Linux Add User To Group

How can I add a user to a group under Linux operating system using command line options? How to add an existing user into a group in Linux using command line options?

Tutorial details
Difficulty	Intermediate (rss)
Root privileges	Yes
Requirements	usermod/useradd
Time	5 minutes

You can use the useradd or usermod commands to add a user to a group. The useradd command creates a new user or updates default new user information. The usermod command modifies a user account, and it is useful to add a user to existing groups. There are two types of groups on Linux operating systems:

1. Primary user group. – It is the group that applied to you when login. Typically it is same as your login name. All of your process and files (including directories/folders) would have your primary group as the group membership. The primary group allows private group membership and security features. Your files or process cannot access by other group members or users on the Linux system.
2. Secondary or supplementary user group – Users can be a member of other groups on the Linux system. It is useful for file sharing and other purposes. A sysadmin can fine-tune security too. For example, if you are a member of a secondary group called cdrom, you can mount and unmout cd-rom drive.

All user account related information are stored in the following files:

1. /etc/passwd – Contains one line for each user account.
2. /etc/shadow – Contains the password information in encrypted formatfor the system’s accounts and optional account aging information.
3. /etc/group – Defines the groups on the system.
4. /etc/default/useradd – This file contains a value for the default group, if none is specified by the useradd command.
5. /etc/login.defs – This file defines the site-specific configuration for the shadow password suite stored in /etc/shadow file.

Linux command to add user to group

1. Add a new user called jerry to secondary group named cartoons on Linux:
   sudo useradd -G cartoons jerry
2. Add a new user called tom to primary group called cartoons:
   useradd -g cartoons tom
3. Linux add a existing user named spike to existing group named cartoons:
   useradd -g cartoons spike

How to become a root user

You must run all command as root user. To become a root user run:
su -
Alternatively use sudo command:
sudo -i

Add a new user to secondary group using useradd

You need to the useradd command to add new users to existing group (or create a new group and then add user). If group does not exist, create it. The syntax is as follows:
useradd -G {group-name} username
In this example, create a new user called vivek and add it to group called developers. First, make sure group developers exists using grep command:
# grep "^developers" /etc/group
Sample outputs:

developers:x:1124:

If you do not see any output then you need to add group developers using the groupadd command:
# sudo groupadd developers
Verify that user vivek does not exists:
# grep "^vivek" /etc/passwd
You should not see any outputs from above command. Finally, add a new user called vivek to group developers:
# useradd -G developers vivek
Setup password for user vivek:
# passwd vivek
Ensure that user added properly to group developers:
# id vivek
Sample outputs:

uid=1122(vivek) gid=1125(vivek) groups=1125(vivek),1124(developers)

Please note that capital G (-G) option add user to a list of supplementary groups. Each group is separated from the next by a comma, with no intervening whitespace. For example, add user jerry to groups admins, ftp, www, and developers, enter:
# useradd -G admins,ftp,www,developers jerry

How to add a new user to primary group using useradd

To add a user tony to group developers use the following command:
# useradd -g developers tony
# id tony

Please note that small g (-g) option add user to initial login group (primary group). The group name must exist. A group number must refer to an already existing group.

How to add a existing user to existing group using usermod

Add existing user tony to ftp supplementary/secondary group with the usermod command using the -a option ~ i.e. add the user to the supplemental group(s). Use only with -G option:
# usermod -a -G ftp tony
In this example, change tony user’s primary group to www, enter:
# usermod -g www tony

usermod command options summary

Option	Purpose
-a--append	Add the user to the supplementary group(s). Use only with the -G option.
-g GROUP --gid GROUP	Use this GROUP as the default group.
-G GRP1,GRP2--groups GRP1,GRP2	Add the user to GRP1,GRP2 secondary group.

A note about security

If you add or delete user to existing group, you must change the owner of any crontab files or at jobs manually. You must make any changes involving NIS on the NIS server too.

A note about GUI tool

You will probably find the use of the GUI tool easy. KDE user can use KUser tool and the GNOME user can use users-admin:

One can easily add users as follows from the gnome 3 settings on a Debian/Ubuntu Linux:
Fedora/RHEL/CentOS user can use system-config-users command as follows
# system-config-users
$ ssh -X -t vivek@server1.cyberciti.biz sudo system-config-users
Sample outputs:

Click on the Groups tab to add or view groups:

See also

For more information type the following command at the shell prompt:
$ man usermod
$ man useradd

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