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1 <?xml version='1.0' encoding='UTF-8'?>
2 <!DOCTYPE sections SYSTEM "/dtd/book.dtd">
3
4 <!-- The content of this document is licensed under the CC-BY-SA license -->
5 <!-- See http://creativecommons.org/licenses/by-sa/2.5 -->
6
7 <!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/handbook/hb-working-use.xml,v 1.46 2007/04/14 03:09:30 nightmorph Exp $ -->
8
9 <sections>
10
11 <abstract>
12 USE flags are a very important aspect of Gentoo. In this chapter, you learn to
13 work with USE flags and understand how USE flags interact with your system.
14 </abstract>
15
16 <version>1.37</version>
17 <date>2007-07-04</date>
18
19 <section>
20 <title>What are USE flags?</title>
21 <subsection>
22 <title>The ideas behind USE flags</title>
23 <body>
24
25 <p>
26 When you are installing Gentoo (or any other distribution, or even operating
27 system for that matter) you make choices depending on the environment you are
28 working with. A setup for a server differs from a setup for a workstation.
29 A gaming workstation differs from a 3D rendering workstation.
30 </p>
31
32 <p>
33 This is not only true for choosing what packages you want to install, but also
34 what features a certain package should support. If you don't need OpenGL, why
35 would you bother installing OpenGL and build OpenGL support in most of your
36 packages? If you don't want to use KDE, why would you bother compiling packages
37 with KDE support if those packages work flawlessly without?
38 </p>
39
40 <p>
41 To help users in deciding what to install/activate and what not, we wanted the
42 user to specify his/her environment in an easy way. This forces the user into
43 deciding what they really want and eases the process for Portage, our package
44 management system, to make useful decisions.
45 </p>
46
47 </body>
48 </subsection>
49 <subsection>
50 <title>Definition of a USE flag</title>
51 <body>
52
53 <p>
54 Enter the USE flags. Such a flag is a keyword that embodies support and
55 dependency-information for a certain concept. If you define a certain USE flag,
56 Portage will know that you want support for the chosen keyword. Of course
57 this also alters the dependency information for a package.
58 </p>
59
60 <p>
61 Let us take a look at a specific example: the <c>kde</c> keyword. If you do not
62 have this keyword in your <c>USE</c> variable, all packages that have
63 <e>optional</e> KDE support will be compiled <e>without</e> KDE support. All
64 packages that have an <e>optional</e> KDE dependency will be installed
65 <e>without</e> installing the KDE libraries (as dependency). If you have defined
66 the <c>kde</c> keyword, then those packages <e>will</e> be compiled with KDE
67 support, and the KDE libraries will be installed as dependency.
68 </p>
69
70 <p>
71 By correctly defining the keywords you will receive a system tailored
72 specifically to your needs.
73 </p>
74
75 </body>
76 </subsection>
77 <subsection>
78 <title>What USE flags exist?</title>
79 <body>
80
81 <p>
82 There are two types of USE flags: <e>global</e> and <e>local</e> USE flags.
83 </p>
84
85 <ul>
86 <li>
87 A <e>global</e> USE flag is used by several packages, system-wide. This is
88 what most people see as USE flags.
89 </li>
90 <li>
91 A <e>local</e> USE flag is used by a single package to make package-specific
92 decisions.
93 </li>
94 </ul>
95
96 <p>
97 A list of available global USE flags can be found <uri
98 link="/dyn/use-index.xml">online</uri> or locally in
99 <path>/usr/portage/profiles/use.desc</path>.
100 </p>
101
102 <p>
103 A list of available local USE flags can be found locally in
104 <path>/usr/portage/profiles/use.local.desc</path>.
105 </p>
106
107 </body>
108 </subsection>
109 </section>
110 <section>
111 <title>Using USE flags</title>
112 <subsection>
113 <title>Declare permanent USE flags</title>
114 <body>
115
116 <p>
117 In the hope you are convinced of the importance of USE flags we will now inform
118 you how to declare USE flags.
119 </p>
120
121 <p>
122 As previously mentioned, all USE flags are declared inside the <c>USE</c>
123 variable. To make it easy for users to search and pick USE flags, we already
124 provide a <e>default</e> USE setting. This setting is a collection of USE flags
125 we think are commonly used by the Gentoo users. This default setting is declared
126 in the <path>make.defaults</path> files part of your profile.
