MAILCAP(4) Bellcore Prototype (Release 2) MAILCAP(4)
NAME
mailcap - metamail capabilities file
DESCRIPTION
The mailcap file is read by the metamail program to
determine how to display non-text at the local site.
The syntax of a mailcap file is quite simple, at least
compared to termcap files. Any line that starts with "#" is
a comment. Blank lines are ignored. Otherwise, each line
defines a single mailcap entry for a single content type.
Long lines may be continued by ending them with a backslash
character, \.
Each individual mailcap entry consists of a content-type
specification, a command to execute, and (possibly) a set of
optional "flag" values. For example, a very simple mailcap
entry (which is actually a built-in default behavior for
metamail) would look like this:
text/plain; cat %s
The optional flags can be used to specify additional
information about the mail-handling command. For example:
text/plain; cat %s; copiousoutput
can be used to indicate that the output of the 'cat' command
may be voluminous, requiring either a scrolling window, a
pager, or some other appropriate coping mechanism.
The "type" field (text/plain, in the above example) is
simply any legal content type name, as defined by RFC 822.
In practice, this is almost any string. It is the string
that will be matched against the "Content-type" header (or
the value passed in with -c) to decide if this is the
mailcap entry that matches the current message.
Additionally, the type field may specify a subtype (e.g.
"text/ISO-8859-1") or a wildcard to match all subtypes (e.g.
"image/*").
The "command" field is any UNIX command ("cat %s" in the
above example), and is used to specify the interpreter for
the given type of message. It will be passed to the shell
via the system(3) facility. Semicolons and backslashes
within the command must be quoted with backslashes. If the
command contains "%s", those two characters will be replaced
by the name of a file that contains the body of the message.
If it contains "%t', those two characters will be replaced
by the content-type field, including th e subtype, if any.
(That is, if the content-type was "image/pbm;
opt1=something-else", then "%t" would be replaced by
Page 1 (printed 5/3/99)
MAILCAP(4) Bellcore Prototype (Release 2) MAILCAP(4)
"image/pbm".) If the command field contains "%{" followed
by a parameter name and a closing "}", then all those
characters will be replaced by the value of the named
parameter, if any, from the Content-type header. Thus, in
the previous example, "%{opt1}" will be replaced by
"something-else". Finally, if the command contains "",
those two characters will be replaced by a single % c h
aracter. (In fact, the backslash can be used to quote any
character, including itself.)
If no "%s" appears in the command field, then instead of
placing the message body in a temporary file, metamail will
pass the body to the command on the standard input. This is
helpful in saving /tmp file space, but can be problematic
for window-oriented applications under some window systems
such as MGR.
Two special codes can appear in the viewing command for
objects of type multipart (any subtype). These are "%n" and
"%F". %n will be replaced by the number of parts within the
multipart object. %F will be replaced by a series of
arguments, two for each part, giving first the content-type
and then the name of the temporary file where the decoded
part has been stored. In addition, for each file created by
%F, a second file is created, with the same name followed by
"H", which contains the header informat ion for that body
part. This will not be needed by most multipart handlers,
but it is there if you ever need it.
The "notes=xxx" field is an uninterpreted string that is
used to specify the name of the person who installed this
entry in the mailcap file. (The "xxx" may be replaced by
any text string.)
The "test=xxx" field is a command that is executed to
determine whether or not the mailcap line actually applies.
That is, if the content-type field matches the content-type
on the message, but a "test=" field is present, then the
test must succeed before the mailcap line is considered to
"match" the message being viewed. The command may be any
UNIX command, using the same syntax and the same %-escapes
as for the viewing command, as described above. A command
is considered to succeed if it exits with a zero exit
status, and to fail otherwise.
The "print=xxx" field is a command that is executed to print
the data instead of display it interactively. This behavior
is usually a consequence of invoking metamail with the "-h"
switch.
Page 2 (printed 5/3/99)
MAILCAP(4) Bellcore Prototype (Release 2) MAILCAP(4)
The "textualnewlines" field can be used in the rather
obscure case where metamail's default rules for treating
newlines in base64-encoded data are unsatisfactory. By
default, metamail will translate CRLF to the local newline
character in decoded base64 output if the content-type is
"text" (any subtype), but will not do so otherwise. A
mailcap entry with a field of "textualnewlines=1" will force
such translation for the specified content-type, while
"textualnewlines=0" will guarantee that the translation does
not take place even for textual content-types.
