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書名： Hadoop MapReduce v2 Cookbook（Second Edition）
作者名： Thilina Gunarathne
本章字數： 428字
更新時間： 2021-07-23 20:32:53

HDFS command-line file operations

HDFS is a distributed filesystem, and just like any other filesystem, it allows users to manipulate the filesystem using shell commands. This recipe introduces some of these commands and shows how to use the HDFS shell commands.

It is worth noting that some of the HDFS commands have a one-to-one correspondence with the mostly used Unix commands. For example, consider the following command:

$ bin/hdfs dfs –cat /user/joe/foo.txt

The command reads the /user/joe/foo.txt file and prints it to the screen, just like the cat command in a Unix system.

Getting ready

Start the HDFS server by following the Setting up HDFS recipe or the Setting up Hadoop ecosystem in a distributed cluster environment using a Hadoop distribution recipe.

How to do it...

Run the following command to list the content of your HDFS home directory. If your HDFS home directory does not exist, please follow step 9 of the Setting up Hadoop ecosystem in a distributed cluster environment using a Hadoop distribution recipe to create your HDFS home directory.
```
$ hdfs dfs -ls
```
Run the following command to create a new directory called test inside your home directory in HDFS:
```
$ hdfs dfs -mkdir test
```
The HDFS filesystem has / as the root directory. Run the following command to list the content of the newly created directory in HDFS:
```
$ hdfs dfs -ls test
```
Run the following command to copy the local readme file to test:
```
$ hdfs dfs -copyFromLocal README.txt test
```

Run the following command to list the test directory:

$ hdfs dfs -ls test
Found 1 items
-rw-r--r-- 1 joesupergroup1366 2013-12-05 07:06 /user/joe/test/README.txt

Run the following command to copy the /test/README.txt file back to a local directory:
```
$ hdfs dfs –copyToLocal \
test/README.txt README-NEW.txt
```

How it works...

When the command is issued, the HDFS client will talk to HDFS NameNode on our behalf and carry out the operation. The client will pick up the NameNode from the configurations in the HADOOP_HOME/etc/hadoop/conf directory.

However, if needed, we can use a fully qualified path to force the client to talk to a specific NameNode. For example, hdfs://bar.foo.com:9000/data will ask the client to talk to NameNode running on bar.foo.com at the port 9000.

There's more...

HDFS supports most of the Unix commands such as cp, mv, and chown, and they follow the same pattern as the commands discussed earlier. The following command lists all the available HDFS shell commands:

$ hdfs dfs -help

Using a specific command with help will display the usage of that command.

$ hdfs dfs –help du

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Hadoop MapReduce v2 Cookbook（Second Edition）

HDFS command-line file operations

Getting ready

How to do it...

How it works...

There's more...