The Difference Between 'Hadoop DFS' and 'Hadoop FS'

While exploring HDFS, I came across these two syntaxes for querying HDFS:

> hadoop dfs
> hadoop fs

Initally I couldn't differentiate between the two, and kept wondering why we have two different syntaxes for a common purpose. I found a number of people online with the same question -- their thoughts are below:

Per Chris's explanation: it seems like there's no difference between the two syntaxes. If we look at the definitions of the two commands (hadoop fs and hadoop dfs) in $HADOOP_HOME/bin/hadoop

...
elif [ "$COMMAND" = "datanode" ] ; then
  CLASS='org.apache.hadoop.hdfs.server.datanode.DataNode'
  HADOOP_OPTS="$HADOOP_OPTS $HADOOP_DATANODE_OPTS"
elif [ "$COMMAND" = "fs" ] ; then
  CLASS=org.apache.hadoop.fs.FsShell
  HADOOP_OPTS="$HADOOP_OPTS $HADOOP_CLIENT_OPTS"
elif [ "$COMMAND" = "dfs" ] ; then
  CLASS=org.apache.hadoop.fs.FsShell
  HADOOP_OPTS="$HADOOP_OPTS $HADOOP_CLIENT_OPTS"
elif [ "$COMMAND" = "dfsadmin" ] ; then
  CLASS=org.apache.hadoop.hdfs.tools.DFSAdmin
  HADOOP_OPTS="$HADOOP_OPTS $HADOOP_CLIENT_OPTS"
...

That was his reasoning. Unconvinced, I kept looking for a more persuasive answer, and these excerpts made more sense to me:

FS relates to a generic file system which can point to any file systems like local, HDFS etc. But dfs is very specific to HDFS. So when we use FS it can perform operation with from/to local or hadoop distributed file system to destination. But specifying DFS operation relates to HDFS.

Below are two excerpts from the Hadoop documentation that describe these two as different shells.

FS Shell
The FileSystem (FS) shell is invoked by bin/hadoop fs. All the FS shell commands take path URIs as arguments. The URI format is scheme://autority/path. For HDFS the scheme is hdfs, and for the local filesystem the scheme is file. The scheme and authority are optional. If not specified, the default scheme specified in the configuration is used. An HDFS file or directory such as /parent/child can be specified as hdfs://namenodehost/parent/child or simply as /parent/child (given that your configuration is set to point to hdfs://namenodehost). Most of the commands in FS shell behave like corresponding Unix commands. 

DFShell
The HDFS shell is invoked by bin/hadoop dfs. All the HDFS shell commands take path URIs as arguments. The URI format is scheme://autority/path. For HDFS the scheme is hdfs, and for the local filesystem the scheme is file. The scheme and authority are optional. If not specified, the default scheme specified in the configuration is used. An HDFS file or directory such as /parent/child can be specified as hdfs://namenode:namenodeport/parent/child or simply as /parent/child (given that your configuration is set to point to namenode:namenodeport). Most of the commands in HDFS shell behave like corresponding Unix commands. 

So, based on the above, we can conclude that it all depends on the scheme configuration. When using these two commands with absolute URI (i.e. scheme://a/b) the behavior shall be identical. Only it's the default configured scheme value for file and hdfs for fs and dfs respectively, which is the cause for difference in behavior.