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<?xml version="1.0" encoding="UTF-8"?>
<guide self="ebuild-writing/use-conditional-code/">
<chapter>
<title>USE flag conditional code</title>
<body>
<p>
Often a particular block of code must only be executed if a given USE flag is
set (or unset). For large blocks, <c>if use foo</c> is best, or for inverse tests
either <c>if ! use foo</c> or <c>if use !foo</c> can be used (the former is more
common and is recommended for readability). For single-statement conditions, the
<c>use foo && blah</c> (or <c>use foo || blah</c> for negatives) form is often more
readable.
</p>
<p>
The <c>if [ "`use foo`" ]</c> and <c>if [ -n "`use foo`" ]</c> forms which are
occasionally seen in older code must <b>not</b> be used. This is because, since Portage 2.1, the 'use' Portage helper does not produce any output when the use flag is enabled or disabled so the [ "`use foo`" ] statement is pretty much identical to [ "" ] which always evaluates to false.
</p>
<note>
<c>die</c> will not work as expected within a subshell, so code in the
form <c>use foo && ( blah ; blah )</c> should be avoided in favour of a proper if
statement. See <uri link="::ebuild-writing/error-handling/#die and subshells"/>.
</note>
<codesample lang="ebuild">
# USE conditional blocks...
if use livecd ; then
# remove some extra files for a small livecd install
rm -fr "${vimfiles}"/{compiler,doc,ftplugin,indent} || die
fi
# Inverse USE conditional blocks...
if ! use cscope ; then
# the --disable-cscope configure arg doesn't quite work properly,
# so sed it out of feature.h if we're not USEing cscope.
sed -i -e '/# define FEAT_CSCOPE/d' src/feature.h || die "couldn't disable cscope"
fi
# USE conditional statements...
use ssl && eapply "${FILESDIR}/${P}-ssl.patch"
use sparc && filter-flags -fomit-frame-pointer
# Inverse USE conditional statements...
use ncurses || eapply "${FILESDIR}/${P}-no-ncurses.patch"
</codesample>
<p>
For echoing content based upon a USE flag, there is often a better helper
function available.
</p>
<p>
It is guaranteed that <c>use</c> produces no output. If you need output
displayed, use the <c>usev</c> function; it echoes the USE flag's name
if the condition is met.
The <c>useq</c> function is a deprecated synonym for <c>use</c>, don't
use it in new code.
</p>
</body>
</chapter>
</guide>
|
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