Users and Groups

A running program (a process) has the same permissions as the user who executes it.

There are exceptions to this rule, but it explains why there are more users of the system than root ant its real users (see /etc/passwd).

Users usually belong to one or more groups (see /etc/group).

Users and groups names are human-friendly identifiers. The systems uses integers: UIDs and GIDs.

id

Print real and effective UIDs and GIDs.

su

Change user ID or become super-user.

Every file has:

• a user, the file's owner,
• a group of users (the file's owner must be a member of this group).