src/Pure/System/linux.scala

   def user_home(name: String): String = user_entry(name, 6)
 
   def user_add(name: String,
     description: String = "",
     system: Boolean = false,
-    ssh_setup: Boolean = false)
+    ssh_setup: Boolean = false): Unit =
   {
     require(!description.contains(','), "malformed description")
 
     if (user_exists(name)) error("User already exists: " + quote(name))
 

   /* system services */
 
   def service_operation(op: String, name: String): Unit =
     Isabelle_System.bash("systemctl " + Bash.string(op) + " " + Bash.string(name)).check
 
-  def service_enable(name: String) { service_operation("enable", name) }
-  def service_disable(name: String) { service_operation("disable", name) }
-  def service_start(name: String) { service_operation("start", name) }
-  def service_stop(name: String) { service_operation("stop", name) }
-  def service_restart(name: String) { service_operation("restart", name) }
+  def service_enable(name: String): Unit = service_operation("enable", name)
+  def service_disable(name: String): Unit = service_operation("disable", name)
+  def service_start(name: String): Unit = service_operation("start", name)
+  def service_stop(name: String): Unit = service_operation("stop", name)
+  def service_restart(name: String): Unit = service_operation("restart", name)
 
-  def service_shutdown(name: String)
-  {
+  def service_shutdown(name: String): Unit =
     try { service_stop(name) }
     catch { case ERROR(_) => }
-  }
 
-  def service_install(name: String, spec: String)
+  def service_install(name: String, spec: String): Unit =
   {
     service_shutdown(name)
 
     val service_file = Path.explode("/lib/systemd/system") + Path.basic(name).ext("service")
     File.write(service_file, spec)