Procmail script to execute a BASH script and send an email

For any email sent to a given email address compare the sender to a list of email addresses in a text file in the home directory of the email recipient. The text file will have one email address per line, (with no more than ten email addresses). If the sender of the email matches, execute a BASH script (which is already written), passing the first word of the subject line of the email as a parameter to the BASH script. Before passing the subject line to the script, verify that the contents are only lowercase [a-z][0-9] characters and that the length of the parameter passed is less than 20 characters. The BASH script will either return success or failure; if there is a failure, email the output of the BASH script to the sender of the email. Target system is an Ubuntu 10.10 server. Procmail, mailutils and postfix are installed and configured. Tools used must be standard Linux.