The Emacs Shell: Eshell
***********************

   Eshell is a command shell implemented entirely in Emacs Lisp.  It
invokes no external processes beyond those requested by the user.  It
is intended to be a functional replacement for command shells such as
bash, zsh, rc, 4dos; since Emacs itself is capable of handling most of
the tasks accomplished by such tools.

What does Eshell offer you?
===========================

   Despite the sheer fact that running an Emacs shell can be fun, here
are a few of the unique features offered by Eshell:

   * Integration with the Emacs Lisp programming environment

   * A high degree of configurability

   * The ability to have the same shell on every system Emacs has been
     ported to. Since Eshell imposes no external requirements, and
     relies upon only the Lisp functions exposed by Emacs, it is quite
     operating system independent. Several of the common UNIX commands,
     such as ls, mv, rm, ln, etc., have been implemented in Lisp in
     order to provide a more consistent work environment.

   For those who might be using an older version of Eshell, version 2.1
represents an entirely new, module-based architecture. It supports most
of the features offered by modern shells. Here is a brief list of some
of its more visible features:

   * Command argument completion (tcsh, zsh)

   * Input history management (bash)

   * Intelligent output scrolling

   * Psuedo-devices (such as "/dev/clip" for copying to the clipboard)

   * Extended globbing (zsh)

   * Argument and globbing predication (zsh)

   * I/O redirection to buffers, files, symbols, processes, etc.

   * Many niceties otherwise seen only in 4DOS

   * Alias functions, both Lisp and Eshell-syntax

   * Piping, sequenced commands, background jobs, etc...

Eshell is free software
=======================

   Eshell is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 2, or (at your option) any
later version.

   This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
General Public License for more details.

   You should have received a copy of the GNU General Public License
along with Eshell; see the file COPYING.  If not, write to the Free
Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
02111-1307, USA.

How to begin
============

   To start using Eshell, add the following to your .emacs file:

     (load "eshell-auto")

   This will define all of the necessary autoloads.

   Now type `M-x eshell'.  See the INSTALL file for full installation
instructions.

Philosophy
==========

   A shell is a layer which metaphorically surrounds the kernel, or
heart of an operating system.  This kernel can be seen as an engine of
pure functionality, waiting to serve, while the user programs take
advantage of that functionality to accomplish their purpose.

   The shell's role is to make that functionality accessible to the
user in an unformed state.  Very roughly, it associates kernel
functionality with textual commands, allowing the user to interact with
the operating system via linguistic constructs.  Process invocation is
perhaps the most significant form this takes, using the kernel's `fork'
and `exec' functions.

   Other programs also interact with the functionality of the kernel,
but these user applications typically offer a specific range of
functionality, and thus are not classed as "shells" proper.  (What they
lose in quiddity, they gain in rigidity).

   Emacs is also a user application, but it does make the functionality
of the kernel accessible through an interpreted language--namely, Lisp.
For that reason, there is little preventing Emacs from serving the
same role as a modern shell.  It too can manipulate the kernel in an
unpredetermined way to cause system changes.  All it's missing is the
shell-ish linguistic model.

   Enter Eshell.  Eshell translates "shell-like" syntax into Lisp in
order to exercise the kernel in the same manner as typical system
shells.  There is a fundamental difference here, however, although it
may seem subtle at first....

   Shells like csh and Bourne shell were written several decades ago,
in different times, under more restrictive circumstances.  This
confined perspective shows itself in the paradigm used by nearly all
command-line shells since.  They are linear in conception, byte
stream-based, sequential, and confined to movement within a single host
machine.

   Emacs, on the other hand, is more than just a limited translator
that can invoke subprocesses and redirect file handles.  It also
manages character buffers, windowing frames, network connections,
registers, bookmarks, processes, etc.  In other words, it's a very
multi-dimensional environment, within which eshell emulates a highly
linear methodology.

   Taking a moment, let's look at how this could affect the future of a
shell allowed to develop in such a wider field of play:

   * There is no reason why directory movement should be linear, and
     confined to a single file-system.  Emacs, through w3 and ange-ftp,
     has access to the entire Web.  Why not allow a user to cd to
     multiple directories simultaneously, for example?  It might make
     some tasks easier, such as diff'ing files separated by very long
     pathnames.

   * Data sources are available from anywhere Emacs can derive
     information from: not just from files or the output of other
     processes.

   * Multiple shell invocations all share the same environment--even
     the same process list!  It would be possible to have "process
     views", so that one buffer is watching standard output, another
     standard error, and another the result of standard output grep'd
     through a regular expression...

   * It is not necessary to "leave" the shell, losing all input and
     output history, environment variables, directory stack, etc.
     Emacs could save the contents of your eshell environment, and
     restore all of it (or at least as much as possible) each time you
     restart.  This could occur automatically, without requiring
     complex initialization scripts.

   * Typos occur all of the time; many of them are repeats of common
     errors, such as `dri' for `dir'.  Since executing non-existent
     programs is rarely the intention of the user, eshell could prompt
     for the replacement string, and then record that in a database of
     known misspellings. (Note: The typo at the beginning of this
     paragraph wasn't discovered until two months after I wrote the
     text; it was not intentional).

   * Emacs' register and bookmarking facilities can be used for
     remembering where you've been, and what you've seen--to varying
     levels of persistence.  They could perhaps even be tied to
     specific "moments" during eshell execution, which would include
     the environment at that time, as well as other variables.
     Although this would require functionality orthogonal to Emacs' own
     bookmarking facilities, the interface used could be made to
     operate very similarly.

   This presents a brief idea of what the fuller dimensionality of an
Emacs shell could offer.  It's not just the language of a shell that
determines how it's used, but also the Weltanschauung underlying its
design--and which is felt behind even the smallest feature.  I would
hope the freedom provided by using Emacs as a parent environment will
invite rich ideas from others.  It certainly feels as though all I've
done so far is to tie down the horse, so to speak, so that he will run
at a man's pace.

Influences
==========

   The author of Eshell has been a long-time user of the following
shells, all of which contributed to Eshell's design:

   * rc

   * bash

   * zsh

   * sh

   * 4nt

   * csh