Actions
=======

.. highlight:: dune

``(action ...)`` fields describe user actions.

User actions are always run from the same subdirectory of the current build
context as the ``dune`` file they are defined in, so for instance, an action defined
in ``src/foo/dune`` will be run from ``$build/<context>/src/foo``.

The argument of ``(action ...)`` fields is a small DSL that's interpreted by
Dune directly and doesn't require an external shell. All atoms in the DSL
support :doc:`/concepts/variables`. Moreover, you don't need to specify
dependencies explicitly for the special ``%{<kind>:...}`` forms; these are
recognized and automatically handled by Dune.

The DSL is currently quite limited, so if you want to do something complicated,
it's recommended to write a small OCaml program and use the DSL to invoke it.
You can use `shexp <https://github.com/janestreet/shexp>`__ to write portable
scripts or :ref:`configurator` for configuration related tasks. You can also
use :ref:`dune-action-plugin` to express program dependencies directly in the
source code.

The following constructions are available:

.. grid:: 1 1 2 2

  .. grid-item::
  
    .. toctree::
       :caption: Running commands
    
       run
       system
       bash
       dynamic-run
       chdir
       setenv
       with-accepted-exit-codes

  .. grid-item::

    .. toctree::
       :caption: Input and output
    
       echo
       with-outputs-to
       with-stdin-from
       ignore-outputs
       cat
       copy
       copy#
       write-file
       pipe-outputs

  .. grid-item::

    .. toctree::
       :caption: Comparing files
    
       diff
       diffq
       cmp

  .. grid-item::

    .. toctree::
       :caption: Control structures
    
       progn
       concurrent
       no-infer

Note: expansion of the special ``%{<kind>:...}`` is done relative to the current
working directory of the DSL being executed. So for instance, if you
have this action in a ``src/foo/dune``:

.. code:: dune

    (action (chdir ../../.. (echo %{dep:dune})))

Then ``%{dep:dune}`` will expand to ``src/foo/dune``. When you run various
tools, they often use the filename given on the command line in error messages.
As a result, if you execute the command from the original directory, it will
only see the basename.

To understand why this is important, let's consider this ``dune`` file living in
``src/foo``::

  (rule
   (target blah.ml)
   (deps blah.mll)
   (action
    (run ocamllex -o %{target} %{deps})))

Here the command that will be executed is:

.. code:: console

  $ ocamllex -o blah.ml blah.mll

And it will be executed in ``_build/<context>/src/foo``. As a result, if there
is an error in the generated ``blah.ml`` file, it will be reported as:

::

    File "blah.ml", line 42, characters 5-10:
    Error: ...

Which can be a problem, as your editor might think that ``blah.ml`` is at the root
of your project. Instead, this is a better way to write it::

  (rule
   (target blah.ml)
   (deps blah.mll)
   (action
    (chdir %{workspace_root}
     (run ocamllex -o %{target} %{deps}))))
