User Concept in UNIX and Linux

User Concept in UNIX and Linux

How to Choose a Username and Password

User names must be chosen to consist of the English alphabet letters (a-z, A-Z) and numbers (0-9). The letters used in other alphabets should not be used in user names; Those letters are not accepted by many Linux distributions and other UNIX variants.

It is preferred that the usernames are selected in a manner that will generally invoke the user's real name. Only names, only surnames or suffixes, are common username alternatives.

User passwords must be chosen so complex that they can not be easily guessed by anyone other than the user. Since the checking of the user identity is performed only on a password basis, the password must not be known by anyone other than the user himself or herself.

It is recommended that an active password be at least six letters long and consist of a combination of letters, numbers and punctuation marks. Since the password is based on confidentiality, it must not be stored in paper form or any electronic form.

User names and passwords are case sensitive. In this sense, "wwalter" and "WWalter" are perceived as two different user names.

Criteria for a Good Password 

A good password must be:

• at least 8 characters long
• too complex to predict
• simple enough to remember
• contain at least one case difference
• contain at least one digit (0-9)
• contain at least one punctuation mark
• not contain repetitive numbers or characters 

Periodically, you should change your password regularly. Many system users are forced to change their passwords every 3 months or 6 months (For example banks). 

User Definition File

Traditionally all UNIX settings are stored in text files, this tradition continues under Linux as well. Storing all settings in text files allows all settings to be made with text editors. Any programmer can easily make maintenance tasks by quickly creating programs to organize settings in text files.

 The basic user authentication information is kept in the file /etc/passwd. In this file, each row represents one user: User name, password, user id, group number, personal information (For example; name, surname or phone number), home directory and user shell.  

Example format:

•  username:password:user_id:group_id:additional_info:home_directory:shell

To improve security in modern UNIX variants, encrypted user passwords are kept in the /etc/shadow file, not in the /etc/passwd file. The passwords appear as "x" in the /etc/passwd file. The /etc/shadow file can only be read and written by the system administrator, in other words, "root".


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