EXIM'S USER INTERFACE TO MAIL FILTERING


Exim is a mail transfer agent for Unix systems. This document describes the
user interface to its in-built mail filtering facility, and is copyright
(c) University of Cambridge 1998.
___________________________________________________________________________


1. Introduction

Most Unix mail transport agents (programs that deliver mail) permit
individual users to specify automatic forwarding of their mail, usually by
placing a list of forwarding addresses in a file called .forward in their
home directories. Exim extends this facility by allowing the forwarding
instructions to be a set of rules rather than just a list of addresses, in
effect providing '.forward with conditions'. Operating the set of rules is
called filtering, and the file that contains them is called a filter file.

The ability to use filtering has to be enabled by the system administrator,
and some of the individual facilities can be separately enabled or
disabled. A local document should be provided to describe exactly what has
been enabled. In the absence of this, consult your system administrator.

It is important to realize that no deliveries are actually made while a
filter file is being processed. The result of filtering is a list of
destinations to which a message should be delivered - the deliveries
themselves take place later, along with all other deliveries for the
message. This means that it is not possible to test for successful
deliveries while filtering. It also means that duplicate addresses gener-
ated by filtering are dropped, as with any other duplicate addresses.

This document describes how to use a filter file and the format of its
contents. It is intended for use by end-users. How the system administrator
can set up and control the use of filtering is described in the full Exim
specification.


2. Testing a new filter file

Filter files, especially the more complicated ones, should always be
tested, as it is easy to make mistakes. Exim provides a facility for
preliminary testing of a filter file before installing it. This tests the
syntax of the file and its basic operation, and can also be used with
ordinary .forward files.

Because a filter can do tests on the content of messages, a test message is
required. Suppose you have a new filter file called "new-filter" and a test
message called "test-message". Assuming that Exim is installed with the
conventional path name /usr/sbin/sendmail, the following command can be
used:

  /usr/sbin/sendmail -bf new-filter  to                                        |
  e.g. add 2 to n3                                                           |
                                                                             |
There are 10 user variables of this type, and their values can be obtained   |
by the normal expansion syntax (for example $n3) in other commands. At the   |
start of filtering, these variables all contain zero. Both arguments of the  |
add command are expanded before use, making it possible to add variables to  |
each other. Subtraction can be obtained by adding negative numbers.          |


12. The deliver command

       deliver 
  e.g. deliver "Dr Livingstone "

This provides a forwarding operation. The message is sent on to the given
address, exactly as happens if the address had appeared in a traditional
.forward file. To deliver a copy of the message to your normal mailbox,
your login name can be given. Once a message has been processed by the
filtering mechanism, it will not be so processed again, so doing this does
not cause a loop.

However, if you have a mail alias, you should not refer to it here. For
example, if the mail address "L.Gulliver" is aliased to "lg103" then all
references in Gulliver's .forward file should be to "lg103". A reference to
the alias will not work for messages that are addressed to that alias,
since, like .forward file processing, aliasing is performed only once on an
address, in order to avoid looping.


13. The save command

       save 
  e.g. save $home/mail/bookfolder

This causes a copy of the message to be appended to the given file (that
is, the file is used as a mail folder). If the name does not start with a /
character, then the contents of the $home variable are prepended. The user
must of course have permission to write to the file, and the writing of the
file takes place in a process that is running as the user, under the user's
primary group. Any secondary groups to which the use may belong are not
normally taken into account, though the system administrator can configure
Exim to set them up. In addition, the ability to use this command at all is
controlled by the system administrator - it may be forbidden on some
systems. An optional mode value may be given after the file name, for
example,

       save /some/folder 0640

The value for the mode is interpreted as an octal number, even if it does    |
not begin with a zero. This makes it possible for users to override the      |
system-wide mode setting for file deliveries, which is normally 600. If an
existing file does not have the correct mode, it is changed.

An alternative form of delivery may be enabled on your system, in which
each message is delivered into a new file in a given directory. If this is
the case, this functionality can be requested by giving the directory name
terminated by a slash after the save command, for example

      save separated/messages/

There are two different formats for such deliveries; check with your system
administrator or local documentation to find out which (if any) are
available on your system. If this functionality is not enabled, the use of
a path name ending in a slash causes an error.


14. The pipe command

       pipe 
  e.g. pipe "$home/bin/countmail $sender_address"

This command causes a separate process to be run, and a copy of the message
is passed on its standard input. The process runs as the user, under the
user's primary group. Any secondary groups to which the use may belong are
not normally taken into account, though the system administrator can
configure Exim to set them up.

