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About the installation
Choosing the media
Configuring the network
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Installing stage3
Installing base system
Configuring the kernel
Configuring the system
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Working with Gentoo
Portage introduction
USE flags
Portage features
Initscript system
Environment variables
Working with Portage
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Mixing software branches
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Adding functionality
Dynamic management

What are USE flags

The idea behind USE flags

When installing Gentoo (or any other distribution, or even operating system for that matter) users make choices depending on the environment they 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 to install, but also what features a certain package should support. If there is no need for OpenGL, why would someone bother to install and maintain OpenGL and build OpenGL support in most of the packages? If someone doesn't want to use KDE, why would they bother compiling packages with KDE support if those packages work flawlessly without?

To help users in deciding what to install/activate and what not, Gentoo 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 to make useful decisions.

Definition of a USE flag

Enter the USE flags. Such a flag is a keyword that embodies support and dependency-information for a certain concept. If someone defines a certain USE flag, Portage will know that they want support for the chosen keyword. Of course this also alters the dependency information for a package.

Take a look at a specific example: the kde keyword. If this keyword is not in the 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). When the kde keyword is defined, then those packages will be compiled with KDE support, and the KDE libraries will be installed as dependency.

By correctly defining the keywords the system will be tailored specifically to the user's needs.

What USE flags exist

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 list of available global USE flags can be found on the main site or locally in the /usr/portage/profiles/use.desc file.
  • A local USE flag is used by a single package to make package-specific decisions. A list of available local USE flags can be found on the main site or locally in the /usr/portage/profiles/use.local.desc file.

Using USE flags

Declare permanent 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 that are part of the selected profile.

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

To view the currently active USE flags (completely), use emerge --info:

root #emerge --info | grep ^USE
USE="a52 aac acpi alsa branding cairo cdr dbus dts ..."

As can be seen, this variable already contains quite a lot of keywords. Do not alter any make.defaults file to tailor the USE variable to personal needs though: changes in these file will be undone when the Gentoo repository is updated!

To change this default setting, add or remove keywords to/from the USE variable. This is done globally by defining the USE variable in /etc/portage/make.conf. In this variable one can add the extra USE flags required, or remove the USE flags that are no longer needed. 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/portage/make.conf:

FILE /etc/portage/make.confUpdating USE in make.conf
USE="-kde -qt4 ldap"

Declaring USE flags for individual packages

Sometimes users want to declare a certain USE flag for one (or a couple) of applications but not system-wide. To accomplish this, edit /etc/portage/package.use. This is usually a single file, but can also be a directory; see man portage for more information. The following examples assume package.use is a single file.

For instance, to only have berkdb support for mysql:

FILE /etc/portage/package.useEnabling berkdb support for MySQL
dev-db/mysql berkdb

Similarly it is possible to explicitly disable USE flags for a certain application. For instance, to disable java support in PHP (but have it for all other packages through the USE flag declaration in make.conf):

FILE /etc/portage/package.useDisable java support in PHP
dev-lang/php -java

Declaring temporary USE flags

Sometimes users need to set a USE flag for a brief moment. Instead of editing /etc/portage/make.conf twice (to do and undo the USE changes) just declare the USE variable as an environment variable. Remember that this setting only applies for the command entered; re-emerging or updating this application (either explicitly or as part of a system update) will undo the changes that were triggered through the (temporary) USE flag definition.

The following example temporarily removes java from the USE setting during the installation of seamonkey:

root #USE="-java" emerge seamonkey


Of course there is a certain precedence on what setting has priority over the USE setting. 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. User-defined USE setting in /etc/portage/make.conf
  3. User-defined USE setting in /etc/portage/package.use
  4. 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 their current definition as known to Portage.

root #emerge --info

Adapting the entire system to the new USE flags

After having altered USE flags, the system should be updated to reflect the necessary changes. To do so, use the --newuse option with emerge:

root #emerge --update --deep --newuse @world

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

Running emerge --depclean is a dangerous operation and should be handled with care. Double-check the provided list of "obsoleted" packages to make sure it doesn't remove packages that are needed. In the following example we add the -p switch to have depclean only list the packages without removing them:
root #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 app-portage/gentoolkit package; don't forget to emerge it first.

root #revdep-rebuild

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

Package specific USE flags

Viewing available USE flags

Let's take the example of seamonkey: what USE flags does it listen to? To find out, we use emerge with the --pretend and --verbose options:

root #emerge --pretend --verbose seamonkey
These are the packages that I would merge, in order:
Calculating dependencies ...done!
[ebuild   R   ] www-client/seamonkey-1.0.7  USE="crypt gnome java -debug -ipv6
-ldap -mozcalendar -mozdevelop -moznocompose -moznoirc -moznomail -moznopango
-moznoroaming -postgres -xinerama -xprint" 0 kB

emerge isn't the only tool for this job. In fact, there is a tool dedicated to package information called equery which resides in the app-portage/gentoolkit package

root #emerge --ask app-portage/gentoolkit

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

user $equery --nocolor uses =gnumeric-1.6.3 -a
[ Searching for packages matching =gnumeric-1.6.3... ]
[ Colour Code : set unset ]
[ Legend : Left column  (U) - USE flags from make.conf              ]
[        : Right column (I) - USE flags packages was installed with ]
[ Found these USE variables for app-office/gnumeric-1.6.3 ]
 U I
 - - debug  : Enable extra debug codepaths, like asserts and extra output.
              If you want to get meaningful backtraces see
              http://www.gentoo.org/proj/en/qa/backtraces.xml .
 + + gnome  : Adds GNOME support
 + + python : Adds support/bindings for the Python language
 - - static : !!do not set this during bootstrap!! Causes binaries to be
              statically linked instead of dynamically

Satisfying REQUIRED_USE conditions

Some ebuilds require or forbid certain combinations of USE flags in order to work properly. This is expressed via a set of conditions placed in a REQUIRED_USE expression. This conditions ensure that all features and dependencies are complete and that the build will succeed and perform as expected. If any of these are not met, emerge will alert you and ask you to fix the issue.

Some examples of this REQUIRED_USE expression are given below:

Example Description
REQUIRED_USE="foo? ( bar )" If foo is set, bar must be set.
REQUIRED_USE="foo? ( !bar )" If foo is set, bar must not be set.
REQUIRED_USE="foo? ( || ( bar baz ) )" If foo is set, bar or baz must be set.
REQUIRED_USE="^^ ( foo bar baz )" Exactly one of foo bar or baz must be set.
REQUIRED_USE="|| ( foo bar baz )" At least one of foo bar or baz must be set.
REQUIRED_USE="?? ( foo bar baz )" No more than one of foo bar or baz may be set.