code: private static void appendSystemClasspath(JobConf conf,
                                              String pathSeparator,
                                              StringBuffer classPath) {
      // The alternate runtime can be used to debug tasks by putting a
      // custom version of the mapred libraries. This will get loaded before
      // the TT's jars.
      String debugRuntime = conf.get("mapred.task.debug.runtime.classpath");
      if (debugRuntime != null) {
        classPath.append(pathSeparator);
        classPath.append(debugRuntime);
      }
      // Determine system classpath for tasks. Default to tasktracker's
      // classpath.
      String systemClasspath = System.getenv(
        MAPREDUCE_TASK_SYSTEM_CLASSPATH_PROPERTY);
      if (systemClasspath == null) {
        systemClasspath = System.getProperty("java.class.path");
      }
      if (LOG.isDebugEnabled()) {
        LOG.debug("System classpath " + systemClasspath);
      }
      classPath.append(pathSeparator);
      classPath.append(systemClasspath);
    }
docs: Add the system class path to a class path
  @param conf JobConf
  @param pathSeparator Path separator
  @param classPath Where to add the system class path