A MIME message contains message blocks of a variety of file-types that can be displayed or created using a number of tools. You or your organization may prefer one tool over another. You can also define your own file-types in Ishmail. To handle such flexibility, Ishmail provides mechanisms for defining unique file-types and display methods through the use of the mailcap files.
The purpose of the mailcap is to indicate to
Ishmail what program
to run in order to show user mail in a given format. With
Ishmail,
it is possible to add a new file-type to all of the mail reading
programs at a site simply by adding a line to a the system
mailcap file. This can be overridden by defining the
$SYSMAILCAP
environment variable.
Ishmail also attempts to read a user-provided mailcap file in the user's home directory. This file is $HOME/.mailcap. This file does not have to exist. But, if it does, its entries will take precedence over entries of the same type in the system mailcap file.
The syntax of a mailcap file is fairly simple and straightforward. 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 might look like this:
image/gif; xloadimage %s
The optional flags can be used to specify additional information about the mail-handling command. For example:
image/gif; xloadimage %s; label="A GIF Image"
can be added to use as a default label to display when a MIME message arrives with an image/gif type, that provides no description of itself.
The type field (image/gif, in the above example) is simply any legal content type name, as defined by Internet RFC 1341 (Multipurpose Internet Mail Extension). In practice, this is almost any string. It is the string that will be matched against the "Content-type" header to decide if this is the mailcap entry that matches the current message.
Additionally, the type field may specify a subtype or a wildcard to match all subtypes (for example, "image/*"). A wild card definition should follow specific definitions, because Ishmail will attempt to match the first entry it finds.
The command field is any UNIX command ("xloadimage %s" in the above example), and is used to specify the interpreter for the given type of message. 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/jpeg; opt1=something-else", then "%t" would be replaced by "image/jpeg".)
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 value of the named parameter, if any, from the value of the Content-type header. Thus, in the earlier example, "%{opt1}" will be replaced by "something-else". If a parameter is specified for the command, and the body part does not have a value for it, then "" (two double quotes representing the empty string) is substituted on the command line.
If the command contains a "%e", those two characters will be replaced by the value of the Content-Transfer-Encoding, if there is one. This is useful particularly for external-body types, and the command needs to decode the retrieved body.
There are two additional built-in parameters; %{ruser} and
%{rpwd}. The first,
ruser, is where a user-supplied user account
name is substituted on the command line. This is commonly used
for external-body types that use "ftp" as an access-type. The
rpwd
parameter is for the accompanying password.
If no "%s" appears in the command field, then instead of placing the message body in a temporary file, Ishmail passes the body to the command on the standard input.
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 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 to use when you want to print the file. For instance application/postscript type could have an entry:
application/postscript; ghostview %s; print=lpr -P ps_printer
The "textualnewlines" field can be used in the rather obscure case where Ishmail's default rules for treating newlines in base64-encoded data are unsatisfactory. By default, Ishmail translates 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.
Ishmail handles the following types automatically, thus negating a need for mailcap entries for text/plain, text/richtext, multipart/ mixed, multipart/digest, multipart/alternative, multipart/parallel, message/rfc822, and message/partial. But this does not preclude creating mailcap entries for these types if, for some reason, you'd rather not use the built-in presentation mechanisms for these types.
Ishmail has a default entry for message/external-body in the mailcap shipped with the program. It is recommended that this entry not be changed.
Ishmail also has default entries for image/gif and audio/basic. However, if you have a better application for presenting these types, make the change there.