sysmoocom-backup-data: Optimize taking a backup

With the ro image we have a cheap way to figure out which
files were changed. We can run cmp between the to be backed
up file. This way the stored data will be minimal in the
sense of changed files.

recipes-apps/sysmocom-backup-data/files/sysmocom-backup-data

@@ -16,6 +16,31 @@ fi
 
 DATE=`date +%Y%m%d`
 
+# Called for a file. Compare with the content in /ro-root
+# if this file has been modified and only take it then. In
+# case the file is not present it in /ro-root it will be
+# added to the backup set
+backup_file() {
+	cmp -s /$2 /ro-root/$2
+	if [ $? -ne 0 ]; then
+		echo "Adding $2"
+		tar -rf $1 --transform='s,^,content/,' -C / $2
+	fi
+}
+
+# Check if this is a file, otherwise descend
+handle_file() {
+	if [ -f "/$2" -a -e "/$2" ]; then
+		backup_file $1 $2
+	fi
+	if [ -d "/$2" -a -e "/$2" ]; then
+		for file in /$2/*;
+		do
+			# Construct to have no leading /
+			handle_file $1 $2/`basename $file`
+		done
+	fi
+}
 
 do_backup_files() {
 	BACKUP_FILE="/data/sysmocom-backup_$DATE.tar"
@@ -32,24 +57,22 @@ do_backup_files() {
 	# 2. Add all the files... we need
 	for file in $FILES;
 	do
-		if [ -e "/$file" ]; then
-			tar -rf $BACKUP_FILE --transform='s,^,content/,' -C / $file
-		fi
+		handle_file $BACKUP_FILE $file
 	done
 
-	# 3. Generate more information
-	NAME="/tmp/backup.$RANDOM"
+	# 4. Generate more information
+	NAME="/tmp/backup.$$"
 	mkdir $NAME
 	opkg list_installed > $NAME/installed_packages
 	/sbin/ifconfig | grep HWaddr | cut -d ' ' -f 11 > $NAME/mac_addr
 
-	# 4. Add the more information
+	# 5. Add the more information
 	tar -rf $BACKUP_FILE --transform='s,^,info/,' -C $NAME installed_packages mac_addr
 
-	# 5. Create stable link
+	# 6. Create stable link
 	ln -sf $BACKUP_FILE /data/sysmocom-backup.tar
 
-	# 6.
+	# 76
 	echo "The backup was stored to $BACKUP_FILE"
 }