Command-line interface

This section describe usage of 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. dune init can be used to quickly add new projects, libraries, tests, or executables without having to manually create dune files, or it can be composed to programmatically generate parts of a multi-component project.

Initializing a project

To initialize a new dune project that uses the base and cmdliner libraries and supports inline tests, you can run

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

This will create a new directory called myproj including sub directories 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

To create a new directory src, initialized as a library, you can run:

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

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

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

Consult the manual page dune init --help 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 parent directories.

dune prints out the root when starting if it is not the current directory:

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

More precisely, it will choose the outermost ancestor directory containing a dune-workspace file as root. For instance if you are in /home/me/code/myproject/src, then dune will look for all these files in order:

  • /dune-workspace
  • /home/dune-workspace
  • /home/me/dune-workspace
  • /home/me/code/dune-workspace
  • /home/me/code/myproject/dune-workspace
  • /home/me/code/myproject/src/dune-workspace

The first entry to match in this list will determine the root. In practice this means that if you nest your workspaces, dune will always use the outermost one.

In addition to determining the root, dune will read this file to setup the configuration of the workspace unless the --workspace command line option is used. See the section dune-workspace for the syntax of this file.

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

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 given on the command line. When no targets are specified, dune builds the default alias, see Default alias for more details.


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 will expand it to the corresponding target in all the build contexts where it knows how to build it. When using --verbose, It prints out the actual set of targets when 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


Targets starting with a @ are interpreted as aliases. For instance @src/runtest means the alias runtest in all descendant of src in all build contexts where it is defined. If you want to refer to a target starting with a @, simply write: ./@foo.

To build and run the tests for a particular build context, use @_build/default/runtest instead.

So for instance:

  • dune build @_build/foo/runtest will run the tests only for the foo build context
  • dune build @runtest will run the tests for all build contexts

You can also build an alias non-recursively by using @@ instead of @. For instance to run tests only from the current directory:

dune build @@runtest

Note that it’s currently not possible to build a target directly if that target lives in a directory that starts with the @ character. In the rare cases where you need to do that, you can declare an alias like so:

 (name foo)
 (deps @foo/some.exe))

@foo/some.exe can then be built with:

dune build @foo

Default alias

When no targets are given to dune build, it builds the special default alias. Effectively dune build is equivalent to:

dune build @@default

When a directory doesn’t explicitly define what the default alias means via an alias stanza, the following implicit definition is assumed:

 (name default)
 (deps (alias_rec all)))

Which means that by default dune build will build everything that is installable.

When using a directory as a target, it will be interpreted as building the default target in the directory. The directory must exist in the source tree.

dune build dir

Is equivalent to:

dune build @@dir/default

Built-in Aliases

There’s a few aliases that dune automatically creates for the user

  • default - this alias includes all the targets that dune will build if a target isn’t specified, i.e. $ dune build. By default, this is set to the all alias. Note that for dune 1.x, this was set to the install alias.
  • runtest - this is the alias to run all the tests, building them if necessary.
  • install - build all public artifacts - those that will be installed.
  • doc - build documentation for public libraries.
  • doc-private - build documentation for all libraries - public & private.
  • lint - run linting tools.
  • all - build all available targets in a directory and installable artifacts defined in that directory.
  • check - This alias will build the minimal set of targets required for tooling support. Essentially, this is .cmi, .cmt, .cmti, and .merlin files.

Variables for artifacts

It is possible to build specific artifacts by using the corresponding variable on the command line, e.g.:

dune build '%{cmi:foo}'

See Variables for artifacts for more information.

Finding external libraries

When a library is not available in the workspace, dune will look it up in the installed world, and expect it to be already compiled.

It looks up external libraries using a specific list of search paths. A list of search paths is specific to a given build context and is determined as follows:

  1. if the ocamlfind is present in the PATH of the context, use each line in the output of ocamlfind printconf path as a search path
  2. otherwise, if opam is present in the PATH, use the output of opam config var lib
  3. otherwise, take the directory where ocamlc was found, and append ../lib to it. For instance if ocamlc is found in /usr/bin, use /usr/lib

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. They will run all the tests defined in the current directory and its children 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 is 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 will search (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.

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

You can 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 is not 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 is recommended to use dune-release for this purpose.

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


And that if your project contains several packages, then 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 files of your project before creating a release tarball or when the package is pinned by the user using opam.

This is especially interesting for the VERSION watermark, which gets replaced by the version obtained from the vcs. For instance if you are using git, dune-release invokes this command to find out the version:

$ git describe --always --dirty

Projects using dune usually only need dune-release for creating and publishing releases. However they might still want to substitute the watermarks when the package is pinned by the user. To help with this, dune provides the subst sub-command.

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

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

  • NAME, the name of the project
  • VERSION, output of git describe --always --dirty
  • 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 name of the project 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 in particular 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 is 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 the packages available in the workspace. When several build contexts are specified via a dune-workspace file, the installation will be performed in all the build contexts.

Destination directory

The <prefix> directory is determined as follows for a given build context:

  1. if an explicit --prefix <path> argument is passed, use this path
  2. if opam is present in the PATH and is configured, use the output of opam config var prefix
  3. otherwise, take the parent of the directory where ocamlc was found.

As an exception to this rule, library files might be copied to a different location. The reason for this is that they often need to be copied to a particular location for the various build system used in OCaml projects to find them and this location might be different from <prefix>/lib on some systems.

Historically, the location where to store OCaml library files was configured through findlib and the ocamlfind command line tool was used to both install these files and locate them. Many Linux distributions or other packaging systems are using this mechanism to setup where OCaml library files should be copied.

As a result, if none of --libdir and --prefix is passed to dune install and ocamlfind is present in the PATH, then library files will be copied to the directory reported by ocamlfind printconf destdir. This ensures that dune install can be used without opam. When using opam, ocamlfind is configured to point to the opam directory, so this rule makes no difference.

Note that --prefix and --libdir are only supported if a single build context is in use.

Relocation Mode

The installation can be done in specific mode (--relocation) for creating a directory that can be moved around. In that case the executables installed will lookup the sites (cf How to load additional files at runtime) of the packages relatively to its location. The –prefix directory should be used to specify the destination.

If you are using plugins that depends on installed libraries which are not dependencies of the executables – so libraries that need to be loaded at runtime – you must copy the libraries manually to the destination directory.