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 flawlessly without?

To help users in deciding what to install/activate and what not, we wanted the user to specify his/her environment in an easy way. This forces the user into deciding what they really want and eases the process for Portage, our package managment system, to make useful 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. Of course this also alters the dependency information for a package.

Let us 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-flag 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

A list of available local USE-flags can be found locally in /usr/portage/profiles/use.local.desc.

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 make.defaults files part of your profile.

The profile your system listens to is pointed to by the /etc/make.profile symlink. Each profile works on top of another, larger profile, the end result is therefore the sum of all profiles. The top profile is the base profile (/usr/portage/profiles/base).

Let us take a look at this default setting for the 2004.3 profile:

(This example is the sum of the settings in base, default-linux,
 default-linux/x86 and default-linux/x86/2004.3)
USE="x86 oss apm arts avi berkdb bitmap-fonts crypt cups encode fortran f77
     foomaticdb gdbm gif gpm gtk gtk2 imlib jpeg kde gnome libg++ libwww mad
     mikmod motif mpeg ncurses nls oggvorbis opengl pam pdflib png python qt
     quicktime readline sdl 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 any 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 declare a certain USE flag for one (or a couple) of applications but not system-wide. To accomplish this, you will need to create the /etc/portage directory (if it doesn't exist yet) and edit /etc/portage/package.use.

For instance, if you don't want berkdb support globally but you do want it for mysql, you would add:

dev-db/mysql berkdb

You can of course also explicitly disable USE flags for a certain application. For instance, if you don't want java support in PHP:

dev-php/php -java

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. Remember that, when you re-emerge or update this application (either explicitly or as part of a system update) your changes will be lost!

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

# USE="-java" emerge mozilla

After certain packages are installed, additional USE flags will automatically be enabled for you if you do not explicitly disable them. To view the list of packages that trigger automatic USE-flags, check /etc/make.profile/use.defaults and the use.defaults files of the parent profiles.

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

Of course there is a certain precedence on what setting has priority over the USE setting. You don't want to declare USE="-java" only to see that java is still used due to a setting that has a higher priority. The precedence for the USE setting is, ordered by priority (first has lowest priority):

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

To view the final USE setting as seen by Portage, run emerge --info. This will list all relevant variables (including the USE variable) with the content used by Portage.

# emerge --info

If you have altered your USE flags and you wish to update your entire system to use the new USE flags, use emerge's --newuse option:

# emerge --update --deep --newuse world

Next, run Portage's depclean to remove the conditional dependencies that were emerged on your "old" system but that have been obsoleted by the new USE flags.

# emerge -p --depclean

When depclean has finished, run revdep-rebuild to rebuild the applications that are dynamically linked against shared objects provided by possibly removed packages. revdep-rebuild is part of the gentoolkit package; don't forget to emerge it first.

# revdep-rebuild

When all this is accomplished, your system is using the new USE flag settings.

Let us take the example of mozilla: what USE-flags does it listen to? To find out, we use emerge with the --pretend and --verbose 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 equery which resides in the gentoolkit package. First, install gentoolkit:

# emerge gentoolkit

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

# equery 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)