The command supplied to pipe is split up by Exim into a command name and a
number of arguments, delimited by white space except for arguments enclosed
in double quotes, in which case backslash is interpreted as an escape, or
in single quotes, in which case no escaping is recognized. Note that as the
whole command is normally supplied in double quotes, a second level of
quoting is required for internal double quotes. For example:

       pipe "$home/myscript \"size is $message_size\""

By default, string expansion is performed on the separate components after
the line has been split up and the command is then run directly by Exim; it
is not run under a shell. Therefore, substitution cannot change the number
of arguments, nor can quotes, backslashes or other shell metacharacters in
variables cause confusion.

Documentation for some programs that are normally run via this kind of pipe
often suggest the the command start with

  IFS=" "

This is a shell command, and should not be present in Exim filter files,
since it does not normally run the command under a shell.

However, there is an option that the administrator can set to cause a shell
to be used. In this case, the entire command is expanded as a single string
and passed to the shell for interpretation. It is recommended that this be
avoided if at all possible, since it can lead to problems when inserted
variables contain shell metacharacters.

The default PATH set up for the command is determined by the system
administrator, usually containing at least /usr/bin so that common commands
are available without having to specify an absolute file name. However, it
is possible for the system administrator to restrict the pipe facility so
that the command name must not contain any / characters, and must be found
in one of the directories in the configured PATH. It is also possible for
the system administrator to lock out the use of the pipe command
altogether.

When the command is run, the following environment variables are set up:

  DOMAIN               the local domain of the address
  HOME                 your home directory
  LOCAL_PART           your login name
  LOGNAME              your login name
  MESSAGE_ID           the message's unique id
  PATH                 the command search path
  SENDER               the sender of the message
  SHELL                /bin/sh
  USER                 your login name

If you run a command that is a shell script, be very careful in your use of
data from the incoming message in the commands in your script. RFC 822 is
very generous in the characters that are legally permitted to appear in
mail addresses, and in particular, an address may begin with a vertical bar
or a slash. For this reason you should always use quotes round any
arguments that involve data from the message, like this:

  /some/command "$SENDER"

so that inserted shell meta-characters do not cause unwanted effects.


15. Mail commands

There are two commands which cause the creation of a new mail message,
which does not count as a significant delivery unless the command is
preceded by the word "seen". This is a powerful facility, but it should be
used with care, because of the danger of creating infinite sequences of
messages. The system administrator can forbid the use of these commands
altogether.

To help prevent runaway message sequences, these commands have no effect
when the incoming message is a delivery error message, and messages sent by
this means are treated as if they were reporting delivery errors. Thus they
should never themselves cause a delivery error message to be returned. The
basic mail-sending command is

       mail [to ]
            [cc ]
            [bcc ]
            [subject ]
            [text ]
            [[expand] file ]
            [return message]
            [log ]
            [once ]

  e.g. mail text "Your message about $h_subject has been received"

As a convenience for use in one common case, there is also a command called
vacation. It behaves in the same way as mail, except that the defaults for
the "file", "log", and "once" options are

  expand file .vacation.msg
         log  .vacation.log
         once .vacation

respectively. This mimics the behaviour of the traditional Unix vacation
command. If a file name is given to "vacation", it is expanded only if
explicitly requested.

The key/value argument pairs can appear in any order. At least one of
"text" or "file" must appear (except with "vacation"); if both are present,
the text string appears first in the message. If "expand" precedes "file",
then each line of the file is subject to string expansion as it is included
in the message.

Several lines of text can be supplied to "text" by including the escape      |
sequence '\n' in the string where newlines are required. If the command is   |
output during filter file testing, newlines in the text are shown as '\n'.   |

If no "to" argument appears, the message is sent to the address in the
"Reply_address" variable (see section 6 above). An "In-Reply-To:" header is
automatically included in the created message, giving a reference to the
message identification of the incoming message.

If 'return message' is specified, the incoming message that caused the
filter file to be run is added to the end of the message, subject to a
limitation as the the maximum size that is returned.

If a log file is specified, a line is added to it for each message sent. If
a "once" file is specified, it is used to create a database for remembering
who has received a message, and no more than one message is ever sent to
any particular address.

The file name specified for "once" is used as the base name for direct-
access (DBM) file operations. There are a number of different DBM libraries
in existence. Some operating systems provide one as a default, but even in
this case a different one may have been used when building Exim. With some
DBM libraries, specifying "once" results in two files being created, with
the suffixes ".dir" and ".pag" being added to the given name. With some
others a single file with the suffix ".db" is used, or the name is used
unchanged.


16. Logging commands

A log can be kept of actions taken by a filter file. This facility is
normally available in conventional configurations, but there are some
situations where it might not be. Also, the system administrator may choose
to disable it. Check your local information if in doubt.

