lib/common.sh: Use double brackets

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>

lib/common.sh

@@ -58,14 +58,14 @@ stat_done() {
 }
 
 setup_workdir() {
-	[ -z "$WORKDIR" ] && WORKDIR=$(mktemp -d /tmp/$(basename $0).XXXXXXXXXX)
+	[[ -z $WORKDIR ]] && WORKDIR=$(mktemp -d /tmp/$(basename $0).XXXXXXXXXX)
 }
 
 cleanup() {
 	trap - EXIT INT QUIT TERM
 
-	[ -n "$WORKDIR" ] && rm -rf "$WORKDIR"
-	[ "$1" ] && exit $1
+	[[ -n $WORKDIR ]] && rm -rf "$WORKDIR"
+	[[ $1 ]] && exit $1
 }
 
 abort() {