User names, also called login names, let users access their own systems and remote systems that have the appropriate access privileges. You must choose a user name for each user account you create.
Keep the following guidelines in mind when creating user or role names:
Be unique within your organization, which might span multiple domains
Contain from two to eight letters and numerals. The first character should be a letter and at least one character should be a lowercase letter.
Even though user names can include a period (.), underscore (_), or hyphen (-), using these characters is not recommended because they can cause problems with some software products.
Consider establishing a standard way of assigning user names so they are easier for you to track. Also, names should be easy for users to remember. A simple scheme when selecting a user name is to use the first name initial and first seven letters of the user's last name. For example, Ziggy Ignatz becomes zignatz. If this scheme results in duplicate names, you can use the first initial, middle initial, and the first six characters of the user's last name. For example, Ziggy Top Ignatz becomes ztignatz. If this scheme still results in duplicate names, consider using the following scheme:
The first initial, middle initial, first five characters of the user's last name,
and the number 1, or 2, or 3, and so on, until you have a unique name.
Each new user name must be distinct from any mail aliases known to the system or to an NIS or NIS+ domain. Otherwise, mail might be delivered to the alias rather than to the actual user.