User Administration in Linux

Linux - User
Administration
Samuel OJO

Introduction
In this section, I will discuss in detail about user
administration in Linux.
There are three main types of user accounts on a
Linux Server:
Root, System and User Accounts

ROOT ACCOUNT
This is also called superuser and would
have complete control of the Linux
system. A superuser can run any
commands without any restriction. This
user should therefore not be used to
access the system directly, it should be
assumed as a system administrator.

SYSTEM ACCOUNTS
•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 Linux system, and any
modifications to them could adversely
affect the system.

USER ACCOUNTS
User accounts provide interactive access to the system
for users and groups. General users are typically
assigned to these accounts and usually have limited
access to critical system files and directories.
Linux uses Group Account which logically groups a
number of accounts. Every account would be a part of
another group account. A group plays important role in
handling file permissions and process management.

MANAGING USERS AND GROUPS IN LINUX
There are four main user administration files −
1. /etc/passwd − Keeps the user account and password
information. This file holds the majority of information about
accounts on the Unix system.
2. /etc/shadow − Holds the encrypted password of the
corresponding account. Not all the systems support this file.
3. /etc/group − This file contains the group information for
each account.
4. /etc/gshadow − This file contains secure group account
information.

BASIC USER ADMINISTRATION COMMANDS
1 Useradd: - Adds accounts to the system
2 Usermod: - Modifies account attributes
3 Userdel: - Deletes accounts from the system
4 Groupadd: - Adds groups to the system
5 Groupmod: - Modifies group attributes
6 Groupdel: - Removes groups from the system

CREATE A GROUP
groupadd [-g gid [-o]] [-r] [-f] groupname --OR-- groupadd local_admin
1 -g GID - - The numerical value of the group's ID
2 -o -- This option permits to add group with non-unique GID
3 -r -- This flag instructs groupadd to add a system account
4 -f -- This option causes to just exit with success status
5 groupname --- Actual group name to be created
$ groupadd developers

MODIFY A GROUP
To modify a group, use
the groupmod syntax −
$ groupmod -n new_group_name old_group_name
$ groupmod -n sysadmin local_admin
$ groupmod -g 484 sysadmin

DELETE A GROUP
We will quickly look at how to delete a group. To delete
an existing group, all you need is the groupdel
command and the group name, e.g:
$ groupdel local_admin
This removes only the group, not the files associated with
that group. The files are still accessible by their owners.

CREATE AN ACCOUNT
1 -d homedir
Specifies home directory for the account
2 -g groupname
Specifies a group account for this account
3 -m
Creates the home directory if it doesn't exist
4 -s shell
Specifies the default shell for this account
5 -u userid
You can specify a user id for this account
6 accountname
Actual account name to be created
useradd -d homedir -g groupname -m -s shell -u userid accountname

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.
This is the example that creates an account samojo, setting its home
directory to /home/samojo and the group as sysadmin. This user would
have Korn Shell assigned to it
$ useradd -d /home/samojo -g sysadmin -s /bin/ksh samojo
Before issuing the above command, make sure you already have the
sysadmin group created using the groupadd command.
Once an account is created you can set its password using
the passwd command. E.g passwd samojo

MODIFY AN ACCOUNT
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.
Eg, to change the account name samojo to samojo00 and
to change home directory accordingly
$ usermod -d /home/samojo00 -m -l samojo samojo00

DELETE AN ACCOUNT
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. Eg, to remove account samojo00, type the command:
$ userdel -r samojo00
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

User Administration in Linux

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