User Administration in Linux

User administration is the most important part of Linux system administration. It is one of the most important objective of RHCSA and RHCE certification exam. In Linux Users and Groups can be administered using user/group administration GUI tool, also users can be created through command line as well in Redhat Enterprise Linux. But I will suggest you to use command line user administration commands as much as possible because in production environment GUI won’t be provided. Many people who are learning Linux don’t know all features of user administration commands and related files. In the next few posts we will discuss all about the user administration and features.

There are three types of users in Linux.

1. Superuser: At the time of Linux installation it is create. Root user has right to make other users & his “userid” & “groupid” is zero in “/etc/passwd” file.
2. Systemuser: These users create by System. They can't login because their shell ‘sbin/noloin’ is default in seventh field in `/etc/passwd' file.
3. Normaluser: These users create by superuser.

To add a user from command line we need to use a command useradd/adduser:

For example “useradd pawan”.

adduser is not a binary but it is a symlink to the old unix command useradd which is present in /usr/sbin directory and this command can only be executed by root. In total, 69 things happen when a command useradd is given, we will discuss all.

When useradd command executed, the information of useradd store in two main configuration files ‘passwd’ & ‘group’ these are related with other configuration files.

Configuration files

|----/etc/passwd
|
|----/etc/group
|
|----/etc/shadow
|
|----/etc/gshadow
|
|----/etc/login.defs
|
|----/etc/default/useradd
|
|----/etc/skel

We will discuss about all these files in details in next post.