Improve migration script

* Do not print/do anything if there’s no .metadata.json file
* Do the migration even if there’s already a .old file, by creating a unique backup file name
* Tell the user where the backup is stored

package/plato/package

@@ -62,55 +62,46 @@ package() {
 configure() {
     local mediadir=/home/root/plato-media
 
-    if [ ! -d $mediadir ]; then
+    if [[ ! -d $mediadir ]]; then
         # This directory should not be tracked by opkg
         # so that its contents are kept after removing
-        mkdir $mediadir
+        mkdir "$mediadir"
         echo ""
         echo "Place your media files for plato in '$mediadir'"
         echo ""
     else
-        # Fix breaking change introduced in version 0.9.14
-        # https://github.com/baskerville/plato/releases/tag/0.9.14
+        # Convert old library metadata format for version 0.9.14
+        # (see <https://github.com/baskerville/plato/releases/tag/0.9.14>)
+        local metadata_file="$mediadir"/.metadata.json
+        local key_len
+        [[ -f $metadata_file ]] && key_len="$(jq --raw-output 'keys[]' "$metadata_file" | wc -L)" || key_len=-1
 
-        local metadata_file=$mediadir/.metadata.json
-        local metadata_file_old=$mediadir/.metadata-old.json
-
-        local max_key_len
-        [ -e $metadata_file ] && max_key_len=$(jq --raw-output 'keys[]' < $metadata_file | wc -L) || max_key_len=-1
-        if [ "$max_key_len" -gt 16 ] && [ ! -e $metadata_file_old ]; then
+        if [[ "$key_len" -gt 16 ]]; then
+            local metadata_file_bak
+            metadata_file_bak="$(mktemp "$metadata_file".bak.XXXXXXXX)"
 
             echo ""
             echo "In Plato 0.9.14 the metadata format for libraries changed."
-            echo "Found your outdated metadata file. Will update it automatically..."
+            echo "Found existing metadata file in '$metadata_file'. Will update it automatically..."
 
             # Routine is based on baskerville's migrate-hex_keys.py (from the release page)
             # Rewritten in bash to only require jq rather than python.
-            mv $metadata_file $metadata_file_old
+            mv "$metadata_file" "$metadata_file_bak"
             local change_cmds=""
             local del_list=""
-            for key in $(jq --raw-output 'keys[]' < $metadata_file_old); do
-                hex_key=$(printf %016X "$key")
-                #echo "Key: $key --> $hex_key"
 
-                [ -z "$del_list" ] || del_list="${del_list}, "
-                [ -z "$change_cmds" ] || change_cmds="${change_cmds} | "
-                del_list="${del_list}.[\"${key}\"]"
-                change_cmds="${change_cmds}.[\"$hex_key\"] = .[\"$key\"]"
-            done
+            while read -r key; do
+                hex_key="$(printf %016X "$key")"
+                [[ -z "$del_list" ]] || del_list="$del_list, "
+                del_list="$del_list.[\"$key\"]"
+                [[ -z "$change_cmds" ]] || change_cmds="$change_cmds | "
+                change_cmds="$change_cmds.[\"$hex_key\"] = .[\"$key\"]"
+            done <<< "$(jq --raw-output 'keys[]' "$metadata_file_bak")"
 
             # Create updated json file
-            jq "$change_cmds | del($del_list)" < $metadata_file_old > $metadata_file
-            echo "Update complete. The old file was renamed to '.metadata-old.json'."
-            echo ""
+            jq "$change_cmds | del($del_list)" "$metadata_file_bak" > "$metadata_file"
 
-        elif [ "$max_key_len" -gt 16 ] && [ -e $metadata_file_old ] || [ ! -e $metadata_file ]; then
-            echo ""
-            echo "In Plato 0.9.14 the metadata format for libraries changed."
-            echo "MANUAL INTERVENTION REQUIRED\!\!\!"
-            echo "Please follow the following guide to update your library metadata manually:"
-            echo "https://github.com/baskerville/plato/releases/tag/0.9.14"
-            echo "Please LAUNCH plato ONLY AFTER this was done\!"
+            echo "Update complete. Backup of previous version created in '$metadata_file_bak'."
             echo ""
         fi
     fi