Difference between revisions of "Eshell"
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Revision as of 09:24, 13 March 2014
Work in progress manual: Eshell - the emacs shell
There are several shells for Emacs, but none can match the versatility and integration with Emacs like Eshell. Eshell is a shell written entirely in Emacs-Lisp, and it replicates most of the features and commands from GNU CoreUtils and the Bourne-like shells. So by re-writing common commands like ls and cp in Emacs-Lisp, Eshell will function identically on any environment Emacs itself runs on.
Unlike the other shells in Emacs, Eshell does not inherit from comint-mode, the default mode for interacting with inferior processes in Emacs. But because Eshell is not an inferior process, it does not have to use comint; but while that may seem like a good thing, it does mean that hooks and routines written for comint-mode won’t work with Eshell.
However, almost all the Emacs commands common to comint-mode are reimplemented natively in Eshell — and most share the same keybinds — but there are a few new advances that haven’t been ported over to Eshell, like the spiffy
comint-history-isearch-backward-regexp in Emacs 23.2, bound to M-r.
Eshell works well on any platform Emacs itself runs on, as Eshell interacts with a common middleware (namely the Emacs-Lisp/C source library) and that middleware will in turn communicate with your OS on how to go about copying files and what have you. That middleware support enables Eshell to take advantage of TRAMP as well.
Given Emacs’ UNIX origin, Eshell emulates traditional UNIX shells like bash and the GNU toolchain. This is good news if you are using Windows and cannot be bothered fidgeting with cygwin, or if you require a completely portable Emacs with few or no external dependencies.
And actually, the Windows support in Eshell is a lot better, in many ways, than cygwin’s bash. You do not have the
/cygdrive/c crud to contend with, as Eshell natively supports Windows/MS-DOS drive paths (so
cd D: and
D: both work equally well.)
Despite all the advantages offered by Eshell, there are some points I want to make that seem to confuse some people:
- Eshell is not a terminal emulator. It does not talk to a shell, for it is the shell. Everything it does — from displaying stuff on the screen, to fetching the contents of a directory — it does through Emacs, and Emacs in turn talks to your operating system.
- Because of the way Eshell talks to other processes (asynchronous ones especially) there may be issues with the way it buffers text and how interrupts work.
- Eshell does not support interactive (or “visual” in Eshell parlance) programs, like top, directly; you must tell Eshell to launch them in a separate
- It is not bash or zsh or even csh; do not treat it as such, even though it is heavily inspired by them. To use Eshell effectively you should treat it as if you are using a completely alien shell.
CommandsEshell is capable of invoking almost any elisp function loaded in Emacs. That sort of flexibility is unmatched; there are no shells out there capable of approximating what Eshell can do. In fact, this functionality is heavily used (and encouraged!) by Eshell. If you want to open the file foobar.txt in Emacs you simply invoke
All commands evaluated by Eshell have an evaluation order, which is an ordered list your command must pass through to determine what part of Eshell handles it. If there is nothing on the list that wants to evaluate your command, you will be told your command is invalid.
Assuming you want to execute the command cp, the evaluation order is:
- A full filepath (e.g. /bin/cp) runs cp in /bin
- Look for the command prefix, (default is *), and if it is found then look for the command in the search path.
- Look for a shell-defined alias (alias command)
- Look for cp in the search path, $PATH (or )
- Look for a Lisp function named cp or the elisp function eshell/cp
Eshell has a handful of commands written in Emacs-Lisp that closely emulate a large subset of what the real GNU Coreutils (or your favorite shell) has. Those commands are called "Alias functions."
EShell only implements a subset of the functionality provided by the real commands, but if you pass an unknown argument to Eshell it will defer to the real commandline tool (if it is installed) automatically.
Here’s what Eshell currently re-implements in elisp:
cat, cp, ls, cd, export, dirs, du, echo, env, kill, ln, mkdir, mv, alias, popd, pushd, pwd, rm, rmdir, time, umask.