Logging takes place while the filter file is being interpreted. It does not
queue up for later like the delivery commands. The reason for this is so
that a log file need be opened only once for several write operations.
There are two commands, neither of which constitutes a significant
delivery. The first defines a file to which logging output is subsequently
written:

       logfile 
  e.g. logfile $home/filter.log

The file name may optionally be followed by a mode for the file, which is
used if the file has to be created. For example,

       logfile $home/filter.log 0644

The number is interpreted as octal, even if it does not begin with a zero.   |
The default for the mode is 600. It is suggested that the logfile command    |
normally appear as the first command in a filter file. Once logfile has      |
been obeyed, the logwrite command can be used to write to the log file:      |
                                                                             |
       logwrite ""                                         |
  e.g. logwrite "$tod_log $message_id processed"                             |
                                                                             |
It is possible to have more than one logfile command, to specify writing to  |
different log files in different circumstances. Writing takes place at the   |
end of the file, and a newline character is added to the end of each string  |
if there isn't one already there. Newlines can be put in the middle of the   |
string by using the '\n' escape sequence. Lines from simultaneous deliver-   |
ies may get interleaved in the file, as there is no interlocking, so you     |
should plan your logging with this in mind. However, data should not get     |
lost.                                                                        |
                                                                             |
In earlier versions of Exim the logwrite command was called log, and this    |
name remains available for backwards compatibility. However, it is not       |
possible to use the name log as a command name following a mail command,     |
because it will be interpreted as the log option of that command.            |


17. Other commands

The command "finish", which has no arguments, causes Exim to stop
interpreting the filter file. This is not a significant action unless
preceded by "seen". A filter file containing only "seen finish" is a black
hole.

It is sometimes helpful to be able to print out the values of variables
when testing filter files. The command

       testprint 
  e.g. testprint "home=$home reply_address=$reply_address"

does nothing when mail is being delivered. However, when the filtering code
is being tested by means of the -bf option, the value of the string is
written to the standard output.

When Exim's filtering facilities are being used as a system filter, the
fail and freeze commands are available. However, since they are usable only
by the system administrator and not by ordinary users, they are described
in the main Exim specification rather than in this document.


18. Obeying commands conditionally

Most of the power of filtering comes from the ability to test conditions
and obey different commands depending on the outcome. The "if" command is
used to specify conditional execution, and its general form is

  if    
  then  
  elif  
  then  
  else  
  endif

There may be any number of "elif"-"then" sections (including none) and the
"else" section is also optional. Any number of commands, including nested
"if" commands, may appear in any of the  sections.

Conditions can be combined by using the words "and" and "or", and round
brackets (parentheses) can be used to specify how several conditions are to
combine. Without brackets, "and" is more binding than "or". A condition can
be preceded by "not" to negate it, and there are also some negative forms
of condition that are more English-like.


19. String testing conditions

There are a number of conditions that operate on text strings, using the     |
words 'begins', 'ends', 'is', 'contains' and 'matches'. If the condition     |
names are written in lower-case, the testing of letters is done without      |
regard to case; if they are written in upper-case (for example, 'CONTAINS')  |
then the case of letters is significant.                                     |
                                                                             |
        begins                                                 |
        does not begin                                         |
  e.g. $header_from: begins "Friend@"                                        |
                                                                             |
A 'begins' test checks for the presence of the second string at the start    |
of the first, both strings having been expanded.                             |
                                                                             |
        ends                                                   |
        does not end                                           |
  e.g. $header_from: ends "public.com"                                       |
                                                                             |
An 'ends' test checks for the presence of the second string at the end of    |
the first, both strings having been expanded.                                |

        is 
        is not 
  e.g. $local_part_suffix is "-foo"

An 'is' test does an exact match between the strings, having first expanded
both strings.

        contains 
        does not contain 
  e.g. $header_subject: contains "evolution"

A 'contains' test does a partial string match, having expanded both
strings.

        matches 
        does not match 
  e.g. $sender_address matches "(Bill|John)@"

For a 'matches' test, after expansion of both strings, the second one is
interpreted as a regular expression. Exim uses the PCRE regular expression
library, which provides regular expressions that are compatible with Perl.

Care must be taken if you need a backslash in a regular expression, because
backslashes are interpreted as escape characters both by the string
expansion code and by Exim's normal string reading code. For example, if
you want to test the sender address for a domain ending in ".com" the
regular expression is

  \.com$

The backslash and dollar sign in that expression have to be escaped when
used in a filter command, as otherwise they would be interpreted by the
expansion code. Thus what you actually write is

  if $sender_address matches \\.com\$

However, if the expression is given in quotes (mandatory only if it
contains white space) you have to write

  if $sender_address matches "\\\\.com\\$"

If the regular expression contains bracketed sub-expressions, then numeric
variable substitutions such as $1 can be used in the subsequent actions
after a successful match. If the match fails, the values of the numeric
variables remain unchanged. Previous values are not restored after "endif"
- in other words, only one set of values is ever available. If the
condition contains several sub-conditions connected by "and" or "or", it is
the strings extracted from the last successful match that are available in
subsequent actions. Numeric variables from any one sub-condition are also
available for use in subsequent sub-conditions, since string expansion of a
condition occurs just before it is tested.


