don't error when processing autocomplete commands

The shell autocomplete command will use the binary name as the first
argument which does not show up under the list of subcommands.

main.go

@@ -357,7 +357,12 @@ func wrappedMain() int {
 	// where a command isn't found, because it's likely more helpful to
 	// mention what specifically went wrong, rather than just printing out
 	// a big block of usage information.)
-	if cmd := cliRunner.Subcommand(); cmd != "" {
+
+	// Check if this is being run via shell auto-complete, which uses the
+	// binary name as the first argument and won't be listed as a subcommand.
+	autoComplete := os.Getenv("COMP_LINE") != ""
+
+	if cmd := cliRunner.Subcommand(); cmd != "" && !autoComplete {
 		// Due to the design of cli.CLI, this special error message only works
 		// for typos of top-level commands. For a subcommand typo, like
 		// "terraform state posh", cmd would be "state" here and thus would

main_test.go

@@ -253,6 +253,34 @@ func TestMain_cliArgsFromEnvAdvanced(t *testing.T) {
 	}
 }
 
+// verify that we output valid autocomplete results
+func TestMain_autoComplete(t *testing.T) {
+	// Restore original CLI args
+	oldArgs := os.Args
+	defer func() { os.Args = oldArgs }()
+
+	// Set up test command and restore that
+	Commands = make(map[string]cli.CommandFactory)
+	defer func() {
+		Commands = nil
+	}()
+
+	// Set up test command and restore that
+	Commands["foo"] = func() (cli.Command, error) {
+		return &testCommandCLI{}, nil
+	}
+
+	os.Setenv("COMP_LINE", "terraform versio")
+	defer os.Unsetenv("COMP_LINE")
+
+	// Run it!
+	os.Args = []string{"terraform", "terraform", "versio"}
+	exit := wrappedMain()
+	if exit != 0 {
+		t.Fatalf("unexpected exit status %d; want 0", exit)
+	}
+}
+
 type testCommandCLI struct {
 	Args []string
 }