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At the same time I often get mail that, while it is interesting and important, can easily be filed to be read later, without ever actually having to clutter up my incoming mailbox.
This, then, is what filter does! The filter program allows you to define a set of rules by which all incoming mail is screened, and a subsequent set of actions to perform based on whether the conditions are met or not. Filter also has the ability to mail a summary of what actions it performed on the incoming mail as often as you'd like.
The language for writing filter rules is pretty simple, actually. The fundamental structure is:
if (condition) then action
Where condition is constructed by an arbitrary number of individual conditions of the form field relation value (an optional further type of rule is of the form always action but should only be used as the last rule in the ruleset, for obvious reasons). The field value can be:
subject
sender
from
to
For the field lines, the
relation can be any of the standard
relationships (>, <, >=, <=, != and =). contains
is equivalent to the relation = or you can skip the relationship entirely
(e.g. subject = "joe", subject contains "joe",
and subject "joe" are all equivalent). The value is any quoted string
that is to be matched against or
a number if lines is the field
being considered.
The relation matches (or ~) takes an egrep like regular expression as the value. The regular expression must be enclosed in pairs of /. If you need a / in your expression write it as \/.
Individual conditions are joined together by using the word and, and the logic of a condition can be flipped by using not as the first word (e.g. subject "fun" and not subject "games"). We'll see more examples of this later.
Note that the or logical conjunction isn't a valid part of the filter conditional statement.
Finally, action can be any of:
Description
Deletes the message
Saves a copy of the message in the specified foldername.
Does the same as save, but also puts a copy in your mailbox.
Sends the message to the specified address.
Sends the message to the specified address and leaves a copy in your
mailbox.
Feeds the message to the specified command (or complex sequence
of commands) as standard input.
Like execute but also leaves a copy in your mailbox.
Leaves the message in your mailbox.
Note that the execute command assumes that you are doing 1something useful with incoming mail. The command does not automatically append the message to your mail spool. Use executec for that. A command such as:
if (from = "dave") then execute "echo mail from dave > /dev/tty"
will result in losing your mail. A more suitable use for execute would be:
if (from = "boss") then execute "vacation pat"
Foldernames can contain any of a number of macros, too, as we'll see in the example ruleset below. The macros available for the string fields are:
Meaning day of the month day of the week (0-6) hour of the day (0-23) month of the year (1-12) return address of message subject of original message Re: subject of original message current hour and minute in HH:MM format year (last two digits) the string that matched the last regular expression the corresponding subexpression in the last regular expression that matched
If a message has a subject of This is a test, and the rule is
if (subject matches /([a-z]+) a ([a-z]+)/)
then "echo %& %1 %2"
then %& expands to is a test,
%1 expands to is and %2 expands to test.
Foldernames may contain a leading ~/ which expands to your home directory.
The rules file can also contain comments (any line starting with a #) and blank lines. Metacharacters may be escaped by a preceding \ to remove any special meaning. If you need a % character as part of an argument (such as an address), use \%. If you need a double quote in a double quoted string, use \". If you need a single quote in a single quoted string, use \'. If you need a backslash in any kind of string, use \\..
Unless you specify an alternate file via the -f option, the rules file needs to reside in your .elm directory off your home directory and be called .elm/filter-rules. Here's an example:
# $HOME/.elm/filter-rules
#
# Filter rules for the Elm Filter program. Don't change without some
# serious thought. (remember - order counts)
#
# (for Dave Taylor)
# rule 1
if (from contains "!uucp") then delete
# rule 2
to "postmaster" ? save "/tmp/postmaster-mail.%d"
# rule 3
if (to "culture" and lines > 20) ? save "~/Mail/culture"
# rule 4
subject = "filter test" ? forward "hpldat!test"
# rule 5
if [ subject = "elm" ] savecopy "~/Mail/elm-incoming"
# rule 6
subject = "display-to-console" ? execute "cat - > /dev/console"
(Notice the loose syntax - there are lots of
valid ways
to specify a rule in the filter program!!)
To translate these into English:
1. All messages from uucp should be summarily deleted.
2. All mail to postmaster should be saved in a folder
(file)
called /tmp/postmaster-mail.n, where n is the numeric-day-
of-the-week.
