Document long running processes pitfall in mix cmd, closes #7495

2 files changed, 13 insertions, 1 deletions

diff --git a/lib/elixir/lib/port.ex b/lib/elixir/lib/port.ex
index b035db5a9..06f33aa15 100644
--- a/lib/elixir/lib/port.ex
+++ b/lib/elixir/lib/port.ex
@@ -117,7 +117,7 @@ defmodule Port do
   reimplementing core part of the Runtime System, such as the `:user` and
   `:shell` processes.
 
-  ## Zombie processes
+  ## Zombie OS processes
 
   A port can be closed via the `close/1` function or by sending a `{pid, :close}`
   message. However, if the VM crashes, a long-running program started by the port
diff --git a/lib/mix/lib/mix/tasks/cmd.ex b/lib/mix/lib/mix/tasks/cmd.ex
index f2bbe8b4a..636e80c4e 100644
--- a/lib/mix/lib/mix/tasks/cmd.ex
+++ b/lib/mix/lib/mix/tasks/cmd.ex
@@ -18,6 +18,18 @@ defmodule Mix.Tasks.Cmd do
       mix cmd --app app1 --app app2 echo pwd
 
   Aborts when a command exits with a non-zero status.
+
+  ## Zombie OS processes
+
+  Beware that the Erlang VM does not terminate child processes
+  when it shuts down. Therefore, if you use `mix cmd` to start
+  long running processes and then shutdown the VM, it is likely
+  that those child processes won't be terminated with the VM.
+
+  A solution is to make sure the child processes listen to the
+  stdndard input and terminate when standard input is closed.
+  We discuss this topic at length in the "Zombie OS processes"
+  of the `Port` module documentation.
   """
 
   def run(args) do