USE Flag Conditional Code

Often a particular block of code must only be executed if a given USE flag is set (or unset). For large blocks, if use foo is best, or for inverse tests either if ! use foo or if use !foo can be used (the former is more common and is recommended for readability). For single-statement conditions, the use foo && blah (or use foo || blah for negatives) form is often more readable.

The if [ "`use foo`" ] and if [ -n "`use foo`" ] forms which are occasionally seen in older code must not 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.

	# USE conditional blocks...
	if use livecd ; then
		# remove some extra files for a small livecd install
		rm -fr "${vimfiles}"/{compiler,doc,ftplugin,indent} || die

	# 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"

	# 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"

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

It is guaranteed that use produces no output. If you need output displayed, use the usev function; it echoes the USE flag's name if the condition is met. The useq function is a deprecated synonym for use, don't use it in new code.