20. Numeric testing conditions

The following conditions are available for performing numerical tests:

        above 
        is not above 
        below 
        is not below 
  e.g. $message_size is not above 10k

The  arguments must expand to strings of digits, optionally
followed by one of the letters K or M (upper-case or lower-case) which
cause multiplication by 1024 and 1024x1024 respectively.


21. Testing for personal mail

A common requirement is to distinguish between incoming personal mail and
mail from a mailing list. The condition

       personal

is a shorthand for

       $header_to: contains $local_part@$domain and
       $header_from: does not contain $local_part@$domain and
       $header_from: does not contain server@ and
       $header_from: does not contain daemon@ and
       $header_from: does not contain root@ and
       $header_subject: does not contain "circular" and
       $header_precedence: does not contain "bulk"

The variable "local_part" contains the local part of the mail address of
the user whose filter file is being run - it is normally your login id. The
"domain" variable contains the mail domain. This condition tests for the
appearance of the current user in the "To:" header, checks that the sender
is not the current user or one of a number of common daemons, and checks
the content of the "Subject:" and "Precedence:" headers.

If prefixes or suffixes are in use for local parts - something which         |
depends on the configuration of Exim (see section 25 below) - then the       |
first two tests above are also done with                                     |
                                                                             |
  ${local_part_prefix}${local_part}${local_part_suffix}                      |
                                                                             |
instead of just $local_part. If the system is configured to rewrite local    |
parts of mail addresses, for example, to rewrite 'dag46' as 'Dirk.Gently',
then the rewritten form of the address is also used in the tests.

It is quite common for people who have mail accounts on a number of
different systems to forward all their mail to one system, and in this case
a check for personal mail should test all their various mail addresses. To
allow for this, the personal condition keyword can be followed by

  alias 


any number of times, for example

  personal alias smith@else.where alias jones@other.place

This causes messages containing the alias addresses to be treated as
personal.


22. Testing for significant deliveries

Whether or not any previously obeyed filter commands have resulted in a
significant delivery can be tested by the condition "delivered", for
example:

  if not delivered then save mail/anomalous endif


23. Testing for error messages

The condition "error_message" is true if the incoming message is a mail
delivery error message. Putting the command

  if error_message then finish endif

at the head of your filter file is a useful insurance against things going
wrong in such a way that you cannot receive delivery error reports, and is
highly recommended. Note that error_message is a condition, not an expan-
sion variable, and therefore is not preceded by $.

                                                                             |
24. Testing a list of addresses                                              |
                                                                             |
There is a facility for looping through a list of addresses and applying a   |
condition to each of them. It takes the form                                 |
                                                                             |
  foranyaddress  ()                                       |
                                                                             |
where  is interpreted as a list of RFC 822 addresses, as in a        |
typical header line, and  is any valid filter condition or        |
combination of conditions. The parentheses surrounding the condition are     |
mandatory, to delimit it from possible further sub-conditions of the         |
enclosing if command. Within the condition, the expansion variable           |
$thisaddress is set to the non-comment portion of each of the addresses in   |
the string in turn. For example, if the string is                            |
                                                                             |
  B.Simpson , lisa@springfield.tv (his sister)          |
                                                                             |
then $thisaddress would take on the values 'bart@springfield.tv' and         |
'lisa@springfield.tv' in turn.                                               |
                                                                             |
If there are no valid addresses in the list, the whole condition is false.   |
If the internal condition is true for any one address, the overall           |
condition is true and the loop ends. If the internal condition is false for  |
all addresses in the list, the overall condition is false. This example      |
tests for the presence of an eight-digit local part in any address in a To:  |
header:                                                                      |
                                                                             |
    if foranyaddress $h_to ( $thisaddress matches ^\\d{8}@ ) then ...        |
                                                                             |
Header lines can be joined together if a check is to be applied to more      |
than one of them. For example:                                               |
                                                                             |
    if foranyaddress $h_to:,$h_cc ....                                       |
                                                                             |
Note that the colon that terminates a header name can be omitted only for    |
the last name in cases like this, since RFC 822 header names are permitted   |
to contain a wide range of characters, including commas.                     |


25. Multiple personal mailboxes

The system administrator can configure Exim so that users can set up
variants on their email addresses and handle them separately. Consult your
system administrator or local documentation to see if this facility is
enabled on your system, and if so, what the details are.

The facility involves the use of a prefix or a suffix on an email address.
For example, all mail addressed to lg103- would be the property
of user lg103, who could determine how it was to be handled, depending on
the value of .

There are two possible ways in which this can be set up. The first
possibility is the use of multiple .forward files. In this case, mail to
lg103-foo, for example, is handled by looking for a file called .forward-
foo in lg103's home directory. If such a file does not exist, delivery
fails and the message is returned to its sender.

