ebuild-writing/use-conditional-code/text.xml


1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62

<?xml version="1.0"?>
<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 &amp;&amp; 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.
</p>

<note>
<c>die</c> will not work as expected within a subshell, so code in the
form <c>use foo &amp;&amp; ( blah ; blah )</c> should be avoided in favour of a proper if
statement. See `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}
    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 &amp;&amp; epatch ${FILESDIR}/${P}-ssl.patch
    use sparc &amp;&amp; filter-flags -fomit-frame-pointer

    # Inverse USE conditional statements...
    use ncurses || epatch ${FILESDIR}/${P}-no-ncurses.patch
</codesample>

<p>
For echoing content based upon a USE flag, there is often a better helper
function available.
</p>

<p>
You must not rely upon <c>use</c> producing an output <d/> this no longer happens.
If you really need output displayed, use the <c>usev</c> function. The
<c>useq</c> function is available for explicitly requesting no output.
</p>
</body>

</chapter>
</guide>