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<?xml version='1.0' encoding='UTF-8'?>
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<!DOCTYPE sections SYSTEM "/dtd/book.dtd">
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<!-- The content of this document is licensed under the CC-BY-SA license -->
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<!-- See http://creativecommons.org/licenses/by-sa/2.5 -->
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<!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/handbook/hb-working-use.xml,v 1.51 2012/07/09 04:34:16 nightmorph Exp $ -->
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<sections>
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<abstract>
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USE flags are a very important aspect of Gentoo. In this chapter, you learn to
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work with USE flags and understand how USE flags interact with your system.
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</abstract>
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<version>3</version>
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<date>2012-07-24</date>
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<section>
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<title>What are USE flags?</title>
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<subsection>
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<title>The ideas behind USE flags</title>
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<body>
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<p>
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When you are installing Gentoo (or any other distribution, or even operating
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system for that matter) you make choices depending on the environment you are
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working with. A setup for a server differs from a setup for a workstation.
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A gaming workstation differs from a 3D rendering workstation.
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</p>
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<p>
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This is not only true for choosing what packages you want to install, but also
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what features a certain package should support. If you don't need OpenGL, why
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would you bother installing OpenGL and build OpenGL support in most of your
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packages? If you don't want to use KDE, why would you bother compiling packages
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with KDE support if those packages work flawlessly without?
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</p>
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<p>
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To help users in deciding what to install/activate and what not, we wanted the
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user to specify his/her environment in an easy way. This forces the user into
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deciding what they really want and eases the process for Portage, our package
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management system, to make useful decisions.
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</p>
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</body>
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</subsection>
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<subsection>
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<title>Definition of a USE flag</title>
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<body>
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<p>
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Enter the USE flags. Such a flag is a keyword that embodies support and
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dependency-information for a certain concept. If you define a certain USE flag,
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Portage will know that you want support for the chosen keyword. Of course
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this also alters the dependency information for a package.
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</p>
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<p>
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Let us take a look at a specific example: the <c>kde</c> keyword. If you do not
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have this keyword in your <c>USE</c> variable, all packages that have
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<e>optional</e> KDE support will be compiled <e>without</e> KDE support. All
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packages that have an <e>optional</e> KDE dependency will be installed
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<e>without</e> installing the KDE libraries (as dependency). If you have defined
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the <c>kde</c> keyword, then those packages <e>will</e> be compiled with KDE
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support, and the KDE libraries will be installed as dependency.
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</p>
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<p>
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By correctly defining the keywords you will receive a system tailored
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specifically to your needs.
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</p>
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</body>
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</subsection>
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<subsection>
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<title>What USE flags exist?</title>
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<body>
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<p>
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There are two types of USE flags: <e>global</e> and <e>local</e> USE flags.
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</p>
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<ul>
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<li>
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A <e>global</e> USE flag is used by several packages, system-wide. This is
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what most people see as USE flags.
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</li>
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<li>
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A <e>local</e> USE flag is used by a single package to make package-specific
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decisions.
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</li>
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</ul>
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<p>
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A list of available global USE flags can be found <uri
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link="/dyn/use-index.xml">online</uri> or locally in
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<path>/usr/portage/profiles/use.desc</path>.
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</p>
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<p>
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A list of available local USE flags can be found locally in
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<path>/usr/portage/profiles/use.local.desc</path>.
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</p>
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</body>
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</subsection>
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</section>
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<section>
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<title>Using USE flags</title>
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<subsection>
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<title>Declare permanent USE flags</title>
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<body>
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<p>
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In the hope you are convinced of the importance of USE flags we will now inform
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you how to declare USE flags.
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</p>
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<p>
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As previously mentioned, all USE flags are declared inside the <c>USE</c>
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variable. To make it easy for users to search and pick USE flags, we already
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provide a <e>default</e> USE setting. This setting is a collection of USE flags
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we think are commonly used by the Gentoo users. This default setting is declared
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in the <path>make.defaults</path> files part of your profile.
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</p>
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<p>
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The profile your system listens to is pointed to by the
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<path>/etc/portage/make.profile</path> symlink. Each profile works on top of
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another, larger profile, the end result is therefore the sum of all profiles.
