drush updatedb on multiple sites
Here is a script I wrote for different time consuming tasks to run on a multisite installation 

The file has to be in your sites folder.

The script will loop through all folders (except all and default) to run the command choosen on every sites based on their settings.php file

#!/bin/bash

# Get all Drupal sites
sites=`find . -maxdepth 1 -type d -print | grep -v '/all$' | grep -v '/default$' | grep -v '\.$'`

echo "Choose the commande to execute : "
echo "1. update"
echo "2. put sites offline"
echo "3. put sites online"
echo "4. clear all cache"
echo "5. clear css+js cache"
echo "6. clear specific cache"
echo "7. install specific module"
echo "8. disable specific module"
echo -n "Input [1,2,3,4,5,6,7 or 8] ? "
read choice

if [ $choice -gt 6 ] ; then
  echo -n "Extension (module/theme) name ?"
  read ext
fi

# For each site, execute the command
for site in $sites
do
  echo ----------
  echo $site
  cd $site  
  if [ $choice -eq 1 ] ; then
    drush updatedb
  elif [ $choice -eq 2 ] ; then
    drush vset --always-set maintenance_mode 1
  elif [ $choice -eq 3 ] ; then
    drush vset --always-set maintenance_mode 0
  elif [ $choice -eq 4 ] ; then
    drush cc all
  elif [ $choice -eq 5 ] ; then
    drush cc css+js
  elif [ $choice -eq 6 ] ; then
    drush cc
  elif [ $choice -eq 7 ] ; then
    drush pm-enable -y $ext
  elif [ $choice -eq 8 ] ; then
    drush pm-disable -y $ext
  fi
  cd ../
done