open_wrapper.sh: make the wrapper script POSIX shell compliant

Running open_wrapper.sh was failing with some shells different from
bash, because they don't support substring expansion.

For example, dash was giving this error:

  ./open_wrapper.sh: 35: ./open_wrapper.sh: Bad substitution

While fixing the issue also use a stricter check for the URL: the anchor
should only contain hex digits.

diff --git a/open_wrapper.sh b/open_wrapper.sh
index 6a3d0d3..7f35ce8 100755 (executable)
--- a/open_wrapper.sh
+++ b/open_wrapper.sh
@@ -32,7 +32,7 @@ decrypted_file()
   echo "${TEMPDIR}/${FILENAME}"
 }
 
   echo "${TEMPDIR}/${FILENAME}"
 }
 

-if [ ${URL: -97:1} = "#" ];
+if echo -n "$URL" | grep -q '#[[:xdigit:]]\{96\}$';

 then
   DESTINATION=$(decrypted_file "$URL")
 else
 then
   DESTINATION=$(decrypted_file "$URL")
 else