In this tutorial I’ll discuss about user and group management on linux. When you using linux operating system you must know to manage user and group on your system. In linux system user and group has unique id. For user it called as UID (User ID) and for group is GID (Group ID).

UID has ranges number :

• UID 0 are used for root user
• UID 1-200 are used for system users that are assign as statically to the system process.
• UID 201-999 are used for system users  that are assign as dynamically to the system process that do not own files on the system.
• UID 1000+ are used for regular user.

To create new user you can use useradd or adduser and to create new group you can use groupadd.

If you use linux ubuntu, you can use useradd command for add user, password and home directory as automatically. If you use adduser you have to configure it manually.

root@Taufik:~# adduser user1
Adding user `user1' ...
Adding new group `user1' (1003) ...
Adding new user `user1' (1003) with group `user1' ...
Creating home directory `/home/user1' ...
Copying files from `/etc/skel' ...
New password: type your password user
Retype new password: retype your password user
passwd: password updated successfully
Changing the user information for user1
Enter the new value, or press ENTER for the default
          Full Name []: user1
          Room Number []:
          Work Phone []:
          Home Phone []:
          Other []:
Is the information correct? [Y/n]  enter
root@Taufik:~#

As you shown above, when you add the new user, it will be create new user with UID number is 1003 and it will also create new group with GID number is 1003 and create home directory on /home/user1.

To show user information you can see at /etc/passwd

root@Taufik:~# tail /etc/passwd
systemd-resolve:x:103:104:systemd Resolver,,,:/run/systemd:/usr/sbin/nologin
messagebus:x:104:106::/nonexistent:/usr/sbin/nologin
pulse:x:105:107:PulseAudio daemon,,,:/var/run/pulse:/usr/sbin/nologin
xrdp:x:106:109::/run/xrdp:/usr/sbin/nologin
admin:x:1000:1000:,,,:/home/admin:/bin/bash
usbmux:x:107:46:usbmux daemon,,,:/var/lib/usbmux:/usr/sbin/nologin
sshd:x:108:65534::/run/sshd:/usr/sbin/nologin
user1:x:1003:1003:user1,,,:/home/user1:/bin/bash

To create new group you can use groupadd command :

root@Taufik:~# groupadd NEWGROUP

Show group information list on /etc/group :

root@Taufik:~# tail /etc/group
input:x:110:
kvm:x:111:
render:x:112:
admin:x:1000:
ssh:x:113:
taufik:x:1001:
user12:x:1002:
user1:x:1003:
NEWGROUP:x:1005:

To create new user with exiting group (NEWGROUP) you can use command : useradd –group GID newuser

root@Taufik:~# useradd --group 1005 user2
root@Taufik:~# tail /etc/group
kvm:x:111:
render:x:112:
admin:x:1000:
ssh:x:113:
taufik:x:1001:
user12:x:1002:
user1:x:1003:
NEWG:x:1004:
NEWGROUP:x:1005:user2
user2:x:1006:

To add supplementary group on the existing user you can use command : usermod -aG GID username

root@Taufik:~# usermod -aG 1005 user1
root@Taufik:~# tail /etc/group
kvm:x:111:
render:x:112:
admin:x:1000:
ssh:x:113:
taufik:x:1001:
user12:x:1002:
user1:x:1003:
NEWG:x:1004:
NEWGROUP:x:1005:user2,user1
user2:x:1006: