Module Eio.Process

Managing child processes.


# @@ fun env ->
  let proc_mgr = Eio.Stdenv.process_mgr env in
  Eio.Process.parse_out proc_mgr Eio.Buf_read.line ["echo"; "hello"]

Status and error types

type exit_status = [
  1. | `Exited of int

    Process exited with the given return code.

  2. | `Signaled of int

    Process was killed by the given signal.

type status = [
  1. | exit_status
  2. | `Stopped of int

    Process was stopped (paused) by the given signal.

val pp_status : [< status ] Fmt.t
type error =
  1. | Executable_not_found of string

    The requested executable does not exist.

  2. | Child_error of exit_status

    The process exited with an error status.

type Exn.err +=
  1. | E of error
val err : error -> exn

err e is Eio.Exn.create (E e)

val pp_args : string list Fmt.t

Formats a list of arguments, quoting any that might cause confusion to the reader.

This is intended for use in error messages and logging.


type 'tag ty = [
  1. | `Process
  2. | `Platform of 'tag
type 'a t = [> [> `Generic ] ty ] as 'a Std.r

A process.

type 'tag mgr_ty = [
  1. | `Process_mgr
  2. | `Platform of 'tag
type 'a mgr = 'a Std.r constraint 'a = [> [> `Generic ] mgr_ty ]

A process manager capable of spawning new processes.


val pid : _ t -> int

pid t is the process ID of t.

val await : _ t -> exit_status

await t waits for process t to exit and then reports the status.

val await_exn : ?is_success:(int -> bool) -> _ t -> unit

Like await except an exception is raised if does not return a successful exit status.

  • parameter is_success

    Used to determine if an exit code is successful. Default is Int.equal 0.

val signal : _ t -> int -> unit

signal t i sends the signal i to process t.

If the process has already exited then this does nothing (it will not signal a different process, even if the PID has been reused).

See Sys for the signal numbers.

val spawn : sw:Switch.t -> [> 'tag mgr_ty ] Std.r -> ?cwd:Fs.dir_ty Path.t -> ?stdin:_ Flow.source -> ?stdout:_ Flow.sink -> ?stderr:_ Flow.sink -> ?env:string array -> ?executable:string -> string list -> 'tag ty Std.r

spawn ~sw mgr args creates a new child process that is connected to the switch sw.

The child process will be sent Sys.sigkill when the switch is released.

If the flows stdin, stdout and stderr are not backed by file descriptors then this also creates pipes and spawns fibers to copy the data as necessary. If you need more control over file descriptors, see Eio_unix.Process.

  • parameter cwd

    The current working directory of the process (default: same as parent process).

  • parameter stdin

    The flow to attach to the process's standard input (default: same as parent process).

  • parameter stdout

    A flow that the process's standard output goes to (default: same as parent process).

  • parameter stderr

    A flow that the process's standard error goes to (default: same as parent process).

  • parameter env

    The environment for the process (default: same as parent process).

  • parameter executable

    The path of the executable to run. If not given then the first item in args is used, searching $PATH for it if necessary.

val run : _ mgr -> ?cwd:_ Path.t -> ?stdin:_ Flow.source -> ?stdout:_ Flow.sink -> ?stderr:_ Flow.sink -> ?is_success:(int -> bool) -> ?env:string array -> ?executable:string -> string list -> unit

run does spawn followed by await_exn, with the advantage that if the process fails then the error message includes the command that failed.

When is_success is provided, it is called with the exit code to determine whether it indicates success or failure. Without is_success, success requires the process to return an exit code of 0.

Note: If spawn needed to create extra fibers to copy stdin, etc, then it also waits for those to finish.

val parse_out : _ mgr -> 'a Buf_read.parser -> ?cwd:_ Path.t -> ?stdin:_ Flow.source -> ?stderr:_ Flow.sink -> ?is_success:(int -> bool) -> ?env:string array -> ?executable:string -> string list -> 'a

parse_out mgr parser args runs args and parses the child's stdout with parser.

It also waits for the process to finish and checks its exit status is zero.

Note that parser must consume the entire output of the process (like Buf_read.parse).

To return all the output as a string, use Buf_read.take_all as the parser.

This is a convenience wrapper around run, and the optional arguments have the same meanings.


pipe ~sw mgr creates a pipe backed by the OS.

The flows can be used by spawn without the need for extra fibers to copy the data. This can be used to connect multiple processes together.

module Pi : sig ... end