3. All mail addressed to culture with at least 20 lines should
be automatically appended to the folder $HOME/Mail/culture.
4. All messages that contain the subject filter test should be
forwarded to me, but via the address hpldat!test (to force a
non-user forward).
5. All messages with a subject that contains the
word elm
should be saved in the folder $HOME/Mail/elm-incoming
and
also dropped into my mailbox.
6. Any message with the subject display-to-console
will be
immediately written to the console.
Notice that the order of the rules is very important. If we, for
example, were to get a message from uucp that had
the subject
filter test, the filter program would match rule 1 and delete the
message. It would never be forwarded to hpldat!test.
It is for
this reason that great care should be taken with the ordering of
The rules.
The filter program has a convenient way of check out the rules you have written. Simply invoke it with the -r (rules) option:
% filter -r
Rule 1: if (from = "!uucp") then
Delete
Rule 2: if (to = "postmaster") then
Save /tmp/postmaster-mail.3
Rule 3: if (to = "culture" and lines > 20) then
Save ~/Mail/culture
Rule 4: if (subject = "filter test") then
Forward hpldat!test
Rule 5: if (subject="elm") then
Copy and Save ~/Mail/elm-incoming
Rule 6: if (subject="display-to-console") then
Execute "cat - > /dev/console"
There are a few things to notice - first off, these are the parsed and rebuilt rules, so we see them all in a consistent format. Also, notice on the filename for rule 2 that filter has correctly expanded the %d macro to be the day of the week (in this case, Wednesday, denoted by the digit 3).
It is highly recommended that you always check your ruleset before actually using it to filter your mail!
Now the bad news. If you aren't running sendmail or some other Mail Transport Agent that supports delivery to a program you cannot use this program as currently written. Why? Because the filter program expects to be put in your .forward file (or equivalent concept for MTAs other than sendmail) that causes the messages to be delivered to the filter program. That is something that only sendmail and other smart MTAs look at!
The format for the entry in the .forward file (located in your home directory) is simply:
"|/usr/local/bin/filter"
Alright, it isn't quite that simple! Since filter is invoked by processes that don't know where you are logged in, you need to have some way to trap the error messages. For ease of use, it was decided to have all the messages written to the file specified by -o (or stderr) which means that you have two main choices for the actual entry. Either:
"|/usr/local/bin/filter -o /dev/console"
which will log all errors on the system console (each error is prefixed with filter (username) to distinguish it), or:
"|/usr/local/bin/filter -o /tmp/joe.filter_errors"
if you want to have a copy saved to a file. Note that the quotes are a required part of the line. A possible strategy would be to have the errors written to a file and to then have a few lines in your .login (or equivalent) script like:
if ( -f /tmp/joe.filter_errors) then
echo " "
echo "Filter program errors;"
cat /tmp/joe.filter_errors
echo " "
endif
You can also use the -v option in combination with the above to have a more verbose log file (including action taken with date/time stamp) saved by having in your .forward file:
"|/usr/local/bin/filter -vo /tmp/joe.filter_errors"
Suffice it to say, you can get pretty tricky with all this!!
The filter program can keep a log of all actions performed, including what rules it matched against, in your .elm directory in a file called .elm/filterlog. You can either directly operate on this file, or, much more recommended, you can use one of the two summarization options to filter and let it do the work for you!
The difference between the two is best demonstrated by example:
% filter -s
Summary of Filter Activity
--------------------------
A total of 418 messages were filtered:
The default rule of putting mail into your mailbox
applied 364 times (87%)
Rule #1: (delete message)
applied 1 time (0%)
Rule #2: (save in "~/Filtered-Mail/netnews.12")
applied 8 times (2%)
Rule #3: (save in "~/Filtered-Mail/postmaster.12")
applied 14 times (3%)
Rule #5: (save in "~/Filtered-Mail/risks.12")
applied 3 times (1%)
Rule #6: (save in "~/Filtered-Mail/rays.12")
applied 28 times (7%)
versus:
% filter -S
[the output as listed above, followed by:]
Explicit log of each action;
Mail from taylor about Filter Summary
PUT in mailbox: the default action
Mail from news@hplabsz.hpl.hp.com about Newsgroup comp.editors created
PUT in mailbox: the default action
Mail from root about Log file: cleanuplog
PUT in mailbox: the default action
[etc.]
