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早期freebsd实现

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\section{Introduction}				% mtr
The UCI version of the Rand Message Handling System, \MH/,
is a software system that performs two functions:
\underbar{first},
it interfaces a user to a message transport system,
so the user may receive and send mail;
\underbar{second},
it permits the user to maintain an organized mail environment to facilitate
the composition of new messages and the reading of old messages.
In short,
while not responsible for the delivery of messages,
\MH/ aids the user in handling mail.

\MH/ was originally developed by the Rand Corporation,
and initially was proprietary software.
The Department of Information and Computer Science at
University of California, Irvine,
shortly after joining the Computer Science Network (CSnet),
acquired a copy of \MH/,
and began additional development of the software.
Since that time,
the Rand Corporation has declared \MH/ to be in the public domain,
and the UCI version of \MH/ has passed through four major releases.
The current version, \mh5,
is available from U.C.~Irvine for a nominal distribution fee,
or may be retrieved from the University of Delaware via anonymous FTP.

Much credit must be given to the initial designers and implementors of \MH/:
Bruce Borden, Stockton Gaines, and Norman Shapiro.
Although \MH/ has suffered significant development at UCI
since Rand's initial release,
the fundamental concepts of \MH/'s environs have remained nearly unchanged.
In addition,
the authors of the current release gratefully acknowledge the comments of the
many sites which have run various releases of \MH/ in the past.
In particular,
the dozen or so beta test sites for \mh5
provided tremendous help in stabilizing the current release.

\MH/ runs on different versions of the \unix/ operating system
(such as Berkeley~4.2\bsd/ and various flavors of v7).
In addition,
\MH/ supports four different message transport interfaces:
\SendMail/\cite{EAllm83},
the standard mailer for 4.2\bsd/ systems;
\MMDF/\cite{DCroc79} and \MMDFII/\cite{DKing84},
the Multi-Channel Memo Distribution Facility developed by the University of
Delaware
which forms the software-backbone for CSnet\cite{DCome83} mail relay service;
SMTP,
the ARPA Internet Simple Mail Transfer Protocol\cite{SMTP};
and,
a stand-alone delivery system.

This paper is organized in a straight-forward fashion:
Initially,
the \MH/ philosophy of mail handling is presented,
along with a description of the environment which the \MH/ user is given to
process mail.
Following this,
certain advanced features of \MH/ are discussed in more detail,
such as facilities for selecting messages,
and ``advanced'' concepts in {\it draft} handling.
In addition,
user interface issues in mail handling are addressed,
and the merits of \MH/'s approach is critically examined.
Next,
the \mh5 distribution package is described.
Finally,
we conclude by discussing the authors' experience with \MH/ development
and introducing areas where \MH/ may be further developed.

Although familiarity with \MH/ is not assumed on the part of the reader,
some knowledge of the \unix/ operating system is useful.
Appendix~A gives a short synopsis of the \MH/ commands.

\section{The \MH/ Philosophy}			% mtr
Although \MH/ has many traits which tend to distinguish it from other systems
which handle mail,
there is a single fundamental design decision which influences the interface
between \MH/ and the user:
\MH/ differs from most other systems in that it is composed of many small
programs instead of one very large one.
This architecture gives \MH/ much of its strength,
since intermediate and advanced users are able to take advantage of this
flexibility.

The key to this flexibility is that the \unix/ shell
(usually the {\it C} shell or the {\it Bourne} shell),
is the user's interface to \MH/.
This means that when handling mail,
the entire power of the shell is at the user's disposal,
in addition to the
facilities which \MH/ provides.
Hence,
the user may intersperse mail handling commands with other commands in an
arbitrary fashion,
making use of command handling capabilities which
the user's shell provides.

Furthermore,
rather than storing messages in a complicated data structure
within a monolithic file,
each message in \MH/ is a \unix/ file,
and each folder (an object which holds groups of messages)
in \MH/  is a \unix/ directory.
That is,
the directory- and file-structure of \unix/ is used directly.
As a result,
any \unix/ file-handling command can be applied to any message.

To the novice,
this may not make much sense or may not seem important.
However,
as users of \MH/ become more experienced,
they find this capability attractive.
In addition,
this approach is often quite pleasing to system implementors,
because it minimizes the amount of coding to be performed,
and given a modular design,
changes to the software system can be maintained easily.
There are, however, performance penalties to be paid with this scheme.
This issue is considered later in the paper.

Having described how \MH/ fits into the \unix/ environment,
we now discuss the mail handling environment which is available to the \MH/
user.

\subsection{The \MH/ Environs}			% jlr
In the \file{\$HOME} directory of each \MH/ user, a file named
\profile/ contains static information about the user's \MH/ environment, 
and default arguments for \MH/ programs.
For the latter case,
each line of profile takes the form:
\example program-name:\ options\endexample
Each \MH/ program consults the user's \profile/ for its options.
These options are consulted prior to evaluating any command-line arguments,
and so provide the \MH/ user the capability to customize the defaults for each
command.
Futher, by using the \unix/ link facility,
different names can be given to the same command.
Since each \MH/ command looks
in the \profile/
for a component with the name by which it was invoked,
it's possible to have different defaults for the same program.
For example,
it is not uncommon to link \pgm{prompter}
(a simple prompting editor front-end)
under the name \pgm{rapid} in the
user's \file{bin/} directory, and add to the \profile/:
\example rapid:\ -prepend\ -rapid\endexample
As a result,
when \pgm{prompter} is invoked as \pgm{rapid},
it automatically uses the \switch{prepend} and \switch{rapid} options.

