If you’re having errors with drush since upgrading to Ubuntu 13.10 here are the steps to make it work:
sudo apt-get install php5-json
cp /etc/php5/cli/php.ini ~/.drush/
sudo nano ~/.drush/php.ini
Now editing the file, make sure the “disabled_functions =” list is empty.
There you go, problem solved, drush should throw no more errors.