User & Group Management

To add a New User Account & Set Password

                  To add or create the user account you need to use the "useradd" or "adduser" command with "username". The username is the login name of the user that is used by a user to log-in into system.
                          Syntax:
                                         # useradd username
                                         # passwd username
                          Example:
                                         # useradd Rahul
                                         # passwd Rahul@dev
                          Verify:
                                         # cat /etc/passwd |grep Rahul

To create a user with different Home Directory
                          
                        By default 'useradd' command creates a user's home directory under /home directory with username.
          However the action can be changed by using  (-d) option along with the location of new home directory ( i.e /etc/rahul ). For example the following command will create a user "Amit" with a home directory ' /etc/rahul'.

                            Syntax:
                                         # useradd -d (home directory) (username)
                            Example:
                                         # useradd -d /etc/rahul Amit
                            Verify:                                     
                                        # cat /etc/passwd |grep Amit

Create a user account with specific User ID (UID) & Group ID (GID) 

                   In Linux, every user has its own UID by default, whenever we create a new user account. But we can create user account with custom userid with (-u) option. for example the following command will create a user 'Rahul' with custom UID '1050'

                             Syntax:
                                         # useradd -u (custom UID) (username)
                             Example:
                                         # useradd -u 1050 Rahul
                             Verify:                                     
                                        # cat /etc/passwd |grep Rahul

               Similarly, Every user has its own GID. We can creates user with specific group ID as well with (-g) option.

                              Syntax:
                                         # useradd -u (custom UID) -g (custom GID) (username)
                             Example:
                                         # useradd -u 1050 -g 1010 Rahul
                             Verify:                                     
                                        # cat /etc/passwd | grep Rahul
                                        # cat /etc/group | grep Rahul

Add a User to multiple Group

                        The (-G) option is used to add a user to additional group. Each group name is separated bye comma with no intervening spaces.
         In the  given example, I am going to add user "Rahul" into multiple group like admin1,admin2 & admin3.

                      # useradd -G admin1,admin2,admin3 Rahul

Now execute one of the following command to verify the multiple groups assigned to the user :
                                   # id Rahul
                                   # groups Rahul

Add a user without Home Directory

                           To create user without their home directories, (-M) option is used with useradd command. For example, the following command will create a user "Rahul" without home directory.
                                    Syntax:
                                                 # useradd -M (username)
                                    Example:
                                        # useradd -M Rahul
                                    Verify:
                                       # cat /etc/passwd |grep Rahul
                                      # ls -l /home/Rahul

Create a user with account expiry date 

                            By default when we add user's with "useradd" command, user account never get expires. i.e there expiry date is set to 0 ( means never expired)
        However we can set the expiry date using (-e) option that sets date in YYYY-MM-DD format. This is helpful for creating temporary account for a specific period of time.

Now we create a user "Rahul" with account expiry date i.e 3rd may 2018.

                                 # useradd -e 2018-05-03 Rahul

To verify the age of account and password, execute the following command

                               # chage -l Rahul

Create a user account with password expiry date

                                The (-f) option is used to define the number of days after a password expires. A value of "0" inactive the user account as soon as the password has expired. By default, the password expiry value set to "-1" means never expire.
     Example:
                      we will set an account password expiry date i.e. 2 days on a user "Rahul" using "-f " options.
                               # useradd -f 2 Rahul

To verify the user password expiry date, execute on of the command:
                           
                             # cat /etc/shadow |grep Rahul
or

                            # passwd -S Rahul

Add a user with custom comments

                               The  (-c) option allow you to add custom comments, such a user's full name, phone no, etc to '/etc/passwd' file. The comment can be added as a single line without any spaces.
for example:
                       The following command will add user "Rahul" and would insert the user's full name,"RahulDev" into the comment field.

                          # useradd -c "RahulDev" Rahul

Now you can see your comments in

                        # cat /etc/passwd |grep Rahul

Change user account login shell 

                                           sometimes, we add users which has nothing to do with login shell or sometimes we require to assign different shells to our users. we can assign different login shell to each user with (-s) option.
                           In the given example, I am going to add a user 'Rahul' without login shell.
i.e /sbin/nologin shell.

                              # useradd -s /sbin/nologin Rahul

To verify the assigned shell to the user in

                             # cat /etc/passwd /Rahul