MAILCAP(5) File Formats Manual MAILCAP(5)
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 term-
cap files. Any line that starts with "#" is a comment. Blank lines are
ignored. Otherwise, each line defines a single mailcap entry for a sin-
gle 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 vo-
luminous, 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 "im-
age/pbm; opt1=something-else", then "%t" would be replaced by "im-
age/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 multi-
part 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 consid-
ered 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 conse-
quence 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 lo-
cal 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 mul-
tiple 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 re-
placed by the name of a file to which the composed data is to be written
by the specified composing program, thus allowing the 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 sim-
pler, and is preferred for use with existing (non-mail-oriented) pro-
grams for composing data in a given format. The "composetyped" field is
necessary when the Content-type information must include auxiliary para-
meters, 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, "com-
pose" 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 win-
dow-oriented mail reader under X11) this will require the cre-
ation of a new terminal emulation window, while in most environ-
ments it will not. If the mailcap entry specifies "needstermi-
nal" and metamail is not running on a terminal (as determined by
isatty(3), the -x option, and the MM_NOTTTY environment vari-
able) then metamail will try to run the command in a new termi-
nal 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 copi-
ousoutput, and pagination has been requested via the "-p" com-
mand, 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 multi-
part/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/mail-
cap -- default path for mailcap files.
SEE ALSO
run-mailcap(1), mailcap.order(5), update-mime(8)
RFC 1524 (<http://tools.ietf.org/html/rfc1524>)
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 copy-
right 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 REPRESENTA-
TIONS ABOUT THE ACCURACY OR SUITABILITY OF THIS MATERIAL FOR ANY PUR-
POSE. IT IS PROVIDED "AS IS", WITHOUT ANY EXPRESS OR IMPLIED WAR-
RANTIES.
AUTHOR
Nathaniel S. Borenstein
Bellcore Prototype Release 2 MAILCAP(5)
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