A binary is present inside the bin directory. We can call the following Hadoop command if we need to know all the commands available:

<Hadoop directory path>bin/hadoop

In the version previous to Hadoop v1, we can use the preceding command. However, in the later versions, we have to use the following command:

<Hadoop directory path>bin/hdfs

A binary without any parameter will display the list of available commands. We can check the actual implementation of Hadoop shell and its Java source at https://github.com/shot/hadoop-source-reading/blob/master/src/core/org/apache/hadoop/fs/FsShell.java.