metamail(1)

metamail -- infrastructure for mailcap-based multimedia mail handling

Synopsis

metamail [-b] [-B] [-c contenttype . . .] [-d] [-e] [-E encoding] [-f from-name] [-h] [-m mailer-name] [-p] [-P] [-r] [-s subject] [-q] [-w] [-x] [-y] [-z] [filename]

Description

The metamail program reads a mailcap file to determine how to display non-text at the local site. Every mail-reading interface needs to call metamail whenever non-text mail is being viewed, unless the mail is of a type that is already understood by the mail-reading program.

metamail consults the mailcap file(s) to determine what program to use to show the message to the user.

At a site where all mail reading interfaces have been modified to call metamail for non-text mail, extending the local email system to handle a new media type in the mail becomes a simple matter of adding a line to a mailcap file.

Although this manual page discusses only mail, metamail is equally useful in adding multimedia support to news and bulletin board reading programs, assuming those programs preserve the ``Content-type'' header or some other indication of the ``Content-type'' of the messages.

In general, users never run metamail directly. Instead, metamail is invoked automatically on behalf of the user by the user's mail reading program, whenever a non-text message is to be viewed.

This manual page, therefore, is directed not at end users, but at two categories of readers: those who are adding metamail support to a particular mail-reading program, and those who are adding lines to a mailcap file. The former need only be concerned with the command line syntax of metamail. The latter may ignore the command line syntax and need only be concerned with the mailcap file syntax, as described in a later section.

Note: metamail determines the type of a message using the ``Content-type'' header, as defined in RFC 1049 and RFC 1341 (MIME). However, using the -b and -c options, metamail can be made to work with mail that is not in Internet format, including X.400 messages. Note also that metamail automatically decodes mail that has been encoded for 7-bit transport if the mail includes a ``Content-Transfer-Encoding'' header as specified by RFC 1341. If data has been encoded via the "base64" encoding, it will map CRLF to local newlines for textual data, but not for other data, unless instructed otherwise by a textualnewlines field in a mailcap entry.

Files

The default path for mailcap files is:

   HOME/.mailcap:/etc/mailcap:/usr/etc/mailcap:/usr/local/etc/mailcap

Options

When called with no options or arguments, metamail expects to receive an RFC 822 format message on its standard input. The following options can alter that expectation:

-b: Tells metamail that the message is not in RFC 822 format, but instead is only the body of the message (that is, there are no message headers). The use of -b requires the use of -c.
-B: Tells metamail that the message is to be displayed in the background, if it is non-interactive (that is, it does not have the needsterminal attribute in the mailcap file). It cannot be used with -p or -P.
-c contenttype: Tells metamail to use the specified ``~Content-type'' rather than the one in the headers, if any.
-d: This option tells metamail not to ask any questions before running an interpreter to view the message. (By default, metamail always asks before running almost any interpreter, if it is running in an interactive terminal and the MM_NOASK environment variable is not set. However, it does not ask about the ``Content-type'' of text, that is, the default value for MM_NOASK is ``text,text/plain''.)
-e: Tells metamail to suppress leading newline characters in message bodies. This is particularly useful for MH-format mail.
-f address: Specifies the name of the sender of the message. Otherwise, this is determined from the header, if possible. This information will be placed in the environment to make it available to any interpreters called by metamail.
-h: Specifies that metamail is being used for printing a message. In particular, this means that the normal command field in mailcap will not be executed but instead the command specified in the ``prints'' field will be executed. (If there is nothing in the ``print'' field, the mailcap entry will be ignored and the search will continue for a matching mailcap entry that does have a ``print'' field.) The -h option automatically turns on the -d option.
-m mailername: Specifies the name of the mail program that called metamail. This information will be placed in the environment to make it available to any interpreters called by metamail.
-p: Specifies that, if necessary, output should be shown to the user one page at a time. By default, this will cause such output to be piped through the more command, but the environment variable METAMAIL_PAGER can be used to specify an alternative command to use. Note that one should use -p rather than piping the output of metamail through a pager, because some interpreters called by metamail might be interactive rather than requiring pagination. Metamail can tell whether or not to use a pager from information in the mailcap file. This option cannot be used with -B.
-P: Just like -p, except that it also causes metamail to print ``Press <Return> to go on'' and await a <Return> after it has finished with the message. This is intended for use only when metamail calls itself recursively in a new terminal window created only for that purpose. This option cannot be used with -B.
-q: Tells metamail to be quiet. By default, metamail prints a few key message headers (controllable with the KEYHEADS environment) and some other informative information, on stdout before running the interpreter, but this behavior is suppressed with -q.
-r: Specifies that it is permissible to run as root. By default, metamail refuses to run if the real or effective user id is root. You can get the same effect using the MM_RUNASROOT environment variable.
-R: Specifies that the tput clear should be executed to reset the terminal state, before any other I/O activity.
-s subject: Specifies the subject of the mail message. By default, this information is obtained from the headers. This information will be placed in the environment to make it available to any interpreters called by metamail.
-w: Tells metamail that instead of consulting a mailcap file to determine how to display the data, it should simply decode each part and write it to a file in its raw (possibly binary) format. Depending on the circumstances in which it is called, metamail may derive the filename to use from the message headers, by asking the user, or by generating a unique temporary file name.
-x: Tells metamail that it is definitely not running on a terminal, no matter what isatty indicates. This is necessary when metamail is actually running on a pseudo-terminal and isatty(3C) returns TRUE but there is really no terminal on which to interact with the user. The same effect as -x can also be obtained with the environment variable MM_NOTTTY.
-y: Tells metamail to try to ``yank'' a MIME-format message from the body of the message. It is useful when a MIME-format has been rejected by a mail delivery system that does not now how to format the rejection in a MIME-compliant manner. (For the convenience of those who cannot control how metamail is called from their mail reader, this can also be set with the MM_YANKMODE variable.) If you use yank mode on messages that really are in MIME format, or on messages that do not contain a MIME message in the body, the effects could be strange.
-z: Tells metamail to delete its input file when finished. The -z option requires that a filename be given as an argument to metamail, (because it is not reading stdin.
-T: Used by metamail recursively, to turn off the effect of the MM_TRANSPARENT environment variable. It should only be used when the metamail program restarts itself in a terminal emulator window.
filename: Any argument that does not start with minus (-) to indicate an option is interpreted as the name of a file to read instead of standard input.

