Automated Mail Purging for SMTP Mail

If your Linux system actively handles electronic mail, especially for several users, you may discover that mail consumes a lot of space where it resides, usually in one of /var/mail (System V-compliant systems), /usr/mail (BSD), /usr/spool/mail or /var/spool/mail (where my Debian GNU/Linux system stores it). The collection of Bash scripts presented here provides a way to reduce the space used by mail files by purging old messages.

For several months I worked as a contractor at a company that used a Unix system to process all electronic mail for its employees. Several hundred users retrieved their mail from a single host that served as both Simple Mail Transfer Protocol (SMTP) server (for general mail transport) and Post Office Protocol (POP3) server (for clients to retrieve mail without logging into the host). While it had a home-grown mail purge system, users could inadvertently defeat the system, allowing files to grow without end.

The existing mail purge system ran as a nightly cron job. It would determine the date 60 days before and construct a string in SMTP mail header format. It would then loop through all the user mail files in /var/mail using the grep command to find that string and note the lines on which it appeared. For each file that contained the string, it would use the tail command to discard everything prior to the line containing the target date string.

If messages arrived undamaged, stayed intact, always appeared in date sequence and cron called this job daily, this method would usually work. However, sometimes cron doesn't run a job, messages do not always arrive in date order and other software run against mail files might reorder messages. Because of the existing solution's all-or-nothing keep-or-discard method, it had the following problems:

To find a more reliable solution, I sought an existing free program that would address my needs. I asked my peers, including those on Internet mailing lists, but got no useful response. A search of Internet Unix archives revealed nothing, either. Thus, I was left to create a solution.

To ensure acceptable handling of messages, I settled on these requirements:

I discovered that formail, part of the procmail mail-handling package, can split an SMTP mail file into individual messages and repair damaged headers in one pass. This ability enables individual examination of each message to decide whether to keep or discard it. Since each message would undergo separate examination, date order would not be important.

The first script, mailrm.sh (see Listing 1), requires one command-line argument, naming the number of days of messages to preserve. When run, it sets needed variables and starts checking each mail file in $MAILDIR. It uses formail, located in $FORMAIL, to verify, repair and split each mail file into individual messages.

Each message is then examined by the mailage.sh script (see Listing 2) to determine whether to keep it. First, it checks the message header's “From” line for the date, moving fields as necessary. (If formail has to repair a message date, the resulting date doesn't have a time zone in it.) Then it compares the message date with the value computed for today minus the number of days to retain messages. If the message is newer, the script concatenates the message onto STDOUT, saving it to a temporary file. If the message is older, the script exits with no output.

Afterward, if the new output file has a different number of lines from the original, it is moved into the original file's place, its ownership is reset and its permissions are restricted. If the original mail file is now zero length, mailrm.sh removes it. If a user has been removed from a system leaving his mail behind, this script deletes his mail file after all the messages in it have expired.

The third script, maildate.sh (see Listing 3), returns the integer number of days since 1900 of an input date in the form “MMM DD YYYY”. The returned integer is useful for calculating the difference between two dates.