127 </p>
128
129 <p>
130 The profile your system listens to is pointed to by the
131 <path>/etc/make.profile</path> symlink. Each profile works on top of another,
132 larger profile, the end result is therefore the sum of all profiles. The top
133 profile is the <path>base</path> profile
134 (<path>/usr/portage/profiles/base</path>).
135 </p>
136
137 <p>
138 Let us take a look at this default setting for the 2004.3 profile:
139 </p>
140
141 <pre caption="Cumulative make.defaults USE variable for the 2004.3 profile">
142 <comment>(This example is the sum of the settings in base, default-linux,
143 default-linux/x86 and default-linux/x86/2004.3)</comment>
144 USE="x86 oss apm arts avi berkdb bitmap-fonts crypt cups encode fortran f77
145 foomaticdb gdbm gif gpm gtk imlib jpeg kde gnome libg++ libwww mad
146 mikmod motif mpeg ncurses nls oggvorbis opengl pam pdflib png python qt
147 quicktime readline sdl spell ssl svga tcpd truetype X xml2 xmms xv zlib"
148 </pre>
149
150 <p>
151 As you can see, this variable already contains quite a lot of keywords. Do
152 <b>not</b> alter any <path>make.defaults</path> file to tailor
153 the <c>USE</c> variable to your needs: changes in this file will be undone when
154 you update Portage!
155 </p>
156
157 <p>
158 To change this default setting, you need to add or remove keywords to the
159 <c>USE</c> variable. This is done globally by defining the <c>USE</c> variable
160 in <path>/etc/make.conf</path>. In this variable you add the extra USE flags you
161 require, or remove the USE flags you don't want. This latter is done by
162 prefixing the keyword with the minus-sign ("-").
163 </p>
164
165 <p>
166 For instance, to remove support for KDE and QT but add support for ldap, the
167 following <c>USE</c> can be defined in <path>/etc/make.conf</path>:
168 </p>
169
170 <pre caption="An example USE setting in /etc/make.conf">
171 USE="-kde -qt3 -qt4 ldap"
172 </pre>
173
174 </body>
175 </subsection>
176 <subsection>
177 <title>Declaring USE flags for individual packages</title>
178 <body>
179
180 <p>
181 Sometimes you want to declare a certain USE flag for one (or a couple) of
182 applications but not system-wide. To accomplish this, you will need to create
183 the <path>/etc/portage</path> directory (if it doesn't exist yet) and edit
184 <path>/etc/portage/package.use</path>.
185 </p>
186
187 <p>
188 For instance, if you don't want <c>berkdb</c> support globally but you do want
189 it for <c>mysql</c>, you would add:
190 </p>
191
192 <pre caption="/etc/portage/package.use example">
193 dev-db/mysql berkdb
194 </pre>
195
196 <p>
197 You can of course also explicitly <e>disable</e> USE flags for a certain
198 application. For instance, if you don't want <c>java</c> support in PHP:
199 </p>
200
201 <pre caption="/etc/portage/package.use 2nd example">
202 dev-php/php -java
203 </pre>
204
205 </body>
206 </subsection>
207 <subsection>
208 <title>Declare temporary USE flags</title>
209 <body>
210
211 <p>
212 Sometimes you want to set a certain USE setting only once. Instead of editing
213 <path>/etc/make.conf</path> twice (to do and undo the USE changes) you can just
214 declare the USE variable as environment variable. Remember that, when you
215 re-emerge or update this application (either explicitly or as part of a system
216 update) your changes will be lost!
217 </p>
218
219 <p>
220 As an example we will temporarily remove java from the USE setting
221 during the installation of seamonkey.
222 </p>
223
224 <pre caption="Using USE as environment variable">
225 # <i>USE="-java" emerge seamonkey</i>
226 </pre>
227
228 </body>
229 </subsection>
230 <subsection>
231 <title>Precedence</title>
232 <body>
233
234 <p>
235 Of course there is a certain precedence on what setting has priority over the
236 USE setting. You don't want to declare <c>USE="-java"</c> only to see that
237 <c>java</c> is still used due to a setting that has a higher priority.
238 The precedence for the USE setting is, ordered
239 by priority (first has lowest priority):
240 </p>
241
242 <ol>
243 <li>
244 Default USE setting declared in the <path>make.defaults</path> files part of
245 your profile
246 </li>
247 <li>
248 User-defined USE setting in <path>/etc/make.conf</path>
249 </li>
250 <li>
251 User-defined USE setting in <path>/etc/portage/package.use</path>
252 </li>
253 <li>
254 User-defined USE setting as environment variable
255 </li>
256 </ol>
257
258 <p>
259 To view the final <c>USE</c> setting as seen by Portage, run <c>emerge
260 --info</c>. This will list all relevant variables (including the <c>USE</c>
261 variable) with the content used by Portage.
