linux.md

Linux

Account

User

useradd USER -m -s /bin/bash -G sudo 
passwd USER                              # interactively
echo "USER:PASSWORD" | sudo chpasswd     # in shell script

usermod USER -aG sudo           
userdel -r USER    # -r for remove all directories related to USER

who        # show all login users
whoami     # show my username

Change login account

su -        # change to root
su - USER

Group

groupadd GROUP
groupmod GROUP -n NEWGROUPNAME
groupdel GROUP
gpasswd GROUP [-a|-d] USER    # add/delete USER from GROUP

groups USER    # print the groups that USER is in

Example usage

1. add user into a group

   usermod USER -aG GROUP
   gpasswd GROUP -a USER

2. remove user from a group

   gpasswd GROUP -d USER

3. list USER's groups

   groups USER

4. list all group members in GROUP

   cat /etc/group | grep GROUP    # simple usage
   getent group GROUP         # a command that get entries from system

Networking

Static IP

1. For Ubuntu desktop (using network manager), modify /etc/netplan/<something>.yaml

   # Let NetworkManager manage all devices on this system
   network:
     version: 2
     renderer: NetworkManager
     ethernets:
       enp6s0:  # network card name. acquire the name by `ip a`
         dhcp4: no
         addresses: [192.168.0.5/24]  # ip, mask
         gateway4: 192.168.0.1        # gateway ip
         nameservers:   # dns server (optional)
           addresses: [1.1.1.1, 8.8.8.8]

2. Apply the netplan file

   sudo netplan try
   sudo netplan apply