fix if test

 fontpreview | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fontpreview b/fontpreview
index aae0462..1c66567 100755
--- a/fontpreview
+++ b/fontpreview
@@ -185,7 +185,7 @@ while true; do
     shift
 done
 
-if [ -n $input_file ] ; then
+if [[ $input_file != "" ]] ; then
     if [ -z $output_file ] ; then
         output_file="${input_file}.png"
     fi