Command-Line Interface

This section describes using dune from the shell.

Initializing Components

NOTE: The dune init command is still under development and subject to change.

Dune’s init subcommand provides limited support for generating Dune file stanzas and folder structures to define components. The dune init command can be used to quickly add new projects, libraries, tests, and executables without having to manually create Dune files in a text editor, or it can be composed to programmatically generate parts of a multi-component project.

Initializing a Project

You can run the following command to initialize a new Dune project that uses the base and cmdliner libraries and supports inline tests:

$ dune init proj myproj --libs base,cmdliner --inline-tests --ppx ppx_inline_test

This creates a new directory called myproj, including subdirectories and dune files for library, executable, and test components. Each component’s dune file will also include the declarations required for the given dependencies.

This is the quickest way to get a basic dune project up and building.

Initializing an Executable

To add a new executable to a dune file in the current directory (creating the file if necessary), run

$ dune init exe myexe --libs base,containers,notty --ppx ppx_deriving

This will add the following stanza to the dune file:

 (name main)
 (libraries base containers notty)
  (pps ppx_deriving)))

Initializing a Library

Run the following command to create a new directory src, initialized as a library:

$ dune init lib mylib src --libs core --inline-tests --public

This will ensure the file ./src/dune contains the below stanza (creating the file and directory, if necessary):

 (public_name mylib)
 (name mylib)
 (libraries core)
  (pps ppx_inline_tests)))

Initializing Components in a Specified Directory

All init subcommands take an optional PATH argument, which should be a path to a directory. When supplied, the component will be created in the specified directory. E.g., to initialize a project in the current working directory, run

$ dune init proj my_proj .

To initialize a project in a directory in some nested path, run

$ dune init proj my_proj path/to/my/project

If the specified directory does not already exist, it will be created.

Learning More About the init Commands

Consult the manual page using the `dune init --help command for more details.

Finding the Root

The root of the current workspace is determined by looking up a dune-workspace or dune-project file in the current directory and its parent directories. Dune requires at least one of these two files to operate.

If it isn’t in the current directory, Dune prints out the root when starting:

$ dune runtest
Entering directory '/home/jdimino/code/dune'

This message can be suppressed with the --no-print-directory command line option (as in GNU make).

More precisely, Dune will choose the outermost ancestor directory containing a dune-workspace file, which is used to mark the root of the current workspace. If no dune-workspace file is present, the same strategy applies with dune-project files.

In case of a mix of dune-workspace and dune-project files, workspace files take precedence over project files in the sense that if a dune-workspace file is found, only parent dune-workspace files will be considered when looking for the root; however, if a dune-project file is found both parent dune-workspace and dune-project files will be considered.

A dune-workspace file is also a configuration file. Dune will read it unless the --workspace command line option is used. See dune-workspace for the syntax of this file. The scope of dune-project files is wider than the scope dune-workspace files. For instance, a dune-project file may specify the name of the project which is a universal property of the project, while a dune-workspace file may specify an opam switch name which is valid only on a given machine. For this reason, it is common and recommended to commit dune-project files in repositories, while it is less common to commit dune-workspace files.

Current Directory

If the previous rule doesn’t apply, i.e., no ancestor directory has a file named dune-workspace, then the current directory will be used as root.

Forcing the Root (for Scripts)

You can pass the --root option to dune to select the root explicitly. This option is intended for scripts to disable the automatic lookup.

Note that when using the --root option, targets given on the command line will be interpreted relative to the given root, not relative to the current directory, as this is normally the case.

Interpretation of Targets

This section describes how Dune interprets the targets provided on the command line. When no targets are specified, Dune builds the default alias.


All targets that Dune knows how to build live in the _build directory. Although, some are sometimes copied to the source tree for the need of external tools. These includes <package>.install files when either -p or --promote-install-files is passed on the command line.

As a result, if you want to ask Dune to produce a particular .exe file you would have to type:

$ dune build _build/default/bin/prog.exe

However, for convenience, when a target on the command line doesn’t start with _build, Dune expands it to the corresponding target in all the build contexts that Dune knows how to build. When using --verbose, it prints out the actual set of targets upon starting:

$ dune build bin/prog.exe --verbose
Actual targets:
- _build/default/bin/prog.exe
- _build/4.03.0/bin/prog.exe
- _build/4.04.0/bin/prog.exe

If a target starts with the @ sign, it is interpreted as an alias. See Aliases.

