Why is there no 'hadoop fs -head' shell command?
I would say it's more to do with efficiency - a head can easily be replicated by piping the output of a hadoop fs -cat through the linux head command.
hadoop fs -cat /path/to/file | headThis is efficient as head will close out the underlying stream after the desired number of lines have been output
Using tail in this manner would be considerably less efficient - as you'd have to stream over the entire file (all HDFS blocks) to find the final x number of lines.
hadoop fs -cat /path/to/file | tailThe hadoop fs -tail command as you note works on the last kilobyte - hadoop can efficiently find the last block and skip to the position of the final kilobyte, then stream the output. Piping via tail can't easily do this.