Deal with relative paths in mix cmd, closes #14787 (#14788)

Author: josevalim

lib/mix/lib/mix/tasks/cmd.ex

@@ -75,9 +75,19 @@ defmodule Mix.Tasks.Cmd do
     end
 
     if apps == [] or Mix.Project.config()[:app] in apps do
+      path = hd(args)
+      rest = tl(args)
+
+      path =
+        if String.contains?(path, ["/", "\\"]) and Path.type(path) != :absolute do
+          Path.expand(path, Keyword.get(opts, :cd, "."))
+        else
+          path
+        end
+
       cmd_opts = Keyword.take(opts, [:cd])
 
-      case Mix.shell().cmd({hd(args), tl(args)}, cmd_opts) do
+      case Mix.shell().cmd({path, rest}, cmd_opts) do
         0 -> :ok
         status -> exit(status)
       end

lib/mix/test/mix/tasks/cmd_test.exs

@@ -14,6 +14,21 @@ defmodule Mix.Tasks.CmdTest do
     assert_received {:mix_shell, :run, ["hello world\n"]}
   end
 
+  @tag :unix
+  test "supports relative paths" do
+    in_tmp("cmd-relative", fn ->
+      File.mkdir_p!("priv")
+      File.write!("priv/world.sh", "#!/bin/sh\necho world")
+      File.chmod!("priv/world.sh", 0o755)
+
+      Mix.Task.run("cmd", ["priv/world.sh"])
+      assert_received {:mix_shell, :run, ["world\n"]}
+
+      Mix.Task.run("cmd", ["--cd", "priv", "./world.sh"])
+      assert_received {:mix_shell, :run, ["world\n"]}
+    end)
+  end
+
   test "runs the command for each app" do
     in_fixture("umbrella_dep/deps/umbrella", fn ->
       Mix.Project.in_project(:umbrella, ".", fn _ ->