The "compose" field may be used to specify a program that
can be used to compose a new body or body part in the given
format. Its intended use is to support mail composing
agents that support the composition of multiple types of
mail using external composing agents. As with the view-
command, the compose command will be executed after
replacing certain escape sequences starting with "%". In
particular, %s should be replaced by the name of a file to
which the composed data is to be written by the specified
composing program, thus allowing th3e calling program (e.g.
metamail) to tell the called program where to store the
composed data. If %s does not appear, then the composed
data will be assumed to be written by the composing programs
to standard output. The result of the composing program
may be data that is NOT yet suitable for mail transport --
that is, a Content-Transfer-Encoding may still need to be
applied to the data.
The "composetyped" field is similar to the "compose" field,
but is to be used when the composing program needs to
specify the Content-type header field to be applied to the
composed data. The "compose" field is simpler, and is
preferred for use with existing (non-mail-oriented) programs
for composing data in a given format. The "composetyped"
field is necessary when the Content-type information must
include auxilliary parameters, and the composition program
must then know enough about mail formats to pro duce output
that includes the mail type information, and to apply any
necessary Content-Transfer-Encoding. Conceptually,
"compose" specifies a program that simply outputs the
specified type of data in its raw form, while "composetyped"
specifies a program that outputs the data as a MIME object,
with all necessary Content-* headers already in place.
needsterminal
If this flag is given, the named interpreter needs
to interact with the user on a terminal. In some
environments (e.g. a window-oriented mail reader
under X11) this will require the creation of a new
terminal emulation window, while in most
Page 3 (printed 5/3/99)
MAILCAP(4) Bellcore Prototype (Release 2) MAILCAP(4)
environments it will not. If the mailcap entry
specifies "needsterminal" and metamail is not
running on a terminal (as determined by isatty(3),
the -x option, and the MM_NOTTTY environment
variable) then metamail will try to run the command
in a new terminal emulation
window. Currently, metamail knows how to create
new windows under the X11, SunTools, and WM window
systems.
copiousoutput
This flag should be given whenever the interpreter
is capable of producing more than a few lines of
output on stdout, and does no interaction with the
user. If the mailcap entry specifies copiousoutput,
and pagination has been requested via the "-p"
command, then the output of the command being
executed will be piped through a pagination program
("more" by default, but this can be overridden with
the METAMAIL_PAGER environment variable).
BUILT-IN CONTENT-TYPE SUPPORT
The metamail program has built-in support for a few key
content-types. In particular, it supports the text type,
the multipart and multipart/alternative type, and the
message/rfc822 types. This support is incomplete for many
subtypes -- for example, it only supports US-ASCII text in
general. This kind of built-in support can be OVERRIDDEN by
an entry in any mailcap file on the user's search path.
Metamail also has rudimentary built-in support for types
that are totally unrecognized -- i.e. for which no
mailcap entry or built-in handler exists. For such
unrecognized types, metamail will write a file with a
"clean" copy of the data -- i.e. a copy in which all mail
headers have been removed, and in which any 7-bit transport
encoding has been decoded.
FILES
$HOME/.mailcap:/etc/mailcap:/usr/etc/mailcap:/usr/local/etc/mailcap
-- default path for mailcap files.
SEE ALSO
metamail(1)
COPYRIGHT
Copyright (c) 1991 Bell Communications Research, Inc.
(Bellcore)
Permission to use, copy, modify, and distribute this
material for any purpose and without fee is hereby granted,
provided that the above copyright notice and this permission
notice appear in all copies, and that the name of Bellcore
Page 4 (printed 5/3/99)
MAILCAP(4) Bellcore Prototype (Release 2) MAILCAP(4)
not be used in advertising or publicity pertaining to this
material without the specific, prior written permission of
an authorized representative of Bellcore. BELLCORE MAKES NO
REPRESENTATIONS ABOUT THE ACCURACY OR SUITABILITY OF THIS
MATERIAL FOR ANY PURPOSE. IT IS PROVIDED "AS IS", WITHOUT
ANY EXPRESS OR IMPLIED WARRANTIES.
AUTHOR
Nathaniel S. Borenstein
Page 5 (printed 5/3/99)