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The top profile is the <path>base</path> profile
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(<path>/usr/portage/profiles/base</path>).
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</p>
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<p>
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Let us take a look at this default setting for the 10.0 profile:
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</p>
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<pre caption="Cumulative make.defaults USE variable for the 10.0 profile">
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<comment>(This example is the sum of the settings in base, default/linux,
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default/linux/x86 and default/linux/x86/10.0/)</comment>
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USE="a52 aac acpi alsa branding cairo cdr dbus dts dvd dvdr emboss encode exif
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fam firefox flac gif gpm gtk hal jpeg lcms ldap libnotify mad mikmod mng mp3
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mp4 mpeg ogg opengl pango pdf png ppds qt3support qt4 sdl spell
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startup-notification svg tiff truetype vorbis unicode usb X xcb x264 xml xv
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xvid"
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</pre>
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<p>
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As you can see, this variable already contains quite a lot of keywords. Do
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<b>not</b> alter any <path>make.defaults</path> file to tailor
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the <c>USE</c> variable to your needs: changes in this file will be undone when
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you update Portage!
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</p>
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<p>
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To change this default setting, you need to add or remove keywords to the
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<c>USE</c> variable. This is done globally by defining the <c>USE</c> variable
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in <path>/etc/portage/make.conf</path>. In this variable you add the extra USE
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flags you require, or remove the USE flags you don't want. This latter is done
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by prefixing the keyword with the minus-sign ("-").
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</p>
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<p>
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For instance, to remove support for KDE and QT but add support for ldap, the
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following <c>USE</c> can be defined in <path>/etc/portage/make.conf</path>:
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</p>
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<pre caption="An example USE setting in /etc/portage/make.conf">
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USE="-kde -qt4 ldap"
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</pre>
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</body>
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</subsection>
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<subsection>
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<title>Declaring USE flags for individual packages</title>
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<body>
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<p>
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Sometimes you want to declare a certain USE flag for one (or a couple) of
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applications but not system-wide. To accomplish this, you will need to create
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the <path>/etc/portage</path> directory (if it doesn't exist yet) and edit
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<path>/etc/portage/package.use</path>. This is usually a single file, but can
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also be a directory; see <c>man portage</c> for more information. The following
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examples assume <path>package.use</path> is a single file.
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</p>
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<p>
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For instance, if you don't want <c>berkdb</c> support globally but you do want
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it for <c>mysql</c>, you would add:
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</p>
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<pre caption="/etc/portage/package.use example">
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dev-db/mysql berkdb
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</pre>
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<p>
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You can of course also explicitly <e>disable</e> USE flags for a certain
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application. For instance, if you don't want <c>java</c> support in PHP:
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</p>
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<pre caption="/etc/portage/package.use 2nd example">
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dev-php/php -java
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</pre>
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</body>
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</subsection>
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<subsection>
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<title>Declare temporary USE flags</title>
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<body>
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<p>
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Sometimes you want to set a certain USE setting only once. Instead of editing
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<path>/etc/portage/make.conf</path> twice (to do and undo the USE changes) you
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can just declare the USE variable as environment variable. Remember that, when
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you re-emerge or update this application (either explicitly or as part of a
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system update) your changes will be lost!
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</p>
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<p>
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As an example we will temporarily remove java from the USE setting
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during the installation of seamonkey.
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</p>
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<pre caption="Using USE as environment variable">
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# <i>USE="-java" emerge seamonkey</i>
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</pre>
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</body>
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</subsection>
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<subsection>
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<title>Precedence</title>
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<body>
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<p>
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Of course there is a certain precedence on what setting has priority over the
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USE setting. You don't want to declare <c>USE="-java"</c> only to see that
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<c>java</c> is still used due to a setting that has a higher priority.
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The precedence for the USE setting is, ordered
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by priority (first has lowest priority):
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</p>
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<ol>
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<li>
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Default USE setting declared in the <path>make.defaults</path> files part of
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your profile
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</li>
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<li>
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User-defined USE setting in <path>/etc/portage/make.conf</path>
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</li>
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<li>
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User-defined USE setting in <path>/etc/portage/package.use</path>
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</li>
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<li>
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User-defined USE setting as environment variable
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</li>
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</ol>
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<p>
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To view the final <c>USE</c> setting as seen by Portage, run <c>emerge
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--info</c>. This will list all relevant variables (including the <c>USE</c>
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variable) with the content used by Portage.