Unrecognized mail

From time to time, metamail may generate a message:

   **** Unrecognized mail type: 'sensor-vision'.
   Writing to file /tmp/metamail.1234 ****

This means that your are trying to read a message that contains data that is marked as being in ``sensor-vision'' format, but that your site has not yet configured metamail to properly display that type of data. In the general case, such configuration is accomplished using the mailcap file mechanism, as described in the next section.

For unrecognized ``types'', metamail simply removes all header and encoding information from the data, and writes it out to a temporary file. (If running interactively, it will offer you more alternatives -- writing it to a temporary file, viewing it as text, or abandoning the process.) It is up to the user to delete such files when they are no longer needed.

Mailcap

The primary purpose of the metamail program is to allow diverse mail reading programs to centralize their access to multimedia information. If all the mail reading programs call a single program to handle non-text mail, then only that program needs to know about the diverse types of non-text mail that might be received.

The metamail program is made more flexible in this role through the mechanism of one or more mailcap files. The purpose of the mailcap files is to tell metamail what program to run in order to show the user mail in a given format. Thus it becomes possible to add a new media type to all of the mail reading programs at a site simply by adding a line to a mailcap file.

metamail uses a search path to find the mailcap file(s) to consult. Unlike many path searches, if necessary metamail will read all the mailcap files on its path. That is, it will keep reading mailcap files until it runs out of them, or until it finds a line that tells it how to handle the piece of mail it is looking at. If it finds a matching line, it will execute the command that is specified in the mailcap file.

The default search path is equivalent to:

   $HOME/.mailcap:/etc/mailcap:/usr/etc/mailcap:/usr/local/etc/mailcap

It can be overridden by setting the MAILCAPS environment variable. Note: metamail does not actually interpret environment variables such as HOME or the ~ syntax in this path search.

Non-ascii header

metamail has rudimentary built-in support for the emerging Internet standards for non-ASCII data in mail headers. What this means is that such data will be recognized, decoded, and sent to the terminal. This behavior may be more or less reasonable, depending on the character set in the header data and the capability of the user's terminal, but it will rarely be any worse than showing such data in its encoded form.

Environment

METAMAIL_TMPDIR

If set, this variable overrides /tmp as the name of the directory in which metamail and associated programs will create temporary files on UNIX.

MM_NOASK

If set to 1, metamail will never ask the user for confirmation before running an interpreter. Otherwise, MM_NOASK may be set to a comma-separated list of type names (without white space) for which the user does not desire confirmation. (If the -d command line option is given, MM_NOASK is set to 1 for spawned processes, allowing -d to work recursively.)

KEYHEADS

May be set to a colon-separated list of header names which are the only headers that metamail will print out. By default, the behavior is as if KEYHEADS were set to:

Date:From:Subject:To:CC:Content-Description

If KEYHEADS is set to the empty string, no header are printed out. If it is set to an asterisk (

), all headers are printed out.

KEYIGNHEADS

May be set to a colon-separated list of header names which are the headers that metamail will not print out. This variable is only examined if KEYHEADS is not set. If KEYIGNHEADS is set to the empty string, all headers are printed out. If it is set to an asterisk (

), no headers will be printed out.

MM_NOTTTY

If MM_NOTTTY is set to any nonzero value, metamail will assume that it is not running in a terminal window. MM_NOTTTY implies setting MM_NOASK to 1. If -z is given, MM_NOTTTY is set for spawned processes, allowing -z to work recursively.