There is a big emphasis on adhering to the original GNU functionality, so the fact they are emulated is unlikely to cause you any trouble.
Eshell has a cool mechanism where certain commands are intercepted and passed on to Emacs proper. This enables you to invoke a command like man or ls and have Emacs’s built-in man formatter handle it instead. This functionality is especially important for interactive commands (as they will not work properly in Eshell) as Eshell would not be able to call them otherwise.
But where the feature really shines is with complex commands like grep, or diff as Emacs comes with awesome grep and diff tools built in. This feature alone shows the power of Eshell.
The following commands are redirected to Emacs proper:
agrep, diff, egrep, fgrep, glimpse, grep, info, jobs, locate, man, occur, su, sudo, whoami.
The commands su, sudo and whoami are TRAMP aware commands (in Emacs 23.2), so if you are connected to a remote shell they work as expected.
You can use $() to in-line elisp calls and use their output as arguments, in much the same way as you would in bash. The only caveat here is you cannot use the backquote (backtick) to spawn a subshell, but that syntax was never universally supported anyway. It’s also possible (though I would not recommend it, for there are cases where it does not work) to use a standard elisp form like this: (form ...) — so the same as the subshell syntax I explained before, but without the $.
Useful Elisp Commands
Eshell comes with a selection of helper functions that make your day-to-day life just . That, combined with the power to invoke almost any elisp function, means you have incredible flexibility and control over your shell. Some of the commands I’ve listed in the table below were written for Eshell specifically, and the rest are elisp commands I find useful.
I’ve compiled a table of elisp functions (some are made for Eshell; others are not.); then enter the text in that file's own buffer.
|listify ARGS||Parses an argument string into elisp list notation and prints it to the screen. It’s clever enough to handle both MS-DOS/Windows and POSIX-style argument syntax.|
|addpath PATH||Adds the argument, which must be a path, to the $PATH environment variable. If no argument is specified the existing paths are pretty-printed to the screen.|
|unset ENV-VAR||Unsets an existing environment variable|
|find-file FILE||Finds the file FILE and opens it in Emacs. This function is TRAMP aware and will therefore work remotely.|
|dired DIRECTORY||Opens a dired buffer in DIRECTORY.|
|calc-eval EXPR||Runs EXPR through the Emacs calculator.|
|upcase STR/downcase STR||Converts STR to upper- or lowercase.|
|vc-dir DIRECTORY||Reports the status of a version controlled directory (equivalent to the status command in most VCS)|
|ediff-files FILE1 FILE2||Diffs FILE1 and FILE2 using ediff, Emacs’ diff engine.|
Aliasing in Eshell works in much the same way as it does in other mainstream shells, except you can freely mix elisp and Eshell commands. The command alias takes an alias-name and a definition. The definition must be surrounded by single quotes. You can use the usual argument references known from other shells: $1 for the first argument, $2 for the second, …, or $* to use all arguments, or omit them entirely as Eshell will magically append them on to the end of a command if they weren’t referenced in the definition.
alias ff 'find-file $1'
alias d 'dired $1'
To delete an alias, simply leave out the definition argument and it will be removed automagically. To list all the aliases, leave out both arguments.
Eshell will write the alias definitions to eshell-aliases-file, which in turn is governed by the Eshell-directory-name and that put together means your alias file will be put in
~/.Eshell/alias by default. This is done every time you alter an alias.
Another useful thing to know is the auto-correcting aliasing. If you type an invalid command too many times (governed by eshell-bad-command-tolerance, which is 3 by default) Eshell will offer to alias it to its intended command for you. If you don’t like that, you can bump up the aforementioned variable to a large number.
Some commands are too complex to be displayed by Eshell directly, and require special handling. An example would be top, a program that won’t work with a dumb terminal. To support these commands Eshell will run a term session when you invoke a command Eshell considers visual.
To modify the list of visual commands, you can alter eshell-visual-commands.