# Libtmux_unix.Client.select_layout

- **Module:** Libtmux_unix
- **Package:** libtmux-unix
- **Language:** OCaml
- **Kind:** function
- **Source:** https://github.com/libtmux/libtmux-ocaml/blob/205dfda49742592b479247049d186956e4a4cfe9/unix/libtmux_unix.mli#L684
- **Public owner:** Libtmux_unix
- **Page:** https://libtmux.org/en/ocaml/latest/reference/libtmux_unix-client-select_layout/

```
val select_layout :
  ?cancel:Cancellation.t ->
  t ->
  Libtmux.Window.target -> Libtmux.Layout.t -> (unit, Libtmux.Error.t) result
```

Apply a preset or validated custom layout to the window. Pane geometry follows tmux layout rules; capture again to observe the result.

## Examples

Apply a layout. This complete program creates a private daemon, checks its result and closes its clients. Use OCaml 5.5.2 on Linux with tmux 3.3a through 3.8. Run it from the source checkout after following the installation guide.

[Source example](https://github.com/libtmux/libtmux-ocaml/blob/205dfda49742592b479247049d186956e4a4cfe9/examples/showcase_layout.ml).

```ocaml
open Libtmux
module Client = Libtmux_unix.Client
module Workspace = Libtmux_unix.Workspace
module Config = Libtmux_workspace.Config
module Plan = Libtmux_workspace.Plan
module Execution = Libtmux_workspace.Execution

let get = function
  | Ok value -> value
  | Error error -> failwith (Format.asprintf "%a" Error.pp error)

let set_layout client (initial : Created.t) spawn =
  ignore
    (get
       (Client.split_pane client initial.pane ~direction:Geometry.Right ~spawn));
  ignore
    (get
       (Client.split_pane client initial.pane ~direction:Geometry.Right ~spawn));
  get (Client.select_layout client initial.window Layout.Even_horizontal);
  let window =
    Snapshot.find_window
      (get (Client.capture client))
      (Window.Target.id initial.window)
    |> Option.get
  in
  let panes = Window.panes window in
  assert (Iarray.length panes = 3);
  assert (
    Iarray.for_all
      (fun pane -> Pane.top pane = 0 && Pane.height pane = Window.height window)
      panes);
  assert (
    Iarray.fold_left (fun width pane -> width + Pane.width pane) 2 panes
    = Window.width window);
  print_endline "layout: 3 horizontal panes"

let with_demo binary use =
  let directory = Filename.temp_dir "libtmux-ocaml-showcase-" "" in
  Fun.protect
    ~finally:(fun () ->
      Sys.readdir directory
      |> Array.iter (fun name -> Sys.remove (Filename.concat directory name));
      Unix.rmdir directory)
    (fun () ->
      let socket = Filename.concat directory "server" in
      (* Give this private daemon a lifetime even if startup fails. *)
      let config_file = Filename.concat directory "tmux.conf" in
      Out_channel.with_open_text config_file (fun output ->
          output_string output "set-option -s exit-unattached on\n");
      let wrapper = Filename.concat directory "tmux" in
      Out_channel.with_open_text wrapper (fun output ->
          Printf.fprintf output "#!/bin/sh\nexec %s -f %s \"$@\"\n"
            (Filename.quote binary)
            (Filename.quote config_file));
      Unix.chmod wrapper 0o700;
      let channel = Unix.open_process_args_in binary [| binary; "-V" |] in
      let version =
        Fun.protect
          ~finally:(fun () ->
            if Unix.close_process_in channel <> Unix.WEXITED 0 then
              failwith "tmux version discovery failed")
          (fun () -> String.trim (input_line channel))
      in
      if not (String.starts_with ~prefix:"tmux " version) then
        failwith "invalid tmux version";
      let version = String.sub version 5 (String.length version - 5) in
      let ready = Filename.concat directory "ready.txt" in
      Out_channel.with_open_text ready (fun output ->
          output_string output "showcase ready\n");
      let spawn =
        Spawn.make (Exec { program = "/bin/cat"; args = [ "-u"; ready; "-" ] })
      in
      let config =
        match
          Config.make ~directory ~session:"showcase"
            [ Config.window ~name:"editor" [ Config.pane ~spawn () ] ]
        with
        | Ok value -> value
        | Error error -> failwith (Format.asprintf "%a" Config.pp_error error)
      in
      let plan =
        match
          Plan.make
            ~endpoint:(Endpoint.socket_path socket)
            ~version ~observed:None config
        with
        | Ok value -> value
        | Error error -> failwith (Format.asprintf "%a" Plan.pp_error error)
      in
      let operation = get (Workspace.submit_bootstrap ~binary:wrapper plan) in
      Fun.protect
        ~finally:(fun () -> get (Workspace.close operation))
        (fun () ->
          let receipt = get (Workspace.await operation) in
          if receipt.status <> Execution.Succeeded then
            failwith "workspace setup failed";
          let client = Option.get (Workspace.client operation) in
          let initial = List.hd (Iarray.get receipt.steps 0).created in
          Printf.printf "socket=%s\n%!" socket;
          let pane =
            Snapshot.find_pane
              (get (Client.capture client))
              (Pane.Target.id initial.pane)
            |> Option.get
          in
          ignore
            (get
               (Client.wait_for_text ~timeout:0.5 client pane
                  ~text:"showcase ready"));
          use client initial spawn))

let () =
  let binary = if Array.length Sys.argv = 2 then Sys.argv.(1) else "tmux" in
  with_demo binary set_layout
```

