don't assign customizeable variables if they're in the user's environment

yt

@@ -20,10 +20,12 @@ set -e
 
 argv0="$0"
 com="$1"
-
-cachefile="$XDG_CACHE_HOME/yt.cache"
 FMT='%(title)s – %(channel)s (%(duration>%H:%M:%S)s) [%(webpage_url)s]'
-WBAPI='https://archive.org/wayback/available?url='
+
+test -n "$cachefile" \
+	|| cachefile="$XDG_CACHE_HOME/yt.cache"
+test -n "$WBAPI" \
+	|| WBAPI='https://archive.org/wayback/available?url='
 
 add() { # adds a video to a playlist file
 	while test -n "$2"; do