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Genspio: Generate Shell Phrases In OCaml

Genspio is a typed EDSL to generate shell scripts and commands from OCaml.

The idea is to build values of type 'a EDSL.t with the combinators in the Genspio.EDSL module, and compile them to POSIX shell scripts (or one-liners) with functions from Genspio.Compile. See the file src/examples/ which generates a useful list of usage examples, or the section “Getting Started” below.

The tests run the output of the compiler against a few shells that it tries to find on the host (e.g. dash, bash, busybox, mksh, zsh … cf. the example test results summary below).

If you have any questions, do not hesitate to submit an issue.

Genspio's documentation root is at


You can install the library though opam:

opam install genspio

Or get the development version with opam pin:

opam pin add genspio

You can also build locally:

You need OCaml ≥ 4.03.0 together with base, fmt, and dune:

dune build @install

Getting Started

Here is a quick example:

utop> open Genspio.EDSL;;

let c =
  let username_one_way : str t =
    (* We lift the string "USER" to EDSL-land and use function `getenv`: *)
    getenv (str "USER") in
  let username_the_other_way : str t =
    (* The shell-pipe operator is `||>` *)
    (exec ["whoami"] ||> exec ["tr"; "-d"; "\\n"])
    (* `get_stdout` takes `stdout` from a `unit t` as a `byte_array t` *)
    |> get_stdout
  let my_printf : string -> str t list -> unit t = fun fmt args ->
    (* The function `call` is like `exec` but operates on `str t` values
       instead of just OCaml strings: *)
    call (str "printf" :: str fmt :: args) in
  (* The operator `=$=` is `str t` equality, it returns a `bool t` that
     we can use with `if_seq`: *)
  if_seq Str.(username_one_way =$= username_the_other_way)
        my_printf "Username matches: `%s`\\n" [username_one_way];
        my_printf "Usernames do not match: `%s` Vs `%s`\\n"
          [username_one_way; username_the_other_way];
val c : unit t

utop> Sys.command (Genspio.Compile.to_one_liner c);;
Username matches: `smondet`
- : int = 0

Important Modules

  • Genspio.EDSL provides the Embedded Domain Specific Language API to build shell script expressions (there is also a lower-level, not recommended, Genspio.EDSL_v0 API).
  • Genspio.Compile has the 3 “compilers” provided by the library:
    • The pretty printer outputs 'a EDSL.t values as expressions of a lisp-like pseudo-language.
    • The default “To_posix” compiler generates POSIX-compliant shell scripts (with the option of avoiding new-lines).
      ⤷ Note that MacOSX's default bash version is buggy and has been witnessed to choke on generated POSIX-valid scripts.
    • The newer “To_slow_flow” compiler generates POSIX shell scripts which are much simpler, hence more portable across shell implementations, but use (a lot of) temporary files and are generally slower.
  • Genspio.Transform implements code transformations:
    • The module Visitor provides an extensible AST visitor.
    • The module Constant_propagation does some basic constant propagation (using the visitor).

More Examples

  • There are many examples in src/examples/ which are used to generate the usage examples documentation webpage.
  • The file src/examples/ is the code generator for the “COSC” project (Github: smondet/cosc), a family of scripts which manage long-running processes in a GNU-Screen session.
  • The file src/examples/ contains another big example: a script that downloads and unpacks archives from URLs.
  • The file src/examples/ is a “Makefile + scripts” generator to setup Qemu virtual machines, they can be for instance used to run the tests on more exotic platforms.
  • The project hammerlab/secotrec is a real-world, larger-scale use of Genspio (uses Genspio version 0.0.0).

Additional Documentation

From here, one can explore:

  • Some implementation notes.
  • More information on testing, e.g. on more exotic operating systems.
  • The module Genspio.EDSL_v0 is an older version of the API, which can still be useful as it is lower-level: it gives full access to the two “string-like” types, byte-arrays and C-strings while of course becoming more cumbersome to use.


To run the tests you also need make and there is an additional dependency on the uri library, see:

dune build $genspio_test
$genspio_test --help

Try this:

$genspio_test --important-shells bash,dash /tmp/gtests/
cd /tmp/gtests/
make run-all # Attempts to run all the tests on all the shells
make check   # Checks that all the tests for the important ones succeeded

You can generate a markdown report with make report and check

Some failures are expected with not-really-POSIX or buggy shells like KSH93, or on some corner cases cf. #35.

You can check failures in the <shell-test>/ files, see for instance ksh-StdML/ for the failures of the “KSH with standard Genspio compilation to multi-line scripts” (similarly there are <shell-test>/ files).

Building The Documentation

To build the documentation one needs pandoc and caml2html:

sh tools/

The build of the whole website, including the web-based demo, happens in a different repository:


It's Apache 2.0.

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