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| System and Process Management Commands | File and Directory Operations | Automation and Scripting Basics | Package Management Commands | Networking and Connectivity Commands | User and Permission Management | Disk and File System Management | DevOps and Automation Specific Commands | Docker Commands | Kubernetes Commands | Security and Encryption |

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System and Process Management Commands

System Monitoring:

    top -> Displays system tasks, CPU, and memory usage in real-time.
    
    htop -> An interactive version of top with more visual details (if installed).
    
    uptime -> Shows how long the system has been running and the load averages.

    df -h -> Reports disk space usage in a human-readable format.
    
    free -m -> Displays available and used memory in megabytes.
    
    vmstat -> Provides information about system processes, memory, paging, block IO, traps, and CPU activity.

Process Management:

    ps -aux -> Lists all running processes with detailed information.
    
    kill -9 PID -> Forces termination of a process using its Process ID (PID).
    
    pkill process_name -> Kills processes by name.
    
    nohup command & -> Runs a command in the background, unaffected by hangups.
    
    nice -n 10 command -> Runs a command with a lower priority.

File and Directory Operations

File Manipulation:

    touch file.txt -> Creates an empty file or updates the timestamp of an existing file.
    
    cp file1.txt /destination/ -> Copies files from one location to another.
    
    mv file.txt /destination/ -> Moves or renames files.
    
    rm -rf /directory -> Recursively removes directories (be cautious with this command).
    
    cat file.txt -> Displays file contents.
    
    head -n 10 file.txt -> Shows the first 10 lines of a file.
    
    tail -n 10 file.txt -> Shows the last 10 lines of a file.
    
    less file.txt -> Paginates through a file.

Directory Operations:

    mkdir my_folder -> Creates a new directory.
    
    rmdir my_folder -> Removes an empty directory.
    
    cd /path/to/directory -> Changes the working directory.
    
    pwd -> Prints the current working directory.
    
    ls -al -> Lists all files and directories with detailed permissions and sizes.

Automation and Scripting Basics

Variables:

  NAME="Weronika"
  echo "Hello, $NAME"

Conditionals:

if [ -f /etc/passwd ]; then
    echo "File exists"
else
    echo "File does not exist"
fi

Loops:

for i in {1..5}; do
    echo "Number: $i"
done

Functions:

    my_function() {
        echo "This is a function"
    }
    my_function

Package Management Commands

Debian-based Systems (e.g., Ubuntu):

    apt update -> Updates package index.
    
    apt upgrade -y -> Upgrades installed packages.
    
    apt install package_name -y -> Installs a new package.
    
    apt remove package_name -y -> Removes a package.

Red Hat-based Systems (e.g., CentOS, Fedora):

    yum update -> Updates all packages.
    
    yum install package_name -> Installs a new package.
    
    yum remove package_name -> Removes a package.

Networking and Connectivity Commands

Network Configuration:

    ifconfig or ip addr -> Displays network interface information.
    
    ping google.com -> Tests network connectivity.
    
    netstat -tuln -> Shows active network connections and listening ports.
    
    curl -I http://example.com -> Sends a request to a URL and shows the HTTP headers.
    
    wget https://example.com/file.zip -> Downloads files from the internet.

Firewall Management:

    ufw status -> Checks the status of the firewall (Ubuntu/Debian).
    
    ufw allow 22 -> Allows traffic on port 22 (SSH).
    
    iptables -L -> Lists all current firewall rules (Red Hat-based systems).

User and Permission Management

User Commands:

    adduser username -> Creates a new user.
    
    passwd username -> Changes the password for a user.
    
    usermod -aG sudo username -> Adds a user to the sudo group.
    
    whoami -> Displays the current logged-in user.

Permission Commands:

    chmod 755 file.txt -> Sets file permissions.
    
    chown user:group file.txt -> Changes the owner of a file.
    
    sudo -> Runs commands with elevated privileges.

Disk and File System Management

Disk Operations:

    lsblk -> Lists block devices.
    
    fdisk -l -> Displays partition information.
    
    mount /dev/sda1 /mnt -> Mounts a partition.
    
    umount /mnt -> Unmounts a partition.

File System Management:

    du -sh /path -> Shows disk usage for a directory.
    
    df -h -> Displays disk space usage for all mounted filesystems.
    
    fsck /dev/sda1 -> Checks and repairs a filesystem.

DevOps and Automation Specific Commands

Cron Jobs:

    crontab -e -> Edits the crontab for scheduling tasks.

# Example crontab entry:
0 2 * * * /path/to/backup.sh

# Runs a backup script every day at 2 AM.

Service Management:

    systemctl status nginx -> Checks the status of the Nginx service.
    
    systemctl start nginx -> Starts the Nginx service.
    
    systemctl enable nginx -> Enables the service to start on boot.

Log Management:

    tail -f /var/log/syslog -> Follows the system log in real-time.
    
    journalctl -u nginx.service -> Shows logs for the Nginx service.

Docker Commands

Container Operations:

    docker run -d --name my_app nginx -> Runs a Docker container in the background.
    
    docker ps -a -> Lists all containers.
    
    docker exec -it my_app /bin/bash -> Accesses a running container.

Image Management:

    docker images -> Lists Docker images.
    
    docker build -t my_app . -> Builds an image from a Dockerfile.
    
    docker rmi my_app -> Removes a Docker image.

Kubernetes Commands

Basic Operations:

    kubectl get pods -> Lists all pods in the cluster.
    
    kubectl get services -> Lists all services.
    
    kubectl apply -f deployment.yaml -> Applies a configuration file.

Namespace Management:

    kubectl get namespaces -> Lists all namespaces.
    
    kubectl create namespace my-namespace -> Creates a new namespace.

Scaling and Rolling Updates:

    kubectl scale deployment my-deployment --replicas=3 -> Scales a deployment to 3 replicas.
    
    kubectl rollout status deployment/my-deployment -> Checks the rollout status.

Security and Encryption

SSH Commands:

    ssh user@host -> Connects to a remote host via SSH.
    
    scp file.txt user@host:/destination/ -> Securely copies files between hosts.

File Encryption:

    gpg -c file.txt -> Encrypts a file using GPG.
    
    gpg file.txt.gpg -> Decrypts a GPG-encrypted file.