For example, a manager of a company may hold projected salary increases in a file located in his or her home directory. It is unlikely that the manager would like the employees to be able to have access to this file. This could be valuable to someone, which might want to use this information to allow competitors to grant competitive salaries and steal employees from this company. The manager might also have proprietary information, which is still under patent approval that an unwanted guest might have a strong motive to obtain. To battle this, Unix provides separation of users and access privileges to protect others from gaining or altering information that does not belong to them.
Users gain access to their files by authenticating themselves to the operating system.
Once they have authenticated, or provided credentials proving their identity, they must now be authorized to perform a specific task. Authentication is usually done by providing a login name and a password, which the system shares secretly with the user. This guarantees that the person attempting to authenticate knows the login and the password but this does not mean that the person is who the system believes it is. It is common for users to release their login and password to other users for many reasons. One reason is that another user may have had their account revoked and need access to the system so they ask their friend or co-worker if they can borrow theirs.