Variables for Artifacts

It’s possible to build specific artifacts by using the corresponding variable on the command line. For example:

dune build '%{cmi:foo}'

See Variables for Artifacts for more information.

Finding External Libraries

When a library isn’t available in the workspace, Dune will search for it in the installed world and expect it to be already compiled.

It looks up external libraries using a specific list of search paths, and each build context has a specific list of search paths.

When running inside an opam environment, Dune will look for installed libraries in $OPAM_SWITCH_PREFIX/lib. This includes both opam build context configured via the dune-workspace file and the default build context when the variable $OPAM_SWITCH_PREFIX is set.

Otherwise, Dune takes the directory where ocamlc was found and appends ../lib` to it. For instance, if ocamlc is found in /usr/bin, Dune looks for installed libraries in /usr/lib.

In addition to the two above rules, Dune always inspects the OCAMLPATH environment variable and uses the paths defined in this variable. OCAMLPATH always has precedence and can have different values in different build contexts. For instance, you can set it manually in a specific build context via the dune-workspace file.

Running Tests

There are two ways to run tests:

  • dune build @runtest

  • dune test (or the more explicit dune runtest)

The two commands are equivalent, and they will run all the tests defined in the current directory and its children directories recursively. You can also run the tests in a specific sub-directory and its children by using:

  • dune build @foo/bar/runtest

  • dune test foo/bar (or dune runtest foo/bar)

Watch Mode

The dune build and dune runtest commands support a -w (or --watch) flag. When it’s passed, Dune will perform the action as usual and then wait for file changes and rebuild (or rerun the tests). This feature requires inotifywait or fswatch to be installed.

Launching the Toplevel (REPL)

Dune supports launching a utop instance with locally defined libraries loaded.

$ dune utop <dir> -- <args>

Where <dir> is a directory under which Dune searches (recursively) for all libraries that will be loaded. <args> will be passed as arguments to the utop command itself. For example, dune utop lib -- -implicit-bindings will start utop, with the libraries defined in lib and implicit bindings for toplevel expressions.

Dune also supports loading individual modules unsealed by their signatures into the toplevel. This is accomplished by launching a toplevel and then asking dune to return the toplevel directives needed to evaluate the module:

$ utop
# use_output "dune ocaml top-module path/to/";;

Requirements & Limitations

  • Utop version >= 2.0 is required for this to work.

  • This subcommand only supports loading libraries. Executables aren’t supported.

  • Libraries that are dependencies of utop itself cannot be loaded. For example Camomile.

  • Loading libraries that are defined in different directories into one utop instance isn’t possible.

Restricting the Set of Packages

Restrict the set of packages from your workspace that Dune can see with the --only-packages option:

$ dune build --only-packages pkg1,pkg2,... @install

This option acts as if you went through all the Dune files and commented out the stanzas referring to a package that isn’t in the list given to dune.

Distributing Projects

Dune provides support for building and installing your project; however, it doesn’t provide helpers for distributing it. It’s recommended to use dune-release for this purpose.

The common defaults are that your projects include the following files:




If your project contains several packages, all the package names must be prefixed by the shortest one.

dune subst

One of the features dune-release provides is watermarking; it replaces various strings of the form %%ID%% in all your project files before creating a release tarball or when the opam user pins the package.

This is especially interesting for the VERSION watermark, which gets replaced by the version obtained from the Version-Control System (VCS). For instance, if you’re using Git, dune-release invokes this command to find out the version:

$ git describe --always --dirty --abbrev=7

If no VCS is detected, dune subst will do nothing.

Projects using Dune usually only need dune-release for creating and publishing releases. However, they may still substitute the watermarks when the user pins the package. To help with this, Dune provides the subst sub-command.

dune subst performs the same substitution that dune-release does with the default configuration, i.e., calling dune subst at the root of your project will rewrite all your project files.

