There are three types of user we can see in the linux system
1) root : 0
2) System User : 1-999
3) Regular user: 1000+
tail /etc/paswd
tail /etc/shadow
grep [userName] /etc/passwd
id root
id [regular user name]
id mail
Here, mail is a system user
useradd [userName]
useradd -u [UID] [userName]
usermod -c "fullName" [userName]
passwd [userName]
passwd -d [userName]
userdel [userName]
userdel -r [userName]
tail /etc/group
grep [groupName] /etc/group
groupadd [groupName]
useradd -G [groupName] [userName]
usermod -G [groupName] [userName]
usermod -G [groupName-1,groupName-2] [userName]
groupadd -g [GID] [groupName]
groupmod -n [newGroupName] [oldGroupName]
gpasswd -d [userName] [groupName]
groupdel [groupName]