User Administration

There are two types of user:

1) System users 
2) Normal users

The system users have ID values from 0 to 499  & the normal users can have ID values from 500 to 60,000.

The users and groups are maintained by the four databases files. These are:

1) /etc/passwd : This databases file maintains the user information like UID, GID, User name etc.
2) /etc/shadow : This file maintains user password related information like uname, encrypted pwd, etc. The pwd's are encrypted in the pwd binary file. To encrypt the passwords, MD5sum, DES algorithms are used.
3) /etc/group : It maintains group related information like group name, GID, etc.
4) /etc/gshadow : It maintains the group password related information. 

User administration :

For user administration five commands are used.

1) useradd : This command is used to create a new user with default values.
$useradd <options> <user-name>

where the options can be,

-U = User id
-g = group ID (primary)
-G = group ID (secondary)
-c = comment
-d = directory 
-s  = shell 
-f  = inactive days
-e = expire date (YYYY MM DD) 

Lets create an account 

                                     In this case useradd is told to create users  the primary group sith belongs to (-g), and assign rbash as primary command shell (-s) ,Home directory under sith name (/home/sith) is created by default.Also add the userid 880 (-u).Here in the last command you can see the result that the UID = 880 and groups is users.


enter the command #getent passwd
 

 

In creating username ,the command performs several actions

1.   Reads /etc/login.defs and /etc/default/useradd files to get default values .

2.   Check command line parameters to find out which default values override.

3.   Create user entry in /etc/passwd and /etc/shadow

4.   Create any new group entries in the /etc/group file.

5.   Create a home directory based on the user’s name, in the /home directory

6.   Copies any files located within /etc/skel directory to the new home directory.This usually includes login and application startup scripts.

        

If you want to see the default settings

 #useradd –D

   Group=100

   Home=/home

   INACTIVE=1

   EXPIRE=

   SHELL=/bin/bash

   SKEL=/etc/skel

How to change default values of useradd command ?

To change the default home directory location for all new users

# useradd -D -b /opt/users
# useradd -D | grep HOME
HOME=/opt/users

To change the default login shell

 

# useradd -D -s /bin/sh
# useradd -D | grep -i shell
SHELL=/bin/sh

 

Create multiple users with same UID.

# useradd -o deepak -u 501
# useradd -o deep -u 501
# useradd -o user -u 501
 

2) usermod : This command is used to modify the user accounts. Only the administrator can use this command. It's syntax is,
$usermod <options> <user-name>
  
where the options can be,        

-l = to change the user name
-L = to lock the user account
-U = to unlock the user account

  # usermod -s /bin/csh Jedi
  # usermod -Ga sales,marketing  jedi

the first example changes the shell to the csh shell for the user named jedi.In the second example,supplementary groups are added for the user jedi.The -a option (-Ga) makes sure that the supplementary groups are added to any existing groups for the user jedi.If the -a is not used,existing supplementary groups for jedi are erased and the new list of groups includes the only supplementary groups assigned to that user.

3) passwd :  This command is used to generate the passwords for users account.
$passwd < user-name> 
                    
To disable a password,
$passwd -d <user-name>

4) userdel : This command is used to delete a user account.
$userdel <user-name>

5) change : This command is used to change the password expiry information. 
$chage <user name>

Add a User to Multiple Groups

usermod -a -G ftp,admins,othergroup <username>

For instance, lets say you wanted to add a new user named jsmith to the ftp group:

useradd -G ftp jsmith