The alternative approach is to pass all messages through a single .forward
file, which must be a filter file in order to distinguish between the
different cases by referencing the variables local_part_prefix or
local_part_suffix, as in the final example in section 27 below. If the
filter file does not handle a prefixed or suffixed address, delivery fails
and the message is returned to its sender.

It is possible to configure Exim to support both schemes at once. In this
case, a specific .forward-foo file is first sought; if it is not found, the
basic .forward file is used.

The "personal" test (see section 21) includes prefixes and suffixes in its   |
checking.                                                                    |


26. Ignoring delivery errors

As was explained above, filtering just sets up addresses for delivery - no
deliveries are actually done while a filter file is active. If any of the
generated addresses subsequently suffers a delivery failure, an error
message is generated in the normal way. However, if the filter command
which sets up a delivery is preceded by the word "noerror", then errors for
that delivery, and any deliveries consequent on it (that is, from alias,
forwarding, or filter files it invokes) are ignored.


27. Examples of filter commands

Simple forwarding:

  # Exim filter
  deliver baggins@rivendell.middle.earth

Vacation handling using traditional means, assuming that the .vacation.msg
and other files have been set up in your home directory:

  # Exim filter
  unseen pipe "/usr/ucb/vacation \"$local_part\""

Vacation handling inside Exim, having first created a file called
.vacation.msg in your home directory:

  # Exim filter
  if personal then vacation endif

File some messages by subject:

  # Exim filter
  if $header_subject: contains "empire" or
     $header_subject: contains "foundation"
  then
     save $home/mail/f&e
  endif

Save all non-urgent messages by weekday:

  # Exim filter
  if $header_subject: does not contain "urgent" and
     $tod_full matches "^(...),"
  then
    save $home/mail/$1
  endif

Throw away all mail from one site, except from postmaster:

  # Exim filter
  if $reply_address contains "@spam.site" and
     $reply_address does not contain "postmaster@"
  then
     seen finish
  endif

Handle multiple personal mailboxes

  # Exim filter
  if $local_part_suffix is "-foo"
  then
    save $home/mail/foo
  elif $local_part_suffix is "-bar"
  then
    save $home/mail/bar
  endif


28. More about string expansion

The description which follows in the next section is an excerpt from the
full specification of Exim, except that it lists only those expansion
variables that are likely to be useful in filter files.

Expanded strings are copied verbatim except when a dollar or backslash
character is encountered. A dollar specifies the start of a portion of the
string which is interpreted and replaced as described below.

An uninterpreted dollar can be included in the string by putting a
backslash in front of it - if the string appears in quotes, two backslashes
are required because the quotes themselves cause interpretation of
backslashes when the string is read in. A backslash can be used to prevent
any character being treated specially in an expansion, including itself.


29. Expansion items

The following items are recognized in expanded strings. White space may be
used between sub-items that are keywords or sub-strings enclosed in braces
inside an outer set of braces, to improve readability.

$ or ${}

   Substitute the contents of the named variable; the latter form can be
   used to separate the name from subsequent alphameric characters. The
   names of the variables are given in section 33 below. If the name of a
   non-existent variable is given, the expansion fails.

$header_
: or $h_
:

   Substitute the contents of the named message header, for example

     $header_reply-to:

   This particular expansion is intended mainly for use in filter files.
   The header names follow the syntax of RFC 822, which states that they
   may contain any printing characters except space and colon.
   Consequently, curly brackets do not terminate header names. Upper-case
   and lower-case letters are synonymous in header names. If the following
   character is white space, the terminating colon may be omitted. The
   white space is included in the expanded string. If the message does not
   contain the given header, the expansion item is replaced by an empty
   string. (See the def condition in section 32 for a means of testing for   |
   the existence of a header.) If there is more than one header with the     |
   same name, they are all concatenated to form the substitution string,
   with a newline character between each of them.

${:}

   The string is first itself expanded, and then the operation specified by
    is applied to it. A list of operators is given in section 31 below.
   The string starts with the first character after the colon, which may be
   leading white space.

${if  {}{}}

   If  is true,  is expanded and replaces the whole
   item; otherwise  is used. The second string need not be
   present; if it is not and the condition is not true, the item is
   replaced with nothing. Alternatively, the word 'fail' may be present
   instead of the second string (without any curly brackets). In this case,
   the expansion fails if the condition is not true. The available
   conditions are described in section 32 below.

${lookup{}  {} {} {}}

${lookup  {} {} {}}

   These items specify data lookups in files and databases, as discussed in
   chapter 6 of the main Exim specification. The first form is used for
   single-key lookups, and the second is used for query-style lookups. The
   , , and  strings are expanded before use.

   If the lookup succeeds, then  is expanded and replaces the
   entire item. During its expansion, a variable called $value is avail-
   able, containing the data returned by the file lookup. If the lookup
   fails,  is expanded and replaces the entire item. It may be
   omitted, in which case the replacement is null.

