https://devconnected.com/how-to-list-users-and-groups-on-linux/

https://www.section.io/engineering-education/user-groups-and-permissions-linux/

• List All the Users in Linux

  • $ cat /etc/passwd
  • root:x:0:0:root:/root:/bin/bash
  • daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
  • bin:x:2:2:bin:/bin:/usr/sbin/nologin

• List all the groups

  • $ cat /etc/group
  • root:x:0:
  • lxd:x:131:sachin
  • sachin:x:1000:
  • adm:x:4:syslog,sachin

• All the groups the user sachin belongs to

  • $ groups sachin
  • sachin : sachin adm cdrom sudo dip plugdev lpadmin lxd sambashare

• Groups simply allow a user that's a member of group X, access to files and directories that are present on the disk which are assigned the group X

• Add a Group

  • $ sudo groupadd docker

• Add your user to the docker group

  • $ sudo usermod -aG docker sachin
  • $ cat /etc/group | grep -i docker
  • docker:x:998:sachin

• Which process is listening on a port

  • $ netstat -nlp | grep 3005
  • n = numeric
  • l = only listening process
  • p = process id