262 </p>
263
264 <pre caption="Running emerge --info">
265 # <i>emerge --info</i>
266 </pre>
267
268 </body>
269 </subsection>
270 <subsection>
271 <title>Adapting your Entire System to New USE Flags</title>
272 <body>
273
274 <p>
275 If you have altered your USE flags and you wish to update your entire system to
276 use the new USE flags, use <c>emerge</c>'s <c>--newuse</c> option:
277 </p>
278
279 <pre caption="Rebuilding your entire system">
280 # <i>emerge --update --deep --newuse world</i>
281 </pre>
282
283 <p>
284 Next, run Portage's depclean to remove the conditional dependencies that
285 were emerged on your "old" system but that have been obsoleted by the new USE
286 flags.
287 </p>
288
289 <warn>
290 Running <c>emerge --depclean</c> is a dangerous operation and should be handled
291 with care. Double-check the provided list of "obsoleted" packages to make sure
292 it doesn't remove packages you need. In the following example we add the
293 <c>-p</c> switch to have depclean only list the packages without removing them.
294 </warn>
295
296 <pre caption="Removing obsoleted packages">
297 # <i>emerge -p --depclean</i>
298 </pre>
299
300 <p>
301 When depclean has finished, run <c>revdep-rebuild</c> to rebuild the
302 applications that are dynamically linked against shared objects provided by
303 possibly removed packages. <c>revdep-rebuild</c> is part of the
304 <c>gentoolkit</c> package; don't forget to emerge it first.
305 </p>
306
307 <pre caption="Running revdep-rebuild">
308 # <i>revdep-rebuild</i>
309 </pre>
310
311 <p>
312 When all this is accomplished, your system is using the new USE flag settings.
313 </p>
314
315 </body>
316 </subsection>
317 </section>
318 <section>
319 <title>Package specific USE flags</title>
320 <subsection>
321 <title>Viewing available USE flags</title>
322 <body>
323
324 <p>
325 Let us take the example of <c>seamonkey</c>: what USE flags does it listen to? To
326 find out, we use <c>emerge</c> with the <c>--pretend</c> and <c>--verbose</c>
327 options:
328 </p>
329
330 <pre caption="Viewing the used USE flags">
331 # <i>emerge --pretend --verbose seamonkey</i>
332 These are the packages that I would merge, in order:
333
334 Calculating dependencies ...done!
335 [ebuild R ] www-client/seamonkey-1.0.7 USE="crypt gnome java -debug -ipv6
336 -ldap -mozcalendar -mozdevelop -moznocompose -moznoirc -moznomail -moznopango
337 -moznoroaming -postgres -xinerama -xprint" 0 kB
338 </pre>
339
340 <p>
341 <c>emerge</c> isn't the only tool for this job. In fact, we have a tool
342 dedicated to package information called <c>equery</c> which resides in the
343 <c>gentoolkit</c> package. First, install <c>gentoolkit</c>:
344 </p>
345
346 <pre caption="Installing gentoolkit">
347 # <i>emerge gentoolkit</i>
348 </pre>
349
350 <p>
351 Now run <c>equery</c> with the <c>uses</c> argument to view the USE flags of a
352 certain package. For instance, for the <c>gnumeric</c> package:
353 </p>
354
355 <pre caption="Using equery to view used USE flags">
356 # <i>equery --nocolor uses =gnumeric-1.6.3 -a</i>
357 [ Searching for packages matching =gnumeric-1.6.3... ]
358 [ Colour Code : set unset ]
359 [ Legend : Left column (U) - USE flags from make.conf ]
360 [ : Right column (I) - USE flags packages was installed with ]
361 [ Found these USE variables for app-office/gnumeric-1.6.3 ]
362 U I
363 - - debug : Enable extra debug codepaths, like asserts and extra output.
364 If you want to get meaningful backtraces see
365 http://www.gentoo.org/proj/en/qa/backtraces.xml .
366 + + gnome : Adds GNOME support
367 + + python : Adds support/bindings for the Python language
368 - - static : !!do not set this during bootstrap!! Causes binaries to be
369 statically linked instead of dynamically
370 </pre>
371
372 </body>
373 </subsection>
374 </section>
375 </sections>

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