With-Editor User Manual

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With-Editor User Manual

The library with-editor makes it easy to use the Emacsclient as the $EDITOR of child processes, making sure they know how to call home. For remote processes a substitute is provided, which communicates with Emacs on standard output instead of using a socket as the Emacsclient does.

This library was written because Magit has to be able to do the above to allow the user to edit commit messages gracefully and to edit rebase sequences, which wouldn’t be possible at all otherwise.

Because other packages can benefit from such functionality, this library is made available as a separate package. It also defines some additional functionality which makes it useful even for end-users, who don’t use Magit or another package which uses it internally.

Copyright (C) 2015-2016 Jonas Bernoulli <jonas@bernoul.li>

You can redistribute this document and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This document 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.


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1 Using the With-Editor package

The With-Editor package is used internally by Magit when editing commit messages and rebase sequences. It also provides some commands and features which are useful by themselves, even if you don’t use Magit.

For information about using this library in you own package, see Using With-Editor as a library.


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1.1 Configuring With-Editor

With-Editor tries very hard to locate a suitable emacsclient executable, so ideally you should never have to customize the option with-editor-emacsclient-executable. When it fails to do so, then the most likely reason is that someone found yet another way to package Emacs (most likely on OS X) without putting the executable on $PATH, and we have to add another kludge to find it anyway.

User Option: with-editor-emacsclient-executable

The emacsclient executable used as the editor by child process of this Emacs instance. By using this executable, child processes can call home to their parent process.

This option is automatically set at startup by looking in exec-path, and other places where the executable could be installed, to find the emacsclient executable most suitable for the current emacs instance.

You should not customize this option permanently. If you have to do it, then you should consider that a temporary kludge and inform the Magit maintainer as described in Debugging.

If With-Editor fails to find a suitable emacsclient on you system, then this should be fixed for all users at once, by teaching with-editor-locate-emacsclient how to so on your system and system like yours. Doing it this way has the advantage, that you won’t have do it again every time you update Emacs, and that other users who have installed Emacs the same way as you have, won’t have to go through the same trouble.

Note that there also is a nuclear option; setting this variable to nil causes the "sleeping editor" described below to be used even for local child processes. Obviously we don’t recommend that you use this except in "emergencies", i.e. before we had a change to add a kludge appropriate for you setup.

Function: with-editor-locate-emacsclient

The function used to set the initial value of the option with-editor-emacsclient-executable. There’s a lot of voodoo here.

The emacsclient cannot be used when using Tramp to run a process on a remote machine. (Theoretically it could, but that would be hard to setup, very fragile, and rather insecure).

With-Editor provides an alternative "editor" which can be used by remote processes in much the same way as local processes use an emacsclient executable. This alternative is known as the "sleeping editor" because it is implemented as a shell script which sleeps until it receives a signal.

User Option: with-editor-sleeping-editor

The sleeping editor is a shell script used as the editor of child processes when the emacsclient executable cannot be used.

This fallback is used for asynchronous process started inside the macro with-editor, when the process runs on a remote machine or for local processes when with-editor-emacsclient-executable is nil.

Where the latter uses a socket to communicate with Emacs’ server, this substitute prints edit requests to its standard output on which a process filter listens for such requests. As such it is not a complete substitute for a proper Emacsclient, it can only be used as $EDITOR of child process of the current Emacs instance.

Some shells do not execute traps immediately when waiting for a child process, but by default we do use such a blocking child process.

If you use such a shell (e.g. csh on FreeBSD, but not Debian), then you have to edit this option. You can either replace sh with bash (and install that), or you can use the older, less performant implementation:

"sh -c '\
echo \"WITH-EDITOR: $$ OPEN $0\"; \
trap \"exit 0\" USR1; \
trap \"exit 1\" USR2; \
while true; do sleep 1; done'"

This leads to a delay of up to a second. The delay can be shortened by replacing sleep 1 with sleep 0.01, or if your implementation does not support floats, then by using nanosleep 0.01 instead.


