Hadoop comes with a distributed filesystem called HDFS, which stands for Hadoop Distributed File System. Although Hadoop supports many other filesystems (e.g., Amazon S3), HDFS is the most popular choice and will be used throughout this bootcamp. Therefore, in this section, you will learn how to move data between your local filesystem and HDFS.
Hadoop provides a command line utility hdfs to interact with HDFS. Basic operations are placed under hdfs dfs subcommand. Let's play with some basic operations.
When you use HDFS for the first time, it's likely that your home directory in HDFS has not been created yet. Your home directory in HDFS is /user/<username>/ by default. In the environment that we provide, there's a special user hdfs who is an HDFS administrator and has the permission to create new home directories.
First, you will need to switch to the hdfs user via
> sudo su - hdfs
Then, you can create a directory and change ownership of the newly created folder
> hdfs dfs -mkdir -p /user/<username>
> hdfs dfs -chown <username> /user/<username>
Please remember to change <username> to your actual linux user name. Currently, the default user of our Docker image is root. Since it is a virtual environment, you don't need to worry about using root user and its permission. After you create the folder, switch back to your user with exit.
Similar to creating local directory via linux command mkdir, creating a folder named input in HDFS use
> hdfs dfs -mkdir input
where hdfs is the HDFS utility program, dfs is the subcommand to handle basic HDFS operations, -mkdir means you want to create a directory and the directory name is specified as input. Above commands actually create the input directory in your home directory in HDFS. Of course, you can create it to other place with absolute or relative path.
Suppose you followed previous instructions and created an directory named input, you can then copy data from local file system to HDFS using -put. For example,
> cd /bootcamp/data
> hdfs dfs -put case.csv input
> hdfs dfs -put control.csv input
You can find detailed description about these two files in sample data.
Similar to -put, -get operation will copy data out of HDFS to the local folder. For example
hdfs dfs -get input/case.csv local_case.csv
will copy the input/case.csv file out HDFS into the current working directory using a new name local_case.csv. If you didn't specify local_case.csv, the original name case.csv will be kept. You will be able to verify your copy by -ls and -cat operation described below.
Just like linux ls command, -ls is the operation to list files and folders in HDFS. For example, the following command list items in your home directory of HDFS (i.e /user/<username>/)
> hdfs dfs -ls
Found 1 items
-rwxr-xr-x - hadoop supergroup 0 2015-07-11 06:10 input
You can see the newly created input directory is listed. You can also see the files inside a particular directory
>hdfs dfs -ls input
found 2 items
-rw-r--r-- 1 hadoop supergroup 536404 2015-07-11 06:10 input/case.csv
-rw-r--r-- 1 hadoop supergroup 672568 2015-07-11 06:08 input/control.csv
Actually you don't need to copy files out local in order to see its content, you can directly use -cat to printing the content of files in HDFS. For example, the following command print out content of the one file you just put into HDFS.
> hdfs dfs -cat input/case.csv
...
020E860BD31CAC69,DRUG00440128228,976,60.0
020E860BD31CAC69,DIAG486,907,1.0
020E860BD31CAC69,DIAG7863,907,1.0
020E860BD31CAC69,DIAGV5866,907,1.0
020E860BD31CAC69,DIAG3659,907,1.0
020E860BD31CAC69,DIAGRG199,907,1.0
020E860BD31CAC69,PAYMENT,907,15000.0
020E860BD31CAC69,heartfailure,956,1.0
...
You will find wildcard character very useful since output of MapReduce and other Hadoop-based tools tendsto be directory. For example, to print content of all csv files (the case.csv and control.csv) in the input HDFS folder, you can
hdfs dfs -cat input/*.csv
For more detailed usage of different commands and parameters, you can type
hdfs dfs -help
hdfs dfs -rm -r input
You may miss the -r option and get error. -r tells HDFS to remove recursively. This is similar to linux command rm.