How to assign a group , using useradd

useradd is used to create a new user account , or to update the default options of creating a user .

When creating a user , using useradd , the user is assigned a group , based on the specified options , or on the default system options .

If no group is specified

on the command line , useradd , will check the /etc/logins.defs file , for the variable named : USERGROUPS_ENAB .

@root:~$ cat /etc/login.defs | grep USERGROUPS_ENAB
# grep the USERGROUPS_ENAB variable , 
# from : /etc/login.defs
USERGROUPS_ENAB yes
# In this /etc/login.defs file , 
# USERGROUPS_ENAB value , is set to : 
# yes

If USERGROUPS_ENAB is set to yes , then useradd , will create a group , for that user , which has a name , same as , the user : username . If such group already exists , then the useradd command will fail .

If USERGROUPS_ENAB is set to no , then useradd , will check the value , of the variable : GROUP , defined inside : /etc/default/useradd

@root:~$ cat /etc/default/useradd  | grep GROUP
# grep the value of GROUP
# from /etc/default/useradd 
 
# GROUP=100
 
# The value GROUP is commented 
# out .

If GROUP is defined and not commented out , then useradd , will add the user , to the group , specified by : GROUP .

The GROUP specified in /etc/default/useradd must exist , and it can be either : a number , or a name .

If GROUP is not defined , or is commented out , then useradd , will assign the user , to the group with the group id of : 100 . The group , with the group id of 100 , has a name of : users .

If the -g USER_GROUP , or the --gid USER_GROUP_ID , options are used

then the user , is assigned to the specified USER_GROUP or USER_GROUP_ID. USER_GROUP must be : a group id , or a group name , which exists . And USER_GROUP_ID must be a group id which exists . For example :

@root:~$ /sbin/useradd -g 100 user-1
# Create a user account , with the 
# username of : user-1 
# Using the -g option , assign user-1 to 
# the group , with the group id of : 100 
 
@root:~$ groups user-1
# Print the groups , assigned to :
# user-1
user-1 : users
# user-1 is assigned to the group : 
# users

If the -N , or --no-user-group , options are used

and the -g USER_GROUP , or the --gid USER_GROUP_ID are not used , then the /etc/logins.defs file , is not checked , for the variable USERGROUPS_ENAB , but instead , only the /etc/default/useradd file , is checked for the existence of the variable : GROUP .

If the variable GROUP is defined , then the user will be assigned , to the group defined , in the GROUP variable . The group defined in GROUP , must exist , it will not be created .

If GROUP is not defined , then the user will be assigned to the default group , with an id of : 100 .

If the -N or --no-user-group options are used , with the -g USER_GROUP or the --gid USER_GROUP_ID options , then the value defined in USER_GROUP or USER_GROUP_ID, will be assigned to the user , as its group . The group in USER_GROUP or USER_GROUP_ID , must exist , it will not be created .

@root:~$ /sbin/useradd -N -g daemon daemon-1
# Both the -N and -g options 
# are used . 
# The -g option assign the group 
# daemon , to the user daemon-1 . 
# The -N option is ignored . 
 
@root:~$ groups daemon-1
# Print the user account : daemon-1 , groups 
daemon-1 : daemon
# daemon-1 is assigned to the group daemon 
 
 
@root:~$ cat /etc/login.defs | grep USERGROUPS_ENAB
# grep the USERGROUPS_ENAB variable , 
# from : /etc/login.defs
USERGROUPS_ENAB yes
# USERGROUPS_ENAB is set to yes , as such ,
# if no group options are specified when 
# creating a user account , then  the 
# user account , will be assigned to a 
# group , with the same name , as the 
# user account username . The group 
# will be created , and must not exist .
# If it exists , useradd will fail .
 
 
@root:~$ cat /etc/default/useradd | grep GROUP
GROUP=root
# The group variable in /etc/default/useradd 
# has a value of : root 
 
@root:~$ /sbin/useradd -N user-2 
# The -N option is used , as such /etc/login.defs
# is ignored , and /etc/default/useradd  is 
# consulted for the existence , of the value 
# GROUP . GROUP is set to : root , 
# as such user-2 will be assigned to the group 
# root . The group root must exist , and will 
# not be created . 
 
@root:~$ groups user-2 
# Print the user account : user-2 , groups 
user-2  : root
# user-2 is assigned to the group root

The value GROUP inside the /etc/default/useradd file , can be set by using :

/sbin/useradd -D -g USER_GROUP 
# USER_GROUP can be a group name or id that exists . 
 
/sbin/useradd -D --gid USER_GROUP_ID
# USER_GROUP_ID can be a group id , which exists .
 
# For example 
@root:~$ /sbin/useradd -D -g 100 
# Set the GROUP variable inside 
# /etc/default/useradd , to the 
# value of 100 .  
 
@root:~$ /sbin/useradd -D 
# Display the default options ,
# used by useradd 
GROUP=100
HOME=/home
INACTIVE=-1
EXPIRE=
SHELL=/bin/sh
SKEL=/etc/skel
CREATE_MAIL_SPOOL=no

If the -U , or --user-group , options are used

then a group , with the same user : username , will be created , and assigned to the user . The group must not exist , or the useradd command will fail .

The -U or --user-group options , cannot be used with the -N or --no-user-group options , and cannot be used with the -g USER_GROUP or the --gid USER_GROUP_ID options .

@root:~$ /sbin/useradd -U user-3
# Create a user account , with the 
# username of : user-3
# -U is used , as such a group 
# user-3 will be created , and 
# assigned to the user : user-3 
 
@root:~$ groups user-3
# Print the groups , assigned to :
# user-3
user-3 : user-3
# user-3 is assigned to the group : 
# user-3