How to be a Server Administrator

Create a new user

sudo adduser {username}

Change user password

Change from root

sudo passwd {username}

User change their password

passwd

Get user information

id {userName}

Delete user

sudo userdel {userName}

Create a folder for the user

# create a foler by root
sudo mkdir {folder_path}
# change folder owner and group (-R: recursive)
sudo chown -R {userName}:{group} {folder_path}
# list details to check
ls -la

Add an existing user to sudo

sudo adduser {userName} sudo

Unban IP address (Use fail2ban package)

Check banned ip address

sudo fail2ban-client get sshd banip

Unban specific blocked IP address

sudo fail2ban-client set sshd unbanip {ip_addr}

Unban all blocked IP address

sudo fail2ban-client set sshd unbanip --all