Distcp not working due to NoClassDefFoundError: org/apache/hadoop/tools/DistCp

$HADOOP_COMMON_HOME/bin/hadoop --config $HADOOP_CONF_DIR distcp
hdfs://<NN1>:8020/tmp/input
hdfs://<NN1>:8020/tmp/output
Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/hadoop/tools/DistCp
Caused by: java.lang.ClassNotFoundException: org.apache.hadoop.tools.DistCp
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
Could not find the main class: org.apache.hadoop.tools.DistCp. Program will exit.

it used to work before, do we need to export any environment variable? does distcp changed its place?