Unix / Linux - User Administration

In this chapter, we will discuss in detail about user administration in Unix.
There are three types of accounts on a Unix system −
This is also called superuser and would have complete and unfettered control of the system. A superuser can run any commands without any restriction. This user should be assumed as a system administrator.
System accounts are those needed for the operation of system-specific components for example mail accounts and the sshd accounts. These accounts are usually needed for some specific function on your system, and any modifications to them could adversely affect the system.
User accounts provide interactive access to the system for users and groups of users. General users are typically assigned to these accounts and usually have limited access to critical system files and directories.
Unix supports a concept of Group Account which logically groups a number of accounts. Every account would be a part of another group account. A Unix group plays important role in handling file permissions and process management.
There are four main user administration files −
/etc/passwd − Keeps the user account and password information. This file holds the majority of information about accounts on the Unix system.
/etc/passwd − Keeps the user account and password information. This file holds the majority of information about accounts on the Unix system.
/etc/shadow − Holds the encrypted password of the corresponding account. Not all the systems support this file.
/etc/shadow − Holds the encrypted password of the corresponding account. Not all the systems support this file.
/etc/group − This file contains the group information for each account.
/etc/group − This file contains the group information for each account.
/etc/gshadow − This file contains secure group account information.
/etc/gshadow − This file contains secure group account information.
Check all the above files using the cat command.
The following table lists out commands that are available on majority of Unix systems to create and manage accounts and groups −
useradd
Adds accounts to the system
usermod
Modifies account attributes
userdel
Deletes accounts from the system
groupadd
Adds groups to the system
groupmod
Modifies group attributes
groupdel
Removes groups from the system
You can use Manpage Help to check complete syntax for each command mentioned here.
We will now understand how to create a group. For this, we need to create groups before creating any account otherwise, we can make use of the existing groups in our system. We have all the groups listed in /etc/groups file.
All the default groups are system account specific groups and it is not recommended to use them for ordinary accounts. So, following is the syntax to create a new group account −
 groupadd [-g gid [-o]] [-r] [-f] groupname

The following table lists out the parameters −
-g GID
The numerical value of the group's ID
-o
This option permits to add group with non-unique GID
-r
This flag instructs groupadd to add a system account
-f 
This option causes to just exit with success status, if the specified group already exists. With -g, if the specified GID already exists, other (unique) GID is chosen
groupname
Actual group name to be created
If you do not specify any parameter, then the system makes use of the default values.
Following example creates a developers group with default values, which is very much acceptable for most of the administrators.
$ groupadd developers

To modify a group, use the groupmod syntax −
$ groupmod -n new_modified_group_name old_group_name

To change the developers_2 group name to developer, type −
$ groupmod -n developer developer_2

Here is how you will change the financial GID to 545 −
$ groupmod -g 545 developer

We will now understand how to delete a group. To delete an existing group, all you need is the groupdel command and the group name. To delete the financial group, the command is −
$ groupdel developer

This removes only the group, not the files associated with that group. The files are still accessible by their owners.
Let us see how to create a new account on your Unix system. Following is the syntax to create a user's account −
useradd -d homedir -g groupname -m -s shell -u userid accountname

The following table lists out the parameters −
-d homedir
Specifies home directory for the account
-g groupname
Specifies a group account for this account
-m
Creates the home directory if it doesn't exist
-s shell
Specifies the default shell for this account
-u userid
You can specify a user id for this account
accountname
Actual account name to be created
If you do not specify any parameter, then the system makes use of the default values. The useradd command modifies the /etc/passwd, /etc/shadow, and /etc/group files and creates a home directory.
Following is the example that creates an account mcmohd, setting its home directory to /home/mcmohd and the group as developers. This user would have Korn Shell assigned to it.
$ useradd -d /home/mcmohd -g developers -s /bin/ksh mcmohd

Before issuing the above command, make sure you already have the developers group created using the groupadd command.
Once an account is created you can set its password using the passwd command as follows −
$ passwd mcmohd20
Changing password for user mcmohd20.
New UNIX password:
Retype new UNIX password:
passwd: all authentication tokens updated successfully.

When you type passwd accountname, it gives you an option to change the password, provided you are a superuser. Otherwise, you can change just your password using the same command but without specifying your account name.
The usermod command enables you to make changes to an existing account from the command line. It uses the same arguments as the useradd command, plus the -l argument, which allows you to change the account name.
For example, to change the account name mcmohd to mcmohd20 and to change home directory accordingly, you will need to issue the following command −
$ usermod -d /home/mcmohd20 -m -l mcmohd mcmohd20

The userdel command can be used to delete an existing user. This is a very dangerous command if not used with caution.
There is only one argument or option available for the command .r, for removing the account's home directory and mail file.
For example, to remove account mcmohd20, issue the following command −
$ userdel -r mcmohd20

If you want to keep the home directory for backup purposes, omit the -r option. You can remove the home directory as needed at a later time.

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