   For single-key lookups, the string 'partial-' is permitted to precede
   the search type in order to do partial matching, and * or *@ may follow   |
   a search type to request default lookups if the key does not match (see
   sections 6.1 and 6.5 of the main Exim specification).

   If a partial search is used, the variables $1 and $2 contain the wild
   and non-wild parts of the key during the expansion of the replacement
   text. They return to their previous values at the end of the lookup
   item.

   Instead of {} the word 'fail' can appear, and in this case, if
   the lookup fails, the entire string expansion fails in a way that can be
   detected by the caller. The consequences of this depend on the
   circumstances.

   This example looks up the postmaster alias in the conventional alias
   file.

     ${lookup {postmaster} lsearch {/etc/aliases} {$value}}

   This example uses NIS+ to look up the full name of the user correspond-
   ing to the local part of an address, failing the expansion if it is not
   found.

     "${lookup nisplus {[name=$local_part],passwd.org_dir:gcos}        {$value}fail}"

${lookup{}  {} {} {}}

   This searches for  in the file as described above for single-key
   lookups; if it succeeds, it extracts from the data a subfield which is
   identified by the . The data related to the main key must be of
   the form:

      =    =  ...

   where the equals signs are optional. If any of the values contain white
   space, they must be enclosed in double quotes, and any values that are
   enclosed in double quotes are subject to escape processing as described
   in section 5. For example, if a line in a linearly searched file
   contains

     alice: uid=1984 gid=2001

   then expanding the string

     ${lookup{alice:uid}lsearch{}{$value}}

   yields the string '1984'. If the subkey is not found in , then
   , if present, is expanded and replaces the entire item.
   Otherwise the replacement is null.

${extract{} {}}

   The key and the string are first expanded. Then the subfield identified
   by the key is extracted from the string, exactly as just described for
   lookup items with subkeys. If the key is not found in the string, the
   item is replaced by nothing.

${extract{} {} {}}                               |
                                                                             |
   This is distinguished from the above form of extract by having three      |
   rather than two arguments. It extracts from the string the field whose    |
   number is given as the first argument. The first field is numbered one.   |
   If the number is negative or greater than the number of fields in the     |
   string, the result is empty; if it is zero the entire string is           |
   returned. The fields in the string are separated by any one of the        |
   characters in the separator string. For example:                          |
                                                                             |
     ${extract{3}{:}{exim:x:42:99:& Mailer::/bin/bash}}                      |
                                                                             |
   yields '42'. Two successive separators mean that the field between them   |
   is empty (for example, the sixth field above). If the first argument is   |
   not numeric, the expansion fails.                                         |


30. Expansion operators

The following operations can be performed on portions of an expanded
string:

${domain:}                                                           |
                                                                             |
   The string is interpreted as an RFC 822 address and the domain is         |
   extracted from it. If the string does not parse successfully, the result  |
   is empty.                                                                 |

${expand:}

   The expand operator causes a string to be expanded for a second time.
   For example,

     ${expand:${lookup{$domain}dbm{/some/file}{$value}}}

   first looks up a string in a file while expanding the operand for
   expand, and then re-expands what it has found.

${hash__:}                                                     |
                                                                             |
   The two items  and  are numbers. If  is greater than or equal    |
   to the length of the string, the operator returns the string. Otherwise   |
   it computes a new string of length  by applying a hashing function to  |
   the string. The new string consists of characters taken from the first    |
    characters of the string                                              |
                                                                             |
     abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQWRSTUVWXYZ0123456789         |
                                                                             |
   and if  is not present the value 26 is used, so that only lower case   |
   letters appear. These examples:                                           |
                                                                             |
     ${hash_3:monty}                                                         |
     ${hash_5:monty}                                                         |
     ${hash_4_62:monty python}                                               |
                                                                             |
   yield                                                                     |
                                                                             |
     jmg                                                                     |
     monty                                                                   |
     fbWx                                                                    |
                                                                             |
   respectively. The abbreviation h can be used instead of hash.             |

${lc:}

   This forces the letters in the string into lower-case, for example:

     ${lc:$local_part}

${length_:}

   The length operator can be used to extract the initial portion of a
   string. It is followed by an underscore and the number of characters
   required. For example

     ${length_50:$message_body}

   The result of this operator is either the first  characters or
   the whole string, whichever is the shorter. The abbreviation l can be
   used instead of length.

${local_part:}                                                       |
                                                                             |
   The string is interpreted as an RFC 822 address and the local part is     |
   extracted from it. If the string does not parse successfully, the result  |
   is empty.                                                                 |

${quote:}

   The quote operator puts its argument into double quotes if it contains
   anything other than letters, digits, underscores, full stops (periods),   |
   and hyphens. Any occurrences of double quotes and backslashes are
   escaped with a backslash. For example,

     ${quote:ab*cd}

   becomes

     "ab*cd"

   The place where this is useful is when the argument is a substitution
   from a variable or a message header.