The profile component \eg{Path:} is the path to the user's
\MH/-directory, usually \Mail/.
In addition to containing the user's folders,
the \MH/-directory also contains {\it skeletons} and
{\it templates} used by the \MH/ programs,
and the user's \context/ file.
This latter file has the same format as the user's \profile/,
and contains the dynamic,
context-dependent information about the user's environment.
Whenever \MH/ looks for an \MH/-specific file,
such as a template or skeleton,
it first consults the user's \MH/-directory,
and then a system-wide library area.

The \MH/ user always has a {\it current folder},
which is the folder in which
the user is currently (or was last) working.
Since any \MH/ program which deals with folders implicitly manipulates this
information,
the name of the  current folder is stored in the \file{context}
component \eg{Current-Folder:}.
Every folder has a {\it current message} known as \arg{cur}.
These values are the defaults for \MH/ commands which
accept folder and/or messages arguments.

\MH/ programs make use of a set of envariables
which further customize their behavior.
The \file{\$MH} envariable, if present,
specifies the name of an alternate profile for the user.
This allows a user of \MH/ to
easily maintain multiple mail-handling environments.

In terms of command syntax,
most \MH/ commands accept an optional {\it folder} argument,
such as \arg{+outbox}.
Unlike most \unix/ commands,
all \MH/ commands have switches which are words, rather than single letters.
Switches may be abbreviated to the least unambiguous prefix.
All \MH/ commands also support a \switch{help} switch,
which lists the syntax of the command along with available switches,
and the version number of the command.
Most \MH/ commands also take a \arg{msg} or \arg{msgs} argument
which takes the form of a message number (\eg{1}), a message range (\eg{1-2}),
a standard sequence name (\eg{cur}),
or a user-defined sequence name (\eg{select}).

\tagdiagram{1}{An \MH/ Session}{session}
\subsection{An \MH/ Transcript}			% jlr
Figure~\session\ contains a transcript of a simple \MH/ session.
First, \pgm{inc} is run to incorporate the new mail into the 
user's \eg{+inbox} folder.

A \pgm{scan} listing of the mail is printed while
it is being incorporated.
(The user could run \pgm{scan} explicitly to generate additional \pgm{scan}
listings later on.)
The \pgm{scan} listing gives the message number, followed
by the date, message sender, and subject.
(If the message originated from the user generating the listing,
the \eg{to:} addressee is displayed instead of the sender.)
If the subject is short,
the first part of the message body is displayed after the characters \eg{<<}.
The plus sign (`+') after
the message number indicates the current message.

The user \pgm{show\/}s the message, and decides to \pgm{repl\/}y.
A reply draft
is created using the headers of the message being replied-to,
using the default \file{replcomps} template.
The default editor, \pgm{prompter}, is called to edit the draft.
When an EOT is typed, \pgm{prompter} exits and the
user is left at the \whatnow/ prompt.
The option \pgm{send} is chosen.
Since there were no problems in posting the draft with the message transport
system, 
no additional output is produced.
(\MH/ is not verbose by default.)

The user then decides to compose a new message.
The default skeleton, \file{components}, is copied to the draft,
and \pgm{prompter} is once again called.
After entering the addresses, subject, and body,
the user then \pgm{send\/}s the \file{draft} from the \whatnow/ prompt,
using \eg{send\ -verbose}, which causes
\MH/ to list out the message addresses as it submits them
to the message transport system.

\section{Some \MH/ Features}			% mtr
We now consider certain advanced features in \MH/.
These features have been chosen to demonstrate some useful capabilities
available to the \MH/ user.

\subsection{Message Sequences and Selection}	% jlr
\MH/ has several built-in message sequence names, which may
be used anywhere a \arg{msg} or \arg{msgs} argument is expected.
These are:
\arg{cur}, \arg{next}, \arg{prev}, \arg{first}, \arg{last}, and \arg{all}.
Message ranges may also be specified.
For example, \arg{all} is actually \arg{first-last}, and
\arg{+mh\ last:5} references the last five messages in your
\arg{+mh} folder.
A powerful capability of \MH/ is the ability to use not only the pre-defined
message sequence names,
but also arbitrary user-defined message sequence names.

Although all \MH/ programs recognize user-defined sequences when appropriate, 
the \pgm{pick} and \pgm{mark} commands can create and modify 
user-defined message sequences.
The \pgm{mark} command allows low-level manipulation of sequences,
and is not particularly interesting in our discussion.

The \pgm{pick} command selects certain messages out of a folder.
The criteria used for selection may be a search string and/or a date range.

Searching is performed on either a specific header in the message
(e.g., \eg{To:}),
or anywhere within the message.
By default,
\pgm{pick} lists out the message numbers that matched
the selection criteria.
Thus, \pgm{pick} is useful in backquoted operations to the shell.
For example, to scan all the messages in the current folder from ``frated'',