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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 simple mailcap entry (which is actually a built-in default behavior for metamail) would look like this:
text/plain; cat %sThe optional flags can be used to specify additional information about the mail-handling command. For example:
text/plain; cat %s; copiousoutputcan 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 (for example, text/ISO-8859-1) or a wildcard to match all subtypes (for example, 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(3S) 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 0 exit status, and to fail otherwise.
The textualnewlines
field can be used in the rather obscure case where
the default
metamail rules for treating base64-encoded newlines
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''.
Other flags used in mailcap include:
$HOME/.mailcap:/etc/mailcap:/usr/etc/mailcap:/usr/local/etc/mailcap