Add 'find-executable' function.

src/file.toit

@@ -8,6 +8,7 @@ import io
 import system
 import .directory
 import .stream
+import .os as os
 
 export Stream
 
@@ -567,5 +568,51 @@ update-time path/string --access/Time?=null --modification/Time?=null:
 
   update-times_ path atime-s atime-ns mtime-s mtime-ns
 
+/**
+Searches for an executable file with the given $name in the system PATH.
+
+Returns null if no such executable is found.
+Returns a path relative to the part of the PATH where the executable is found.
+  For example, if the PATH contains `..` and the executable is found there,
+  then `../name` (or `..\name`) is returned.
+*/
+find-executable name/string -> string?:
+  bin-paths := ?
+  extensions := ?
+  if system.platform == system.PLATFORM-WINDOWS:
+    path-env := os.env.get "PATH"
+    if not path-env: return null
+    bin-paths = path-env.split ";"
+    pathext-env := os.env.get "PATHEXT"
+    if not pathext-env:
+      extensions = [".exe", ".bat", ".cmd"]
+    else:
+      extensions = pathext-env.split ";"
+  else:
+    path-env := os.env.get "PATH"
+    if not path-env: return null
+    bin-paths = path-env.split ":"
+    extensions = [""]
+
+  bin-paths.do: | bin-path/string |
+    is-windows := system.platform == system.PLATFORM-WINDOWS
+    separator := is-windows ? "\\" : "/"
+    extensions.do: | ext/string |
+      candidate := if bin-path.ends-with "/" or bin-path.ends-with "\\":
+        "$bin-path$name$ext"
+      else:
+        "$bin-path/$separator$name$ext"
+      if is-file candidate:
+        if is-windows:
+          // On Windows, just being a file is enough.
+          return candidate
+        // On other platforms, it must also be executable.
+        stat := stat candidate
+        // We are a bit loose here and accept any executable bit, even if
+        // it is not for the current user.
+        if stat and stat[ST-MODE] & 0b001001001 != 0:
+          return candidate
+  return null
+
 update-times_ path/string atime-s/int atime-ns/int mtime-s/int mtime-ns/int -> none:
   #primitive.file.update-times

tests/find-executable_test.toit

@@ -0,0 +1,30 @@
+// Copyright (C) 2018 Toitware ApS.
+// Use of this source code is governed by a Zero-Clause BSD license that can
+// be found in the tests/TESTS_LICENSE file.
+
+import expect show *
+
+import host.file
+import host.pipe
+import system
+
+main:
+  shell/string := ?
+  shell-args := ?
+  if system.platform == system.PLATFORM-WINDOWS:
+    // 'cmd.exe' should always be present on Windows systems.
+    shell = "cmd"
+    shell-args = ["/c", "echo hello"]
+  else:
+    // 'sh' should always be present on Unix-like systems.
+    shell = "sh"
+    shell-args = ["-c", "echo hello"]
+
+  path := file.find-executable shell
+  expect-not-null path
+
+  // Run the found executable to verify it works.
+  output := pipe.backticks [path] + shell-args
+  expect-equals "hello" output.trim
+
+  expect-null (file.find-executable "this_executable_does_not_exist")