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| <!-- %MENU% How users are identified and classified --> |
| <h2 class="chapter">29 Users and Groups</h2> |
| |
| <p>Every user who can log in on the system is identified by a unique number |
| called the <dfn>user ID</dfn>. Each process has an effective user ID which |
| says which user's access permissions it has. |
| |
| <p>Users are classified into <dfn>groups</dfn> for access control purposes. Each |
| process has one or more <dfn>group ID values</dfn> which say which groups the |
| process can use for access to files. |
| |
| <p>The effective user and group IDs of a process collectively form its |
| <dfn>persona</dfn>. This determines which files the process can access. |
| Normally, a process inherits its persona from the parent process, but |
| under special circumstances a process can change its persona and thus |
| change its access permissions. |
| |
| <p>Each file in the system also has a user ID and a group ID. Access |
| control works by comparing the user and group IDs of the file with those |
| of the running process. |
| |
| <p>The system keeps a database of all the registered users, and another |
| database of all the defined groups. There are library functions you |
| can use to examine these databases. |
| |
| <ul class="menu"> |
| <li><a accesskey="1" href="User-and-Group-IDs.html#User-and-Group-IDs">User and Group IDs</a>: Each user has a unique numeric ID; |
| likewise for groups. |
| <li><a accesskey="2" href="Process-Persona.html#Process-Persona">Process Persona</a>: The user IDs and group IDs of a process. |
| <li><a accesskey="3" href="Why-Change-Persona.html#Why-Change-Persona">Why Change Persona</a>: Why a program might need to change |
| its user and/or group IDs. |
| <li><a accesskey="4" href="How-Change-Persona.html#How-Change-Persona">How Change Persona</a>: Changing the user and group IDs. |
| <li><a accesskey="5" href="Reading-Persona.html#Reading-Persona">Reading Persona</a>: How to examine the user and group IDs. |
| |
| <li><a accesskey="6" href="Setting-User-ID.html#Setting-User-ID">Setting User ID</a>: Functions for setting the user ID. |
| <li><a accesskey="7" href="Setting-Groups.html#Setting-Groups">Setting Groups</a>: Functions for setting the group IDs. |
| |
| <li><a accesskey="8" href="Enable_002fDisable-Setuid.html#Enable_002fDisable-Setuid">Enable/Disable Setuid</a>: Turning setuid access on and off. |
| <li><a accesskey="9" href="Setuid-Program-Example.html#Setuid-Program-Example">Setuid Program Example</a>: The pertinent parts of one sample program. |
| <li><a href="Tips-for-Setuid.html#Tips-for-Setuid">Tips for Setuid</a>: How to avoid granting unlimited access. |
| |
| <li><a href="Who-Logged-In.html#Who-Logged-In">Who Logged In</a>: Getting the name of the user who logged in, |
| or of the real user ID of the current process. |
| |
| <li><a href="User-Accounting-Database.html#User-Accounting-Database">User Accounting Database</a>: Keeping information about users and various |
| actions in databases. |
| |
| <li><a href="User-Database.html#User-Database">User Database</a>: Functions and data structures for |
| accessing the user database. |
| <li><a href="Group-Database.html#Group-Database">Group Database</a>: Functions and data structures for |
| accessing the group database. |
| <li><a href="Database-Example.html#Database-Example">Database Example</a>: Example program showing the use of database |
| inquiry functions. |
| <li><a href="Netgroup-Database.html#Netgroup-Database">Netgroup Database</a>: Functions for accessing the netgroup database. |
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