User Actions

(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 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 to write portable scripts or Configurator for configuration related tasks. You can also use (Experimental) Dune Action Plugin to express program dependencies directly in the source code.

The following constructions are available:

  • (run <prog> <args>) to execute a program. <prog> is resolved locally if it is available in the current workspace, otherwise it is resolved using the PATH

  • (dynamic-run <prog> <args>) to execute a program that was linked against dune-action-plugin library. <prog> is resolved in the same way as in run

  • (chdir <dir> <DSL>) to change the current directory

  • (setenv <var> <value> <DSL>) to set an environment variable

  • (with-<outputs>-to <file> <DSL>) to redirect the output to a file, where <outputs> is one of: stdout, stderr or outputs (for both stdout and stderr)

  • (ignore-<outputs> <DSL>) to ignore the output, where <outputs> is one of: stdout, stderr, or outputs

  • (with-stdin-from <file> <DSL>) to redirect the input from a file

  • (with-accepted-exit-codes <pred> <DSL>) specifies the list of expected exit codes for the programs executed in <DSL>. <pred> is a predicate on integer values, and it’s specified using the Predicate Language. <DSL> can only contain nested occurrences of run, bash, system, chdir, setenv, ignore-<outputs>, with-stdin-from, and with-<outputs>-to. This action is available since Dune 2.0.

  • (progn <DSL>...) to execute several commands in sequence

  • (concurrent <DSL>...)` to execute several commands concurrently and collect all resulting errors, if any. Warning: The concurrency is limited by the -j flag passed to Dune. In particular, if Dune is running with -j 1, these commands will actually run sequentially, which may cause a deadlock if they talk to each other.

  • (echo <string>) to output a string on stdout

  • (write-file <file> <string>) writes <string> to <file>

  • (cat <file> ...) to sequentially print the contents of files to stdout

  • (copy <src> <dst>) to copy a file. If these files are OCaml sources, you should follow the naming convention to preserve Merlin’s functionality.

  • (copy# <src> <dst>) to copy a file and add a line directive at the beginning

  • (system <cmd>) to execute a command using the system shell: sh on Unix and cmd on Windows

  • (bash <cmd>) to execute a command using /bin/bash. This is obviously not very portable.

  • (diff <file1> <file2>) is similar to (run diff <file1> <file2>) but is better and allows promotion. See Diffing and Promotion for more details.

  • (diff? <file1> <file2>) is similar to (diff <file1> <file2>) except that <file2> should be produced by a part of the same action rather than be a dependency, is optional and will be consumed by diff?.

  • (cmp <file1> <file2>) is similar to (run cmp <file1> <file2>) but allows promotion. See Diffing and Promotion for more details.

  • (no-infer <DSL>) to perform an action without inference of dependencies and targets. This is useful if you are generating dependencies in a way that Dune doesn’t know about, for instance by calling an external build system.

  • (pipe-<outputs> <DSL> <DSL> <DSL>...) to execute several actions (at least two) in sequence, filtering the <outputs> of the first command through the other command, piping the standard output of each one into the input of the next. This action is available since Dune 2.7.

As mentioned, copy# inserts a line directive at the beginning of the destination file. More precisely, it inserts the following line:

# 1 "<source file name>"

Most languages recognize such lines and update their current location to report errors in the original file rather than the copy. This is important because the copy exists only under the _build directory, and in order for editors to jump to errors when parsing the build system’s output, errors must point to files that exist in the source tree. In the beta versions of Dune, copy# was called copy-and-add-line-directive. However, most of time, one wants this behavior rather than a bare copy, so it was renamed to something shorter.

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:

(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:

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

Here the command that will be executed is:

ocamllex -o blah.mll

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

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

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

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