new file

src/Which.fs

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+module Bidello.Which
+
+open Sheller
+open Pentole.Path
+
+let which (executable: string) =
+    let possible_shells = [
+        "/bin/sh"
+        "/bin/bash"
+        "/bin/dash"
+        "/bin/fish"
+        "/usr/bin/sh"
+        "/usr/bin/bash"
+        "/usr/bin/dash"
+        "/usr/bin/fish"
+    ]
+
+    let shell = List.tryFind (fun sh -> sh |> Path.of_string |> Result.isOk) possible_shells
+
+    if shell |> Option.isSome then
+        try
+            Builder
+                .UseShell(shell.Value)
+                .UseExecutable("which")
+                .WithArgument(executable)
+                .ExecuteAsync ()
+                |> Async.AwaitTask
+                |> Async.RunSynchronously
+                |> _.StandardOutput
+                |> _.Trim()
+                |> Path.of_string
+                |> Result.bind FileSystem.resolve
+        with exn -> exn.Message |> Error
+    else
+        Error "Can't find the system shell. Check your system $PATH."