Show CPU temperature on MOTD

lib/nerves_motd.ex

@@ -84,6 +84,7 @@ defmodule NervesMOTD do
     [
       [{"Uptime", uptime()}],
       [{"Clock", Utils.formatted_local_time()}],
+      temperature_row(),
       [],
       [firmware_cell(), applications_cell(apps)],
       [memory_usage_cell(), active_application_partition_cell()],
@@ -108,6 +109,19 @@ defmodule NervesMOTD do
     ["  ", format_cell(col0, 0), format_cell(col1, 1), "\n"]
   end
 
+  defp format_row(nil), do: []
+
+  @spec temperature_row() :: [cell()] | nil
+  defp temperature_row() do
+    case runtime_mod().cpu_temperature() do
+      {:ok, temperature_c} ->
+        [{"Temperature", :erlang.float_to_binary(temperature_c, decimals: 1)}]
+
+      _ ->
+        nil
+    end
+  end
+
   @spec format_cell(cell(), 0 | 1) :: IO.ANSI.ansidata()
   defp format_cell({label, value}, column_index) do
     [format_cell_label(label), " : ", format_cell_value(value, column_index, 24), :reset]

lib/nerves_motd/runtime.ex

@@ -1,6 +1,7 @@
 defmodule NervesMOTD.Runtime do
   @moduledoc false
   @callback applications() :: %{started: [atom()], loaded: [atom()]}
+  @callback cpu_temperature() :: {:ok, float()} | :error
   @callback firmware_valid?() :: boolean()
   @callback load_average() :: [String.t()]
   @callback memory_stats() ::
@@ -33,6 +34,21 @@ defmodule NervesMOTD.Runtime.Target do
     %{started: started, loaded: loaded}
   end
 
+  @impl NervesMOTD.Runtime
+  def cpu_temperature() do
+    # Read the file /sys/class/thermal/thermal_zone0/temp. The file content is
+    # an integer in millidegree Celsius, which looks like:
+    #
+    #     39008\n
+
+    with {:ok, content} <- File.read("/sys/class/thermal/thermal_zone0/temp"),
+         {millidegree_c, _} <- Integer.parse(content) do
+      {:ok, millidegree_c / 1000}
+    else
+      _error -> :error
+    end
+  end
+
   @impl NervesMOTD.Runtime
   def firmware_valid?() do
     Nerves.Runtime.firmware_valid?()

test/nerves_motd_test.exs

@@ -88,6 +88,22 @@ defmodule NervesMOTDTest do
     assert capture_motd() =~ ~r/Clock        : \d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2} \w{3}/
   end
 
+  test "Temperature when available" do
+    NervesMOTD.MockRuntime
+    |> Mox.expect(:applications, 1, default_applications_code())
+    |> Mox.expect(:cpu_temperature, 1, fn -> {:ok, 41.234} end)
+
+    assert capture_motd() =~ ~r/Temperature  : 41.2/
+  end
+
+  test "Temperature when unavailable" do
+    NervesMOTD.MockRuntime
+    |> Mox.expect(:applications, 1, default_applications_code())
+    |> Mox.expect(:cpu_temperature, 1, fn -> :error end)
+
+    refute capture_motd() =~ ~r/Temperature/
+  end
+
   test "Firmware when valid" do
     NervesMOTD.MockRuntime
     |> Mox.expect(:applications, 1, default_applications_code())

test/support/runtime.ex

@@ -21,6 +21,9 @@ defmodule NervesMOTD.Runtime.Host do
   @impl NervesMOTD.Runtime
   def applications(), do: %{loaded: @apps, started: @apps}
 
+  @impl NervesMOTD.Runtime
+  def cpu_temperature(), do: {:ok, 41.234}
+
   @impl NervesMOTD.Runtime
   def firmware_valid?(), do: true