Custode is a "file watcher" for Emacs that runs commands in the background, and only pops up when a command fails.

Automatically running build or tests when saving files has been popularized in the last decade or so, but most tools requires you to keep an eye on a shell window (or buffer), in order to make sure you didn't break something.

Custode does it a bit differently:

• It runs commands on file save in Emacs. 99% of the time, this is what one needs, and for the last 1% there's still the excellent entr.
• It doesn't show any output per default, but only pops up the output buffer when the command failed.

While having a build/test window more or less permanently scrolling does gives a nice "Matrix hacker" feel to developing, it is probably more mentally healthy to not have this possible distraction.

Emacs 28, because that's what I'm using, and it haven't been tested on earlier versions (yet).

Custode uses the built in project and compile Emacs packages for most of the heavy lifting.

Example using use-package, elpaca and doom-modeline:

(use-package custode
  :elpaca (:type git :host github :repo "xendk/custode.el")
  :init
  ;; Enable Custode.
  (global-custode-mode)
  :config
  ;; Bind the keymap in the project prefix map. This will make it 
  ;; available on C-x p u. You can choose whatever key fits you.
  (with-eval-after-load "project"
    (define-key project-prefix-map "u" custode-prefix-map))
  ;; Add lighter to mode-line (this is how doom-modeline) suggests
  ;; adding a lighter for a single minor-mode.
  (add-to-list 'global-mode-string (list t custode-lighter)))

If you're using straight, it replacing :elpaca with :straight should suffice. Users of other package/configuration managers should be able to gleen the important bits from the above.

In a buffer visiting a file that is part of a project recognized by project, you'll have the following commands available on the prefix you configured:

• c: Create a new command for the project. The command should be a command that can be run by the shell. New commands always starts out disabled, so adding a command wont make anything happen in itself.
• k: Remove a command.
• e: Edit command. Allows for changing the command but retain state. When run interactively, this will also delete any existing output buffers and trigger the command if it is being watched (can be suppressed with a prefix argument).
• w: Watch a command. This makes custode run this task whenever saving a file in the project. When run interactively, this will also trigger the command (can be suppressed with a prefix argument).
• W: Stop watching command. Stops a command from running automatically again. Interactively, this will also delete the output buffer, unless called with a prefix argument.
• a: Set command arguments. When arguments has been set, they're added to the shell command when run. A use case is temporarily adding arguments to a test command to only run a subset of tests. Note that arguments only persist in the current Emacs session, they're not saved to the configuration file. Arguments can be unset by supplying an empty string.
• l: Loads commands from the configuration file (.custode per default, can be configured with the custode-save-file custom option) in the root of the project. This should seldom be needed as commands are automatically loaded on project discovery.
• s: Saves configuration to the configuration file in the project root.
• p: Select how to position the output buffer when the command has run and failed. Depending on the run command, start or end of buffer might be most appropriate. This is saved with the configuration.

Most tools these days use colors, but Emacs compilation buffers doesn't support colors per default, but since 28.1 it can be configured to do so:

(require 'ansi-color)
(add-hook 'compilation-filter-hook 'ansi-color-compilation-filter)

Or with use-package:

(use-package ansi-color
    :hook (compilation-filter . ansi-color-compilation-filter)) 

Alternatively, there's a package that adds color support and a few other nifty improvements to compilation buffers: https://codeberg.org/ideasman42/emacs-fancy-compilation

If you're using doom-modeline, you can add the following to your configuration in order to show the Custode lighter in the mode-line without showing all minor modes:

(add-to-list 'global-mode-string custode-lighter)