GNU/Linux Desktop Survival Guide
by Graham Williams
Procmail: Filter and Split Incoming Email
The procmail package provides a powerful mechanism for filtering incoming email, to split email that meet certain criteria into separate folders, to forward different emails on to other users, and to add extra headers to your email (e.g., X-Label headers to classify your email into categories you might define).
The procmail application will be invoked on any email that arrives for you by adding the following to your .forward file:
At the heart of procmail is the .procmailrc file. This contains all the rules that are applied to an email to determine what should happen to that email.
There is potential for procmail to be invoked in rapid succession, and thus if two procmails try to write to the one file there could be a problem. Thus procmail uses lock files to ensure this does not happen. A lock file will slow procmail down ever so slightly, so it is kept as an option specified in the configuration file (.procmailrc) rather than a default.
We can use formail to process an mbox format mail file (containing a collection of email messages) to pipe each message through our procmail processing:
$ formail -s procmail < /var/mail/kayon
A .procmailrc might start with some variables being defined:
PATH=$HOME/bin:/usr/local/bin:/usr/bin:/bin SHELL=/bin/sh MAILDIR=$HOME/Maildir DEFAULT=$MAILDIR/ LOGFILE=$MAILDIR/procmail.`date +%Y.%m`.log
:0 New recipe :0: Create a local lockfile while running :0c Continue processing this email even if this recipe matches :0fhw Filter using a pipe processing just the headers and wait
We now start specifying recipes to handle email. A recipe generally contains a pattern to identify the email messages to which the actions of the recipe are to be applied. Generally, the first recipe that matches an email will process that email and then terminate the procmail processing of that message.
For the especially cautious, or for when setting up procmail and testing it, you may want to keep a copy of every email that comes through. The first recipe might then be:
:0 c: .incoming.`date +%Y.%m`/
All recipes are introduced with the :0. This can be followed by flags to control how the recipe works. The c flag above indicates that the email message is to be processed further by this specific recipe, but also a copy is to be pushed through to the remainder of the recipes within the .procmailrc file. The final : indicates that while procmail is processing the email message with this recipe, the recipe should be locked (so that no other procmail can write to the specified file at the same time).
After introducing the recipe with the :0 line, the next line in the above example is the action to be performed. In this case a file name is specified, beginning with a full stop and calling on the operating system to provide a current year and month string (e.g. 2006.01).
We can specify exactly which emails we wish a recipe to apply to. We do this for a recipe which will forward email on to another user, as well as keeping a copy for ourselves:
:0 c * ^From.*abc.com ! firstname.lastname@example.org
With this recipe the recipe introduction uses the c flag to pass the email message on to following recipes as a carbon copy. Also, there is no trailing colon, so we are not requesting a lockfile for this recipe—in this case we don't need to use a lockfile because two instances of procmail will not interfere with each other when the action is to send the email on to someone else.
A sample .procmailrc:
# DEBIAN :0: * ^Resent-Sender.*email@example.com lists/debian-devel :0: * ^Resent-Sender.*firstname.lastname@example.org lists/debian-user
:0 begins a recipe. The following
: ensures the mail
file is locked. A line beginning with
* begins a condition. You
can have multiple conditions within a recipe. The condition
captures email sent to the debian-devel mailing list. This matches
messages that include
in their header. The final line of a recipe is the mailbox into
which procmail will send the mail.
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