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 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 % ch
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 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 "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.
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