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 informational RFC 1524. 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 the subtype, if any. (That is, if
the content-type was "image/pbm; opt1=something-else", then "%t" would
be replaced by "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 % character. (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 information 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.
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 produce 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
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/share/etc/mailcap:/usr/local/etc/mailcap
-- default path for mailcap files.
SEE ALSO
run-mailcap(1), mailcap.order(5), update-mime(8)
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 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