fix: condensed and simplified os check logic and added validate output

common/shell-local/config.go

@@ -175,7 +175,8 @@ func Validate(config *Config) error {
 				}
 			}
 			if supported_os != true {
-				return fmt.Errorf("Invalid OS specified in only_on: '%s'", provided_os)
+				return fmt.Errorf("Invalid OS specified in only_on: '%s'\n"+
+					"Supported OS names: %v", provided_os, supported_syslist)
 			}
 		}
 	}

common/shell-local/run.go

@@ -29,21 +29,19 @@ type EnvVarsTemplate struct {
 
 func Run(ui packer.Ui, config *Config) (bool, error) {
 	// Check if shell-local can even execute against this runtime OS
-	runCommand := false
 	if len(config.OnlyOn) > 0 {
+		runCommand := false
 		for _, os := range config.OnlyOn {
 			if os == runtime.GOOS {
 				runCommand = true
 				break
 			}
 		}
-	} else {
-		runCommand = true
-	}
-	if !runCommand {
-		ui.Say(fmt.Sprintf("Skipping shell-local due to runtime OS"))
-		log.Printf("[INFO] (shell-local): skipping shell-local due to missing runtime OS")
-		return true, nil
+		if !runCommand {
+			ui.Say(fmt.Sprintf("Skipping shell-local due to runtime OS"))
+			log.Printf("[INFO] (shell-local): skipping shell-local due to missing runtime OS")
+			return true, nil
+		}
 	}
 
 	scripts := make([]string, len(config.Scripts))