Debian

Package Manager

apt update
apt-get update

# Search apt package
apt-cache search --names-only '^openjdk-8*'

Reusable terminal Commands

# General
sudo sh             # switch to root user
cat /etc/passwd     # List all users
pwd                 # Current working Directory
sudo passwd root    # Reset root user password

# Service Commands
sudo service ssh status     # Check the status of the service
sudo service ssh start      # Start the service
sudo service ssh restart    # Restart the service

# Other Service Commands
{start|stop|reload|force-reload|restart|try-restart|status}

# systemctl commands
sudo systemctl status ssh       # Status of SSH service
sudo systemctl status --help    # Help page

# Firewall and Security Settings
sudo ufw enable         # Enable firewall
sudo ufw status         # Check the firewall status for all the ports
sudo ufw allow ssh      # Allow SSH ports

# User Add/update
sudo useradd <username> <groupname>         # Add new user without Home directory. Ex. "sudo useradd foo sudo" (or) "sudo adduser foo -G sudo"; useradd/adduser both are correct.
useradd -m -d /PATH/TO/FOLDER <username>    # Add new user with Home directory. -m for Home dir, -d for home dir path
passwd <username>                           # Set the password for a user
sudo deluser <username>                     # Remove User
sudo deluser --remove-home <username>       # Remove user including home directory. 
usermod -aG sudo <username>                 # Add user to sudo group.

# Get VM Details
cat /etc/os-release
[lsblk](https://www.cyberciti.biz/faq/find-hard-disk-hardware-specs-on-linux/)
lsblk -f                                    # List all disks
sudo blkid
sudo mkfs -t ext4 /dev/sdb1                 # Format to EXT4. Run with caution.
sudo mount /dev/sdb1 /mnt/media             # Mount Disk to OS path
sudo umount DIRECTORY                       # Unmount Directory
sudo umount DEVICE_NAME                     # Unmount Device

# symlink
sudo rm -f /sbin/mkfs.ntfs                  # Remove symlink
sudo ln -s /sbin/mkntfs /sbin/mkfs.ntfs     # Create symlink

Permanent mount

The following line must be added to the /etc/fstab file to automatically the hard disk re-mount after the next restart.

/dev/sdb     /media/sda3     ext4    rw              0       0