Project Layout and Metadata Specification

A typical jbuilder project will have one or more <package>.opam file at toplevel as well as jbuild files wherever interesting things are: libraries, executables, tests, documents to install, etc…

It is recommended to organize your project so that you have exactly one library per directory. You can have several executables in the same directory, as long as they share the same build configuration. If you’d like to have multiple executables with different configurations in the same directory, you will have to make an explicit module list for every executable using modules.

The next sections describe the format of Jbuilder metadata files.

Note that the Jbuilder metadata format is versioned in order to ensure forward compatibility. There is currently only one version available, but to be future proof, you should still specify it in your jbuild files. If no version is specified, the latest one will be used.

Metadata format

Most configuration files read by Jbuilder are using the S-expression syntax, which is very simple. It is described below.

Note that the format is completely static. However you can do meta-programming on jbuilds files by writing them in Diffing and promotion.

Lexical conventions of s-expressions

Whitespace, which consists of space, newline, horizontal tab, and form feed, is ignored unless within an OCaml-string, where it is treated according to OCaml-conventions. The left parenthesis opens a new list, the right one closes it. Lists can be empty.

The double quote denotes the beginning and end of a string using similar lexing conventions to the ones of OCaml (see the OCaml-manual for details). Differences are:

  • octal escape sequences (\o123) are not supported;
  • backslash that’s not a part of any escape sequence is kept as it is instead of resulting in parse error;
  • a backslash followed by a space does not form an escape sequence, so it’s interpreted as is, while it is interpreted as just a space by OCaml.

All characters other than double quotes, left- and right parentheses, whitespace, carriage return, and comment-introducing characters or sequences (see next paragraph) are considered part of a contiguous string.


There are three kinds of comments:

  • line comments are introduced with ;, and end at the newline;
  • sexp comments are introduced with #;, and end at the end of the following s-expression;
  • block comments are introduced with #| and end with |#. These can be nested, and double-quotes within them must be balanced and be lexically correct OCaml strings.

Grammar of s-expressions

S-expressions are either sequences of non-whitespace characters (= atoms), doubly quoted strings or lists. The lists can recursively contain further s-expressions or be empty, and must be balanced, i.e. parentheses must match.


this_is_an_atom_123'&^%!  ; this is a comment
"another atom in an OCaml-string \"string in a string\" \123"

; empty list follows below

; a more complex example
    list in a list  ; comment within a list
    (list in a list in a list)
    42 is the answer to all questions
    #; (this S-expression
         (has been commented out)
    #| Block comments #| can be "nested" |# |#

<package>.opam files

When a <package>.opam file is present, Jbuilder will know that the package named <package> exists. It will know how to construct a <package>.install file in the same directory to handle installation via opam. Jbuilder also defines the recursive install alias, which depends on all the buildable <package>.install files in the workspace. So for instance to build everything that is installable in a workspace, run at the root:

$ jbuilder build @install

Declaring a package this way will allow you to add elements such as libraries, executables, documentation, … to your package by declaring them in jbuild files.

Such elements can only be declared in the scope defined by the corresponding <package>.opam file. Typically, your <package>.opam files should be at the root of your project, since this is where opam pin ... will look for them.

Note that <package> must be non-empty, so in particular .opam files are ignored.


Any directory containing at least one <package>.opam file defines a scope. This scope is the sub-tree starting from this directory, excluding any other scopes rooted in sub-direcotries.

Typically, any given project will define a single scope. Libraries and executables that are not meant to be installed will be visible inside this scope only.

Because scopes are exclusive, if you whish to include the dependencies of the project you are currently working on into your workspace, you may copy them in a vendor directory, or any other name of your choice. Jbuilder will look for them there rather than in the installed world and there will be no overlap between the various scopes.

Package version

Note that Jbuilder will try to determine the version number of packages defined in the workspace. While Jbuilder itself makes no use of version numbers, it can be use by external tools such as ocamlfind.

Jbuilder determines the version of a package by first looking in the <package>.opam for a version variable. If not found, it will try to read the first line of a version file in the same directory as the <package>.opam file. The version file is any file whose name is, in order in which they are looked for:

  • <package>.version
  • version

The version file can be generated by a user rule.

If the version can’t be determined, Jbuilder just won’t assign one.

Note that if you are using Topkg as well in your project, you shouldn’t manually set a version in your <package>.opam file or write/generate on of the file listed above. See the section about Using topkg with jbuilder for more details.

Odig conventions

Jbuilder follows the odig conventions and automatically installs any README*, CHANGE*, HISTORY* and LICENSE* files in the same directory as the <package>.opam file to a location where odig will find them.

Note that this includes files present in the source tree as well as generated files. So for instance a changelog generated by a user rule will be automatically installed as well.


By default Jbuilder traverses the whole source tree, ignoring the following files and directories:

  • any file that start with .#
  • any directory that start with either . or _

To ignore a subtree, simply write a jbuild-ignore file in the parent directory containing the name of the sub-directories to ignore.

So for instance, if you write foo in src/jbuild-ignore, then src/foo won’t be traversed and any jbuild file it contains will be ignored.

jbuild-ignore files contain a list of directory names, one per line.