${rxquote:}

   The rxquote operator inserts a backslash before any non-alphanumeric
   characters in its argument. This is useful when substituting the values
   of variables or headers inside regular expressions.

${substr__:}

   The substr operator can be used to extract more general substrings than
   length. It is followed by an underscore and the starting offset, then a
   second underscore and the length required. For example

     ${substr_3_2:$local_part}

   If the starting offset is greater than the string length the result is
   the null string; if the length plus starting offset is greater than the
   string length, the result is the right-hand part of the string, starting
   from the given offset. The first character in the string has offset
   zero. The abbreviation s can be used instead of substr.

   The substr expansion operator can take negative offset values to count
   from the righthand end of its operand. The last character is offset -1,
   the second-last is offset -2, and so on. Thus, for example,

     ${substr_-5_2:1234567}

   yields '34'. If the absolute value of a negative offset is greater than
   the length of the string, the substring starts at the beginning of the
   string, and the length is reduced by the amount of overshoot. Thus, for
   example,

     ${substr_-5_2:12}

   yields an empty string, but

     ${substr_-3_2:12}

   yields '1'.

   If the second number is omitted from substr, the remainder of the string
   is taken if the offset was positive. If it was negative, all characters
   in the string preceding the offset point are taken. For example, an
   offset of -1 and no length yields all but the last character of the
   string.


31. Expansion conditions

The following conditions are available for testing while expanding strings:

!

   This negates the result of the condition.

def:

   This condition is true if the named expansion variable does not contain
   the empty string, for example

     ${if def:sender_ident {from $sender_ident}}

   Note that the variable name is given without a leading $ character. If
   the variable does not exist, the expansion fails.

def:header_
  or  def:h_


   This condition is true if a message is being processed and the named
   header exists in the message. For example,

     ${if def:header_reply-to:{$h_reply-to:}{$h_from:}}

   Note that no $ appears before header_ or h_ in the condition, and that
   header names must be terminated by colons if white space does not
   follow.

exists {}

   The substring is first expanded and then interpreted as an absolute
   path. The condition is true if the named file (or directory) exists. The
   existence test is done by calling the stat() function.

eq {}{}

   The two substrings are first expanded. The condition is true if the two
   resulting strings are identical, including the case of letters.

match {}{}

   The two substrings are first expanded. The second is then treated as a
   regular expression and applied to the first. Because of the pre-
   expansion, if the regular expression contains dollar or backslash
   characters, they must be escaped with backslashes. If the whole expan-
   sion string is in double quotes, further escaping of backslashes is also
   required.

   The condition is true if the regular expression match succeeds. At the
   start of an "if" expansion the values of the numeric variable substitu-
   tions $1 etc. are remembered. Obeying a "match" condition that succeeds
   causes them to be reset to the substrings of that condition and they
   will have these values during the expansion of the success string. At
   the end of the "if" expansion, the previous values are restored. After
   testing a combination of conditions using "or", the subsequent values of
   the numeric variables are those of the condition that succeeded.

or {{}{}...}

   The sub-conditions are evaluated from left to right. The condition is
   true if any one of the sub-conditions is true. When a true sub-condition
   is found, the following ones are parsed but not evaluated. Thus if there
   are several 'match' sub-conditions the values of the numeric variables
   are taken from the first one that succeeds.

and {{}{}...}

   The sub-conditions are evaluated from left to right. The condition is
   true if all of the sub-conditions are true. When a false sub-condition
   is found, the following ones are parsed but not evaluated.


32. Expansion variables

This list of expansion variable substitutions contains those that are
likely to be of use in filter files. Others that are not relevant at
filtering time, or are of interest only to the system administrator, are
omitted.

$0, $1, etc: When a matches expansion condition succeeds, these variables    |
contain the captured substrings identified by the regular expression during  |
subsequent processing of the success string of the containing "if" expan-    |
sion item. They may also be set externally by some other matching process    |
which precedes the expansion of the string. For example, the commands        |
available in Exim filter files include an "if" command with its own regular  |
expression matching condition.                                               |

$domain: When an address is being directed, routed, or delivered on its
own, this variable contains the domain. In particular, it is set during
user filtering, but not during system filtering, since a message may have
many recipients and the system filter is called just once.

$home: This is set to the user's home directory when user filtering is
configured in the normal way. When running a filter test via the -bf
option, $home is set to the value of the environment variable HOME.

$local_part: When an address is being directed, routed, or delivered on its
own, this variable contains the local part. If a local part prefix or
suffix has been recognized, it is not included in the value.

$local_part_prefix: When an address is being directed or delivered locally,
and a specific prefix for the local part was recognized, it is available in
this variable. Otherwise it is empty.