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</p>
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<pre caption="Running emerge --info">
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# <i>emerge --info</i>
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</pre>
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</body>
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</subsection>
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<subsection>
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<title>Adapting your Entire System to New USE Flags</title>
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<body>
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<p>
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If you have altered your USE flags and you wish to update your entire system to
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use the new USE flags, use <c>emerge</c>'s <c>--newuse</c> option:
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</p>
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<pre caption="Rebuilding your entire system">
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# <i>emerge --update --deep --newuse world</i>
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</pre>
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<p>
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Next, run Portage's depclean to remove the conditional dependencies that
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were emerged on your "old" system but that have been obsoleted by the new USE
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flags.
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</p>
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<warn>
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Running <c>emerge --depclean</c> is a dangerous operation and should be handled
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with care. Double-check the provided list of "obsoleted" packages to make sure
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it doesn't remove packages you need. In the following example we add the
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<c>-p</c> switch to have depclean only list the packages without removing them.
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</warn>
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<pre caption="Removing obsoleted packages">
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# <i>emerge -p --depclean</i>
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</pre>
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<p>
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When depclean has finished, run <c>revdep-rebuild</c> to rebuild the
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applications that are dynamically linked against shared objects provided by
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possibly removed packages. <c>revdep-rebuild</c> is part of the
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<c>gentoolkit</c> package; don't forget to emerge it first.
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</p>
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<pre caption="Running revdep-rebuild">
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# <i>revdep-rebuild</i>
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</pre>
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<p>
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When all this is accomplished, your system is using the new USE flag settings.
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</p>
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</body>
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</subsection>
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</section>
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<section>
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<title>Package specific USE flags</title>
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<subsection>
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<title>Viewing available USE flags</title>
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<body>
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<p>
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Let us take the example of <c>seamonkey</c>: what USE flags does it listen to? To
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find out, we use <c>emerge</c> with the <c>--pretend</c> and <c>--verbose</c>
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options:
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</p>
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<pre caption="Viewing the used USE flags">
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# <i>emerge --pretend --verbose seamonkey</i>
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These are the packages that I would merge, in order:
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Calculating dependencies ...done!
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[ebuild R ] www-client/seamonkey-1.0.7 USE="crypt gnome java -debug -ipv6
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-ldap -mozcalendar -mozdevelop -moznocompose -moznoirc -moznomail -moznopango
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-moznoroaming -postgres -xinerama -xprint" 0 kB
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</pre>
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<p>
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<c>emerge</c> isn't the only tool for this job. In fact, we have a tool
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dedicated to package information called <c>equery</c> which resides in the
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<c>gentoolkit</c> package. First, install <c>gentoolkit</c>:
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</p>
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<pre caption="Installing gentoolkit">
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# <i>emerge gentoolkit</i>
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</pre>
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<p>
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Now run <c>equery</c> with the <c>uses</c> argument to view the USE flags of a
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certain package. For instance, for the <c>gnumeric</c> package:
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</p>
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<pre caption="Using equery to view used USE flags">
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# <i>equery --nocolor uses =gnumeric-1.6.3 -a</i>
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[ Searching for packages matching =gnumeric-1.6.3... ]
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[ Colour Code : set unset ]
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[ Legend : Left column (U) - USE flags from make.conf ]
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[ : Right column (I) - USE flags packages was installed with ]
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[ Found these USE variables for app-office/gnumeric-1.6.3 ]
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U I
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- - debug : Enable extra debug codepaths, like asserts and extra output.
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If you want to get meaningful backtraces see
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http://www.gentoo.org/proj/en/qa/backtraces.xml .
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+ + gnome : Adds GNOME support
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+ + python : Adds support/bindings for the Python language
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- - static : !!do not set this during bootstrap!! Causes binaries to be
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statically linked instead of dynamically
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</pre>
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</body>
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</subsection>
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</section>
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</sections>
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