command: Warn instead of error for empty output

When the output subcommand is called with no arguments, and there are no
outputs to show, we previously rendered an error message but returned a
non-error status code. This is confusing.

This commit changes the text UI to use a warning diagnostic, which makes
it clearer that this is a non-error situation. We do not change the exit
code or the text of the warning, so hopefully this is not considered a
breaking change.

command/output.go

@@ -113,13 +113,17 @@ func (c *OutputCommand) Run(args []string) int {
 	}
 
 	if !jsonOutput && (state.Empty() || len(mod.OutputValues) == 0) {
-		c.Ui.Error(
-			"The state file either has no outputs defined, or all the defined\n" +
-				"outputs are empty. Please define an output in your configuration\n" +
-				"with the `output` keyword and run `terraform refresh` for it to\n" +
-				"become available. If you are using interpolation, please verify\n" +
-				"the interpolated value is not empty. You can use the \n" +
-				"`terraform console` command to assist.")
+		diags = diags.Append(tfdiags.Sourceless(
+			tfdiags.Warning,
+			"No outputs found",
+			"The state file either has no outputs defined, or all the defined "+
+				"outputs are empty. Please define an output in your configuration "+
+				"with the `output` keyword and run `terraform refresh` for it to "+
+				"become available. If you are using interpolation, please verify "+
+				"the interpolated value is not empty. You can use the "+
+				"`terraform console` command to assist.",
+		))
+		c.showDiagnostics(diags)
 		return 0
 	}
 

command/output_test.go

@@ -120,7 +120,7 @@ func TestOutput_json(t *testing.T) {
 	}
 }
 
-func TestOutput_emptyOutputsErr(t *testing.T) {
+func TestOutput_emptyOutputs(t *testing.T) {
 	originalState := states.NewState()
 	statePath := testStateFile(t, originalState)
 
@@ -139,6 +139,9 @@ func TestOutput_emptyOutputsErr(t *testing.T) {
 	if code := c.Run(args); code != 0 {
 		t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
 	}
+	if got, want := ui.ErrorWriter.String(), "Warning: No outputs found"; !strings.Contains(got, want) {
+		t.Fatalf("bad output: expected to contain %q, got:\n%s", want, got)
+	}
 }
 
 func TestOutput_jsonEmptyOutputs(t *testing.T) {