Ubuntu

# Display OS version
lsb_release -a

# Change timezone
timedatectl list-timezones
timedatectl set-timezone time_zone

# Read mail
less /var/mail/user_name

# Delete all mail for that particular user
rm /var/mail/user_name

# List all local OS users
cut -d: -f1 /etc/passwd

# Add new user
useradd user_name

# Delete user
userdel user_name
rm -r /home/user_name

# Change user name
usermod -l new_user_name user_name

# Change user password
passwd new_user_name

# Add user to the sudo group
useradd new_user_name sudo