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 auxiliary
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)
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 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