Run the recorded entry point:

```console
$ opam exec -- dune exec examples/showcase_layout.exe -- tmux
```

Expected outcome line, after the temporary socket path:

```text
layout: 3 horizontal panes
```

## Parameters

- `cancel?` (`Cancellation.t`)
- `arg1` (`t`)
- `arg2` (`Libtmux.Window.target`)
- `arg3` (`Libtmux.Layout.t`)

## tmux command

The tmux commands behind `select_layout`.

- [`select-layout`](/en/tmux/latest/manual/select-layout/)

## tmux C API

The C declarations in tmux that implement these commands.

- [`cmd_select_layout_entry`](/en/tmux/latest/reference/c-variable-cmd_select_layout_entry/) (attribute)
- [`cmd_select_layout_exec`](/en/tmux/latest/reference/c-function-cmd-select-layout-c-cmd_select_layout_exec/) (function)

## In other ports

Set a window's pane layout

| Language | Equivalent |
| --- | --- |
| Python | [`libtmux.Window.select_layout`](/en/py/latest/reference/libtmux-window-select_layout/) (method) |
| Ruby | [`LibTmux::Window#select_layout`](/en/ruby/latest/reference/libtmux-window-select_layout/) (method) |
| Lua | [`libtmux.Window:layout`](/en/lua/latest/reference/libtmux-window-layout/) (method) |
| Erlang | [`tmux_window:select_layout/2`](/en/erlang/latest/reference/tmux_window-select_layout-2/) (function) |
| Haskell | No public Haskell equivalent is implemented; the current modules expose scalar values and version parsing. |
| TypeScript | [`window.Window.selectLayout`](/en/ts/latest/reference/window-window-selectlayout/) (method) |
| Rust | [`window.Window.select_layout`](/en/rs/latest/reference/window-window-select_layout/) (method) |
| Go | [`tmux.Window.SelectLayout`](/en/go/latest/reference/tmux-window-selectlayout/) (method) |
| Java | [`io.github.libtmux.Window.selectLayout`](/en/java/latest/reference/io-github-libtmux-window-window-selectlayout/) (method) |
| C# | [`LibTmux.Window.SelectLayoutAsync`](/en/csharp/latest/reference/libtmux-window-selectlayoutasync/) (method) |
| C++ | [`libtmux::Window::select_layout`](/en/cxx/latest/reference/libtmux-window-select_layout/) (method) |
| Swift | [`Server.selectLayout(_:_:)`](/en/swift/latest/reference/server-selectlayout(_-_-)/) (method) |
| Perl | [`LibTmux::Server::select_layout`](/en/perl/latest/reference/libtmux-server-select_layout/) (method). Explicit Server operation. Captured entities remain local observations; use a new snapshot to observe changes. |

No equivalent is recorded in PowerShell, Julia, Elixir, Common Lisp, Clojure, Kotlin, Scala or F#.