When you are installing Gentoo (or any other distribution, or even operating system for that matter) you make choices depending on the environment you are working with. A setup for a server differs from a setup for a workstation. A gaming workstation differs from a 3D rendering workstation.

This is not only true for choosing what packages you want to install, but also what features a certain package should support. If you don't need OpenGL, why would you bother installing OpenGL and build OpenGL support in most of your packages? If you don't want to use KDE, why would you bother compiling packages with KDE-support if those packages work flawless without?

To help users in deciding what to install/activate and what not, we wanted the user to specify his environment in an easy way. This forces the user into deciding what he really wants and eases the process for Portage, our package managment system, to make usefull decisions.

Enter the USE-flags. Such a flag is a keyword that embodies support and dependency-information for a certain concept. If you define a certain USE-flag, Portage will know that you want support for the chosen keyword. Ofcourse this also alters the dependency information for a package.

Lets take a look at a specific example: the kde keyword. If you do not have this keyword in your USE variable, all packages that have optional KDE support will be compiled without KDE support. All packages that have an optional KDE dependency will be installed without installing the KDE libraries (as dependency). If you have defined the kde keyword, then those packages will be compiled with KDE support, and the KDE libraries will be installed as dependency.

By correctly defining the keywords you will receive a system tailored specifically to your needs.

There are two types of USE-flags: global and local USE-flags.

• A global USE-flag is used by several packages, system-wide. This is what most people see as USE-flags.
• A local USE-fag is used by a single package to make package-specific decisions.

A list of available global USE-flags can be found online or locally in /usr/portage/profiles/use.desc. A short (very incomplete) snippet:

gtk     - Adds support for x11-libs/gtk+ (The GIMP Toolkit)
gtk2    - Use gtk+-2.0.0 over gtk+-1.2 in cases where a program supports both.
gtkhtml - Adds support for gnome-extra/gtkhtml
guile   - Adds support for dev-util/guile (interpreter for Scheme)
icc     - Use the Intel C++ Compiler if the package supports it
icc-pgo - Enable PGO data generation or use when use icc.
imap    - Adds support for IMAP

In the hope you are convinced of the importance of USE-flags we will now inform you how to declare USE-flags.

As previously mentioned, all USE-flags are declared inside the USE variable. To make it easy for users to search and pick USE-flags, we already provide a default USE setting. This setting is a collection of USE-flags we think are commonly used by the Gentoo users. This default setting is declared in the /etc/make.profile/make.defaults file. Let us take a look at this default setting:

USE="x86 oss apm arts avi berkdb crypt cups encode foomaticdb gdbm gif gpm gtk 
     imlib jpeg kde gnome libg++ libwww mad mikmod motif mpeg ncurses nls 
     oggvorbis opengl pam pdflib png python qt quicktime readline sdl slang 
     spell ssl svga tcpd truetype X xml2 xmms xv zlib"

As you can see, this variable already contains quite a lot of keywords. Do not alter the /etc/make.profile/make.defaults file to tailor the USE variable to your needs: changes in this file will be undone when you update Portage!

To change this default setting, you need to add or remove keywords to the USE variable. This is done globally by defining the USE variable in /etc/make.conf. In this variable you add the extra USE-flags you require, or remove the USE-flags you don't want. This latter is done by prefixing the keyword with the minus-sign ("-").

For instance, to remove support for KDE and QT but add support for ldap, the following USE can be defined in /etc/make.conf:

USE="-kde -qt ldap"

Sometimes you want to set a certain USE-setting only once. Instead of editing /etc/make.conf twice (to do and undo the USE-changes) you can just declare the USE-variable as environment variable.

As an example we will temporarily remove java from the USE-setting during the installation of mozilla.

# USE="-java" emerge mozilla

Some packages don't only listen to USE-flags, but also provide USE-flags. When you install such a package, the USE-flag they provide is added to your USE setting. To view the list of packages that provide a USE-flag, check /etc/make.profile/use.defaults:

gnome           gnome-base/gnome
gtk             x11-libs/gtk+
qt              x11-libs/qt
kde             kde-base/kdebase
motif           x11-libs/openmotif

Ofcourse there is a certain precendence on what setting has priority over the USE setting. You don't want to declare USE="-java" only to see that java is declared anyway. The precedence for the USE setting is, ordered by priority (first has lowest priority):

1. Default USE setting declared in /etc/make.profile/make.defaults
2. Inherited USE setting if a package from /etc/make.profile/use.defaults is installed
3. User-defined USE setting in /etc/make.conf
4. User-defined USE setting as environment variable

In the next chapter on Portage and Software we will explain how to manage your installed software and how to work with emerge. However, we will give you a primer on emerge by showing you how to view what USE-flags a package uses.

Let us take the example of mozilla: what USE-flags does it listen to? To find out, we use emerge with the --pretend (don't really do anything) and --verbose (give more output) options:

# emerge --pretend --verbose mozilla
These are the packages that I would merge, in order:

Calculating dependencies ...done!
[ebuild  N    ] net-www/mozilla-1.5-r1 +java +crypt -ipv6 -gtk2 +ssl +ldap 
+gnome -debug +mozcalendar -mozaccess -mozxmlterm -moznoirc -moznomail
-moznocompose -moznoxft 

emerge isn't the only tool for this job. In fact, we have a tool dedicated to package information called etcat which resides in the gentoolkit package. First, install gentoolkit:

# emerge --usepkg gentoolkit

Now run etcat with the uses argument to view the USE-flags of a certain package. For instance, for the gnumeric package:

# etcat uses gnumeric
[ Colour Code : set unset ]
[ Legend      : (U) Col 1 - Current USE flags        ]
[             : (I) Col 2 - Installed With USE flags ]

 U I [ Found these USE variables in : app-office/gnumeric-1.2.0 ]
 - - libgda  : Adds GNU Data Access (CORBA wrapper) support for gnumeric
 - - gnomedb : unknown
 + + python  : Adds support/bindings for the Python language
 + + bonobo  : Adds support for gnome-base/bonobo (Gnome CORBA interfaces)