS

```bash
sudo yum update               # Update packages
sudo yum install <pkg>        # Install package
sudo yum remove <pkg>         # Remove package
```

---

## Common Utility Installation

| Utility | Description | Debian-based | RHEL-based |
| --- | --- | --- | --- |
| `curl` | Transfer data from URLs | `sudo apt install curl` | `sudo yum install curl` |
| `dnsutils` | DNS tools: `dig`, `nslookup` | `sudo apt install dnsutils` | `sudo yum install bind-utils` |
| `git` | Version control system | `sudo apt install git` | `sudo yum install git` |
| `htop` | Interactive process viewer | `sudo apt install htop` | `sudo yum install htop` |
| `iproute2` | Modern networking commands | `sudo apt install iproute2` | `sudo yum install iproute` |
| `jq` | JSON processor | `sudo apt install jq` | `sudo yum install jq` |
| `net-tools` | Legacy networking commands | `sudo apt install net-tools` | `sudo yum install net-tools` |
| `netcat` | TCP/UDP networking tool | `sudo apt install netcat` | `sudo yum install nc` |
| `nmap` | Network scanner | `sudo apt install nmap` | `sudo yum install nmap` |
| `python3` | Python interpreter | `sudo apt install python3` | `sudo yum install python3` |
| `pip3` | Python package manager | `sudo apt install python3-pip` | `sudo yum install python3-pip` |
| `screen` | Terminal multiplexer | `sudo apt install screen` | `sudo yum install screen` |
| `telnet` | Test TCP connectivity | `sudo apt install telnet` | `sudo yum install telnet` |
| `tmux` | Terminal multiplexer | `sudo apt install tmux` | `sudo yum install tmux` |
| `tree` | Show directory tree | `sudo apt install tree` | `sudo yum install tree` |
| `unzip` | Extract `.zip` files | `sudo apt install unzip` | `sudo yum install unzip` |
| `wget` | Download files from web | `sudo apt install wget` | `sudo yum install wget` |
| `zip` | Compress files into `.zip` | `sudo apt install zip` | `sudo yum install zip` |

---

## Process Management

| Command | Description |
| --- | --- |
| `ps aux` | List all processes |
| `top` | Real-time view of processes |
| `htop` | Enhanced interactive process viewer |
| `kill PID` | Terminate a process by PID |
| `kill -9 PID` | Force kill a process |
| `pkill name` | Kill processes by name |
| `jobs` | List background jobs |
| `bg` | Resume a job in the background |
| `fg` | Bring background job to foreground |

---

## Networking

| Command | Description |
| --- | --- |
| `ip a` | Show IP addresses |
| `ip r` | Show routing table |
| `ping host` | Ping a host |
| `traceroute host` | Trace route to a host |
| `netstat -tulnp` | Show listening ports |
| `ss -tuln` | Show socket status |
| `curl ifconfig.me` | Show public IP address |
| `host domain.com` | DNS lookup |
| `dig domain.com` | Detailed DNS info |

---

## Disk Usage

| Command | Description |
| --- | --- |
| `df -h` | Show free space on mounted filesystems |
| `du -sh *` | Show size of directories/files in current dir |
| `lsblk` | Show block devices (disks, partitions) |
| `mount` | Show mounted file systems |

---

## Searching and Grep

| Command | Description |
| --- | --- |
| `find . -name "*.log"` | Find files by name pattern |
| `grep "pattern" file.txt` | Search for a pattern in a file |
| `grep -r "pattern" /dir` | Recursive search |
| `locate filename` | Quickly find files (needs `updatedb`) |
| `updatedb` | Update `locate` database |

---

## Compression and Archiving

| Command | Description |
| --- | --- |
| `tar -cvf archive.tar dir/` | Create a tar archive |
| `tar -xvf archive.tar` | Extract tar archive |
| `tar -czvf archive.tar.gz dir/` | Create a compressed tar.gz |
| `tar -xzvf archive.tar.gz` | Extract tar.gz |
| `zip archive.zip file` | Create a zip archive |
| `unzip archive.zip` | Extract a zip archive |

---

## System Monitoring

| Command | Description |
| --- | --- |
| `uptime` | Show system load and uptime |
| `free -h` | Show memory usage |
| `vmstat` | System performance metrics |
| `iostat` | CPU and I/O usage (from `sysstat` package) |
| `sar -u 1 3` | CPU usage over 3 seconds |
| \`dmesg | tail\` |

---

## Miscellaneous

| Command | Description |
| --- | --- |
| `alias ll='ls -la'` | Create a custom shortcut |
| `history` | Show command history |
| `!!` | Run previous command again |
| `!100` | Run command number 100 from history |
| `Ctrl + R` | Reverse search in history |
| `clear` | Clear the terminal |
| `man ls` | Show manual for `ls` |
| `which bash` | Show full path of command |