More precisely, it replaces the following watermarks in the source files:

  • NAME, the name of the project

  • VERSION, output of git describe --always --dirty --abbrev=7

  • VERSION_NUM, same as VERSION but with a potential leading v or V dropped

  • VCS_COMMIT_ID, commit hash from the vcs

  • PKG_MAINTAINER, contents of the maintainer field from the opam file

  • PKG_AUTHORS, contents of the authors field from the opam file

  • PKG_HOMEPAGE, contents of the homepage field from the opam file

  • PKG_ISSUES, contents of the issues field from the opam file

  • PKG_DOC, contents of the doc field from the opam file

  • PKG_LICENSE, contents of the license field from the opam file

  • PKG_REPO, contents of the repo field from the opam file

The project name is obtained by reading the dune-project file in the directory where dune subst is called. The dune-project file must exist and contain a valid (name ...) field.

Note that dune subst is meant to be called from the opam file and behaves a bit different to other Dune commands. In particular it doesn’t try to detect the root of the workspace and must be called from the root of the project.

Custom Build Directory

By default Dune places all build artifacts in the _build directory relative to the user’s workspace. However, one can customize this directory by using the --build-dir flag or the DUNE_BUILD_DIR environment variable.

$ dune build --build-dir _build-foo

# this is equivalent to:
$ DUNE_BUILD_DIR=_build-foo dune build

# Absolute paths are also allowed
$ dune build --build-dir /tmp/build foo.exe

Installing a Package

Via opam

When releasing a package using Dune in opam, there’s nothing special to do. Dune generates a file called <package-name>.install at the root of the project. This contains a list of files to install, and opam reads it in order to perform the installation.


When not using opam, or when you want to manually install a package, you can ask Dune to perform the installation via the install command:

$ dune install [PACKAGE]...

This command takes a list of package names to install. If no packages are specified, Dune will install all available packages in the workspace. When several build contexts are specified via a dune-workspace file, Dune performs the installation in all the build contexts.

Destination Directory

For a given build context, the installation directories are determined with a single scheme for all installation sections. Taking the lib installation section as an example, the priorities of this scheme are as follows:

  1. if an explicit --lib <path> argument is passed, use this path

  2. if an explicit --prefix <path> argument is passed, use <path>/lib

  3. if --lib <path> argument is passed before during dune compilation to ./configure, use this paths

  4. if OPAM_SWITCH_PREFIX is present in the environment use $OPAM_SWITCH_PREFIX/lib

  5. otherwise, fail

Relocation Mode

The installation can be done in specific mode (--relocation) for creating a directory that can be moved. In that case, the installed executables will look up the package sites (cf How to Load Additional Files at Runtime) relative to its location. The –prefix directory should be used to specify the destination.

If you’re using plugins that depend on installed libraries and aren’t executable dependencies, like libraries that need to be loaded at runtime, you must copy the libraries manually to the destination directory.

Querying Merlin Configuration

Since Version 2.8, Dune no longer promotes .merlin files to the source directories. Instead, Dune stores these configurations in the _build folder, and Merlin communicates directly with Dune to obtain its configuration via the ocaml-merlin subcommand. The Merlin configuration is now stanza-specific, allowing finer control. The following commands aren’t needed for normal Dune and Merlin use, but they can provide insightful information when debugging or configuring non-standard projects.

Printing the Configuration

It’s possible to manually query the generated configuration for debugging purposes:

$ dune ocaml merlin dump-config

This command prints the distinct configuration of each module present in the current directory. This directory must be in a Dune workspace and the project must be already built. The configuration will be encoded as s-expressions, which are used to communicate with Merlin.

Printing an Approximated .merlin

It’s also possible to print the current folder’s configuration in the Merlin configuration syntax by running the following command:

$ dune ocaml dump-dot-merlin > .merlin

In that case, Dune prints only one configuration: the result of the configuration’s coarse merge in the current folder’s various modules. This folder must be in a Dune workspace, and the project must be already built. Preprocessing directives and other flags will be commented out and must be un-commented afterward. This feature doesn’t aim at writing exact or correct .merlin files. Its sole purpose is to lessen the burden of writing the configuration from scratch.

Non-Standard Filenames

Merlin configuration loading is based on filenames, so if you have files that are preprocessed by custom rules before they are built, they should respect the following naming convention: the unprocessed file should start with the name of the resulting processed file followed by a dot. The rest does not matter. Dune uses only the name before the first dot to match with available configurations.

For example, if you use the cppo preprocessor to generate the file, then the source file could be named

Running a Coq Toplevel

See Running a Coq Toplevel.