To actually use either of the summarizing options, there are two ways that are recommended. The preferred way is to have a line in either your crontab file (ask your administrator for help with this) that invokes the filter program as often as you desire with the -s option. For example, I have a summary mailed to me every morning at 8 o'clock: 0 8 * * * "/usr/local/bin/filter -s | elm -s 'Filter Summary' taylor"
An alternative is to have your .login (or equivalent) execute the command each time you log in.
Note that if you want to have your log files cleared out each time the summary is generated you'll need to use the -c option too. Also, if you want to keep a long list of actions performed you can do this by saving it as you display it. A way to do this would be, if you were to have the invocation in your .login (or equivalent) script, to use:
echo "Filter Log;"
filter -c -s | tee -a PERM.filter.log
which would append a copy of all the output to the file PERM.filter.log and would avoid you having to read larger and larger summaries of what the program had done.
If you prefer not to have filter generate log files, use the -q option. Messages that are written to stderr or the -o file are still generated; only message to the log files are quashed.
Using the readmsg command, it is quite easy to test the rules you've written to see if they do what you desire.
For example, we can use the -n option to filter, which means don't actually do this, just tell me what rule you matched, if any, and what action you would have performed (you can see why a single letter option is easier to type in!!), and feed it each message in our mailbox by using a command like:
% set message=1
% set total_messages=`messages`
% while (1)
> if ($message > $total_messages) exit
> readmsg -h $message | filter -n
> @ message++
> end
which will then hand each of the messages in your mailbox to the filter program and display what action would have been taken with that message and why.
For example, if we do this for a few interesting messages in my mailbox, we'd end up with output like:
Mail from taylor about filter test
FORWARDED to hpldat!taylor by rule;
subject="filter test" ? forward "hpldat!test"
Mail from bradley%hplkab@hplabsc
about Re: AI-ED mailing address for HP
PUT in mailbox: the default action
Mail from taylor about display-to-console
EXECUTED "cat - > /dev/console"
(Sharp users will notice that this is exactly the same
format as the longer summary listing.)
When a message is forwarded to another user by the action being specified as forward address, then filter can generate one of two styles of message. If the message is to you, then it simply adds it to your mailbox in such a way as to ensure that the return address is that of the person who sent the message.
If not, then the message is enclosed in a message of the form:
From taylor Thu Oct 2 15:07:04 1986
Date: Thu, 2 Oct 86 15:06:58 pdt
Subject: "filter test"
From: The filter of taylor@hpldat <taylor>
To: hpldat!taylor
X-Filtered-By: filter, version 1.4
-- Begin filtered message --
From taylor Thu Oct 2 15:06:41 1986
Date: Thu, 2 Oct 86 15:06:33 pdt
From: Dave Taylor <taylor>
Subject: filter test
Just a simple test.
-- End of filtered message --
The subject of the actual message is the same as the subject of the message being forwarded, but in quotes. The From: header indicates how the message was sent, and the X-Filtered-By: header identifies what version of filter was used.
While the filter program as presented herein is obviously a nice addition to the set of tools available for dealing with electronic mail, there are some key features that are missing and will be added in the future based on demand.
As I see it, the main things missing are:
Just a simple test.
-- End of filtered message --
The subject of the actual message is the same as the subject of the message being forwarded, but in quotes. The From: header indicates how the message was sent, and the X-Filtered-By: header identifies what version of filter was used.
While the filter program as presented herein is obviously a nice addition to the set of tools available for dealing with electronic mail, there are some key features that are missing and will be added in the future based on demand.
As I see it, the main things missing are:
1. Perhaps more actions available (but what?)
2. Certainly the ability to filter based on
any field or
combination of fields.
Since this is a pretty simple program, there are a few pitfalls, some of which have already been mentioned:
- Order counts in the rules. Beware!
- Matching is pretty simple - make sure your patterns are sufficiently exclusive before having any destructive rules.
- The log files can grow indefinitely unless you use the -c or -q options or you trim the files by other means.
- Finally, as with the rest
of the Elm mail system, I welcome feedback
and suggestion on how to improve this program!!