1. What is Command?

Ans: a command is a directive to a computer program to perform a specific task.

2.    Why we use a command?

Ans: In computing, a command is a directive to a computer program to perform a specific task. It may be issued via a command-line interface, such as a shell, or as input to a network service as part of a network protocol, or as an event in a graphical user interface triggered by the user selecting an option in a menu.

3.   How can we find detail information of any command?

Ans: 

The user command displays the user account attributes. You can use this command to list all attributes of all the system users or all the attributes of specific users.