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1.2 Using With-Editor commands

This section describes how to use the with-editor library outside of Magit. You don’t need to know any of this just to create commits using Magit.

The commands with-editor-async-shell-command and with-editor-shell-command are intended as drop in replacements for async-shell-command and shell-command. They automatically export $EDITOR making sure the executed command uses the current Emacs instance as "the editor". With a prefix argument these commands prompt for an alternative environment variable such as $GIT_EDITOR.

Command: with-editor-async-shell-command

Like async-shell-command, but the command is run with the current Emacs instance exported as $EDITOR.

Command: with-editor-shell-command

Like async-shell-command, but the command is run with the current Emacs instance exported as $EDITOR. This only has an effect if the command is run asynchronously, i.e. when the command ends with &.

To always use these variants add this to you init file:

(define-key (current-global-map)
  [remap async-shell-command] 'with-editor-async-shell-command)
(define-key (current-global-map)
  [remap shell-command] 'with-editor-shell-command)

Alternatively use the global shell-command-with-editor-mode.

Variable: shell-command-with-editor-mode

When this mode is active, then $EDITOR is exported whenever ultimately shell-command is called to asynchronously run some shell command. This affects most variants of that command, whether they are defined in Emacs or in some third-party package.

The command with-editor-export-editor exports $EDITOR or another such environment variable in shell-mode, term-mode and eshell-mode buffers. Use this Emacs command before executing a shell command which needs the editor set, or always arrange for the current Emacs instance to be used as editor by adding it to the appropriate mode hooks:

(add-hook 'shell-mode-hook  'with-editor-export-editor)
(add-hook 'term-exec-hook   'with-editor-export-editor)
(add-hook 'eshell-mode-hook 'with-editor-export-editor)

Some variants of this function exist; these two forms are equivalent:

(add-hook 'shell-mode-hook
	  (apply-partially 'with-editor-export-editor "GIT_EDITOR"))
(add-hook 'shell-mode-hook 'with-editor-export-git-editor)
Command: with-editor-export-editor

When invoked in a shell-mode, term-mode, or eshell-mode buffer, this command teaches shell commands to use the current Emacs instance as the editor, by exporting $EDITOR.

Command: with-editor-export-git-editor

Like with-editor-export-editor but exports $GIT_EDITOR.

Command: with-editor-export-hg-editor

Like with-editor-export-editor but exports $HG_EDITOR.


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2 Using With-Editor as a library

This section describes how to use the with-editor library outside of Magit to teach another package how to have its child processes call home, just like Magit does. You don’t need to know any of this just to create commits using Magit. You can also ignore this if you use with-editor outside of Magit, but only as an end-user.

For information about interactive use and options which affect both interactive and non-interactive use, see Using the With-Editor package.

Macro: with-editor &rest body

This macro arranges for the emacsclient or the sleeping editor to be used as the editor of child processes, effectively teaching them to call home to the current emacs instance when they require that the user edits a file.

This is essentially done by establishing a local binding for process-environment and changing the value of the $EDITOR environment variable. This affects all processes started by forms inside BODY.

Function: with-editor-set-process-filter process filter

This function is like set-process-filter but ensures that adding the new FILTER does not remove the with-editor-process-filter. This is done by wrapping the two filter functions using a lambda, which becomes the actual filter. It calls with-editor-process-filter first, passing t as NO-STANDARD-FILTER. Then it calls FILTER.


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3 Debugging

With-Editor tries very hard to locate a suitable emacsclient executable, and then sets option with-editor-emacsclient-executable accordingly. In very rare cases this fails. When it does fail, then the most likely reason is that someone found yet another way to package Emacs (most likely on OS X) without putting the executable on $PATH, and we have to add another kludge to find it anyway.

If you are having problems using with-editor, e.g. you cannot commit in Magit, then please open a new issue at https://github.com/magit/with-editor/issues and provide information about your Emacs installation. Most importantly how did you install Emacs and what is the output of M-x with-editor-debug?