$local_part_suffix: When an address is being directed or delivered locally,
and a specific suffix for the local part was recognized, it is available in
this variable. Otherwise it is empty.

$key: When a domain list is being searched, this variable contains the
value of the key, so that it can be inserted into strings for query-style
lookups. See chapter 6 of the main Exim specification for details. In other
circumstances this variable is empty.

$message_body: This variable contains the initial portion of a message's
body while it is being delivered, and is intended mainly for use in filter
files. The maximum number of characters of the body that are used is set by
the message_body_visible configuration option; the default is 500. Newlines
are converted into spaces to make it easier to search for phrases that
might be split over a line break.

$message_headers: This variable contains a concatenation of all the header
lines when a message is being processed. They are separated by newline
characters.

$message_id: When a message is being received or delivered, this variable
contains the unique message id which is used by Exim to identify the
message.

$message_precedence: When a message is being delivered, the value of any
Precedence: header is made available in this variable. If there is no such
header, the value is the null string.

$message_size: When a message is being received or delivered, this variable
contains its size in bytes. The size includes those headers that were
received with the message, but not those (such as Envelope-to:) that are
added to individual deliveries.

$n0 - $n9: These variables are counters that can be incremented by means of  |
the add command in filter files.                                             |

$original_domain: When a top-level address is being processed for delivery,
this contains the same value as $domain. However, if an address generated
by an alias, forward, or filter file is being processed, this variable
contains the domain of the original address.

$original_local_part: When a top-level address is being processed for
delivery, this contains the same value as $local_part. However, if an
address generated by an alias, forward, or filter file is being processed,
this variable contains the local part of the original address.

$primary_hostname: The value set in the configuration file, or read by the
uname() function.

$qualify_domain: The value set for this option in the configuration file.

$qualify_recipient: The value set for this option in the configuration
file, or if not set, the value of $qualify_domain.

$received_protocol: When a message is being processed, this variable
contains the name of the protocol by which it was received.

$recipients_count: When a message is being processed, this variable con-
tains the number of envelope recipients that came with the message.
Duplicates are not excluded from the count.

$reply_address: When a message is being processed, this variable contains
the contents of the Reply-to: header if one exists, or otherwise the
contents of the From: header.

$return_path: When a message is being delivered, this variable contains the
return path - the sender field that is sent as part of the envelope. In
many cases, this has the same value as $sender_address, but if, for
example, an incoming message to a mailing list has been expanded by a
director which specifies a specific address for delivery error messages,
then $return_path contains the new error address, while $sender_address
contains the original sender address that was received with the message.

$sender_address: When a message is being processed, this variable contains
the sender's address that was received in the message's envelope.

$sender_address_domain: The domain portion of $sender_address.

$sender_address_local_part: The local part portion of $sender_address.

$sender_fullhost: When a message has been received from a remote host, this
variable contains the host name and IP address in a single string, which
always ends with the IP address in square brackets. The format of the rest
of the string depends on whether the host issued a HELO or EHLO SMTP
command, and whether the host name was verified by looking up its IP
address. A plain host name at the start of the string is a verified host
name; if this is not present, verification either failed or was not
requested. A host name in parentheses is the argument of a HELO or EHLO
command. This is omitted if it is identical to the verified host name or to  |
the host's IP address in square brackets.                                    |

$sender_helo_name: When a message has been received from a remote host that
has issued a HELO or EHLO command, the first item in the argument of that
command is placed in this variable. It is also set if HELO or EHLO is used   |
when a message is received using SMTP locally via the -bs or -bS options.    |

$sender_host_address: When a message has been received from a remote host,
this variable contains the host's IP address.

$sender_host_name: When a message has been received from a remote host,
this variable contains the host's name as verified by looking up its IP
address. If verification failed, or was not requested, this variable
contains the empty string.

$sender_ident: When a message has been received from a remote host, this
variable contains the identification received in response to an RFC 1413
request. When a message has been received locally, this variable contains
the login name of the user that called Exim.

$sn0 - $sn9: These variables are copies of the values of the $n0 - $n9       |
accumulators that were current at the end of the system filter file. This    |
allows a system filter file to set values that can be tested in users'       |
filter files. For example, a system filter could set a value indicating how  |
likely it is that a message is junk mail.                                    |

$tod_bsdinbox: The time of day and date, in the format required for BSD-
style mailbox files, for example: Thu Oct 17 17:14:09 1995.

$tod_full: A full version of the time and date, for example: Wed, 16 Oct
1995 09:51:40 +0100. The timezone is always given as a numerical offset
from GMT.

$tod_log: The time and date in the format used for writing Exim's log
files, which is: 1995-10-12 15:32:29.

$value: This variable contains the result of an expansion lookup operation,
as described above. If used in other circumstances, its contents are null.

$version_number: The version number of Exim.