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Working with HDFS on the Cluster

HDFS is somewhat mis-named: it's not a filesystem in the traditional sense. HDFS doesn't get mounted as a filesystem. You can't use the traditional Unix commands like ls or cp or less on the contents, because they aren't files in the computer's filesystems.

Instead, there are hdfs dfs equivalents that will go out to the cluster and do similar operations.

To list the contents of a directory: (respectively, your home directory, an output directory in your home directory, the directory of data sets for this course)

hdfs dfs -ls
hdfs dfs -ls output
hdfs dfs -ls /courses/353/

To examine the contents of HDFS files (in particular, a job's output), you can pipe the files out of HDFS and into a shell command: (for uncompressed and GZIP-compressed output)

hdfs dfs -cat output/part* | less
hdfs dfs -cat output/part* | gunzip | less

Assuming the files are small enough to be reasonably copied to the gateway's filesystem, you can copy them from HDFS to a local file like this:

hdfs dfs -copyToLocal output

Please occasionally clean up your output files on the cluster, just so they aren't taking up room:

hdfs dfs -rm -r out*

For more information, you can have a look at the HDFS filesystem commands. (Note: hdfs dfs and hadoop fs commands are synonyms. For some reason, their own docs use the older style.)

Updated Wed April 07 2021, 09:16 by ggbaker.