MAILCAPS

This variable can be used to override the default path search for mailcap files.

METAMAIL_PAGER

If set, this variable overrides more as the name of the program to run to paginate output from an interpreter, when pagination has been requested. Note that the normal PAGER variable is not used because many pagers (notably the less pager) interfere with the workings of termcap-based mail viewers.

NOMETAMAIL

This variable is not actually used by metamail, but is used by most metamail-compatible mail reading interfaces. If NOMETAMAIL is set to any value, most mail reading interfaces will never call the metamail program, effectively inhibiting all multimedia functionality.

MM_DEBUG

If set to any value, metamail will produce slightly more verbose output to tell what it is doing.

MM_QUIET

If this variable is set to 1, metamail will produce even less output than usual. In particular, it will suppress the ``Executing...'' line unless MM_DEBUG is set. Otherwise, this variable can be set to a comma-separated list of short commands, and the ``Executing...'' line will be suppressed for those commands only. The default setting for MM_QUIET is cat, which means that the ``Executing...'' line is printed for all commands executed except cat. This makes text support look more natural without sacrificing an understanding of what is going on in more complex circumstances.

MM_YANKMODE

Setting this variable to a non-zero value has the same effect as the -y switch. Be sure to read the caveats attached to the description of -y before you use it. Basically, the only time you would set MM_YANKMODE is in order to re-enter a mail reader in which you cannot control the way metamail is called, just to read a single rejected MIME message that was rejected by a mail agent that does not understand MIME. In such cases, you should read that message, exit, and unset this variable.

MM_TRANSPARENT

If this variable is set, metamail will reproduce the entire raw message on stdout, and will open up a new terminal emulator window in which to do something more intelligent. This option supports certain mail readers, such as mailtool, that depend on the output of the UNIX mail program being the same as the raw message in the database.

MM_CHARSET

The value of this variable affects the printing of character set declarations when mail headers being printed contain text in this character set. For example, if you set MM_CHARSET to ``ISO-8859-8'', it will suppress warnings when header output is produced in that character set. If metamail is invoked by mailx, mailx will pass the MM_CHARSET value to metamail. If you invoke metamail separately, you will have to explicitly set MM_CHARSET if you want suppress the warnings. See the mailx(1) manual page for more information on MM_CHARSET.

DISPLAY

Used to create a terminal window under the X11 window system.

WINDOW_PARENT

Used to create a terminal window under the SunTools window system.

WMHOST

Used to create a terminal window under the old Andrew WM window system.

Interpreter environment

When metamail calls an interpreter specified in a mailcap file, it sets several environment variables which can be used by the interpreter if desired:

MM_HEADERS: This variable is set to the full set of RFC822 headers, if any.
MM_MAILER: This variable is set to the name of the mailer that called metamail, if the -m option was used.
MM_CONTENTTYPE: This variable is set to the Content-type, as named by the ``Content-type'' header or passed in via the -c option. If the ``Content-type'' has a subtype and parameters, these are also included in MM_CONTENTTYPE, for example, ``multipart/mixed; boundary=foobar'',
MM_SUMMARY: This variable is set to an efficient one-line ``caption'' of the message, typically including its sender and subject.
MM_USEPAGER: This variable is set to a non-zero if the use of a pager has been requested for long output (for example. the -p switch was given.) If -p is given, MM_USEPAGER is set for spawned processes, allowing -p to work recursively. This option cannot be used with -B.
TERMINAL_CMD: This variable may be set to a string that is used to start a new terminal window if necessary. The command to be executed in that window will be appended to this command. By default, this is set to something like xterm -e if DISPLAY is set, or shelltool if WINDOW_PARENT is set. Users of Sun's OpenWindows may wish to set TERMINAL_CMD to shelltool if they prefer shelltool over xterm.
MM_RUNASROOT: If set to a non-zero variable, this will allow the metamail program to be run by root, the same effect as the -r switch to metamail.

References

mailto(1), metasend(1), mimencode(1), getfilename(1M), richtext(1M), showexternal(1M), shownonascii(1M), showpartial(1M), showpicture(1M)

Notices

In a multipart/alternative body or body parts, some headers in the embedded part that should be displayed may not be displayed. This will rarely be a problem. Also, in a multipart/alternative, anything with a type of multipart or message is considered to be a recognized part, regardless of the recognizability of its contents. This might be a problem, only further experience will tell.

The ``textualnewlines'' field in mailcap entries affects a global table of exceptions. This means that if there is more than one mailcap entry for a given ``Content-type'', and they have conflicting textualnewlines settings, the wrong value may be used. It is difficult to conceive of a situation where this would be a real problem, because it seems inconceivable that a single ``Content-type'' would ever require newline characters to be treated in two different ways, regardless of the environment.

Author is Nathaniel S. Borenstein, Bell Communications Research, Inc. See copyright page for further information.