Understanding file permissions on Unix: a brief tutorial

http://www.dartmouth.edu/~rc/help/faq/permissions.html

Understanding file permissions on Unix: a brief tutorial

Every user on a Unix system has a unique username, and is a member of at least one group (the primary group for that user). This group information is held in the password file (/etc/passwd). A user can also be a member of one or more other groups. The auxiliary group information is held in the file /etc/group. Only the administrator can create new groups or add/delete group members (one of the shortcomings of the system).
Every directory and file on the system has an owner, and also an associated group. It also has a set of permission flags which specify separate read, write and execute permissions for the 'user' (owner), 'group', and 'other' (everyone else with an account on the computer) The 'ls' command shows the permissions and group associated with files when used with the -l option. On some systems (e.g. Coos), the '-g' option is also needed to see the group information.