code: private void executeTruncate(String columnFamily) throws TException, InvalidRequestException, UnavailableException, TimedOutException
      {
          if (!CliMain.isConnected() || !hasKeySpace())
              return;
  
          // getting CfDef, it will fail if there is no such column family in current keySpace.
          CfDef cfDef = getCfDef(CliCompiler.getColumnFamily(columnFamily, currentCfDefs()));
  
          thriftClient.truncate(cfDef.getName());
          sessionState.out.println(columnFamily + " truncated.");
      }
docs: TRUNCATE <columnFamily>