Using the HDFS Client

Scenario

This section describes how to use the HDFS client in an O&M scenario or service scenario.

Prerequisites

The client has been installed.
For example, the installation directory is /opt/hadoopclient. The client directory in the following operations is only an example. Change it to the actual installation directory.
Service component users are created by the administrator as required. In security mode, machine-machine users need to download the keytab file. A human-machine user needs to change the password upon the first login. (This operation is not required in normal mode.)

Using the HDFS Client

Log in to the node where the client is installed as the client installation user.
Run the following command to go to the client installation directory:

cd /opt/hadoopclient
Run the following command to configure environment variables:

source bigdata_env
If the cluster is in security mode, run the following command to authenticate the user. In normal mode, user authentication is not required.

kinit Component service user
Run the HDFS Shell command. Example:

hdfs dfs -ls /

Common HDFS Client Commands

The following table lists common HDFS client commands.

For more commands, see https://hadoop.apache.org/docs/stable/hadoop-project-dist/hadoop-common/CommandsManual.html#User_Commands.

**Table 1** Common HDFS client commands
Command	Description	Example
hdfs dfs -mkdir Folder name	Used to create a folder.	hdfs dfs -mkdir /tmp/mydir
hdfs dfs -ls Folder name	Used to view a folder.	hdfs dfs -ls /tmp
hdfs dfs -put Local file on the client node Specified HDFS path	Used to upload a local file to a specified HDFS path.	hdfs dfs -put /opt/test.txt /tmp Upload the /opt/test.txt file on the client node to the /tmp directory of HDFS.
hdfs dfs -get Specified file on HDFS Specified path on the client node	Used to download the HDFS file to the specified local path.	hdfs dfs -get /tmp/test.txt /opt/ Download the /tmp/test.txt file on HDFS to the /opt path on the client node.
hdfs dfs -rm -r -f Specified folder on HDFS	Used to delete a folder.	hdfs dfs -rm -r -f /tmp/mydir
hdfs dfs -chmod Permission parameter File directory	Used to configure the HDFS directory permission for a user.	hdfs dfs -chmod 700 /tmp/test

Client-related FAQs

What do I do when the HDFS client exits abnormally and error message "java.lang.OutOfMemoryError" is displayed after the HDFS client command is running?
This problem occurs because the memory required for running the HDFS client exceeds the preset upper limit (128 MB by default). You can change the memory upper limit of the client by modifying CLIENT_GC_OPTS in <Client installation path>/HDFS/component_env. For example, if you want to set the upper limit to 1 GB, run the following command:
```
CLIENT_GC_OPTS="-Xmx1G"
```
After the modification, run the following command to make the modification take effect:

source <Client installation path>//bigdata_env
How do I set the log level when the HDFS client is running?
By default, the logs generated during the running of the HDFS client are printed to the console. The default log level is INFO. To enable the DEBUG log level for fault locating, run the following command to export an environment variable:

export HADOOP_ROOT_LOGGER=DEBUG,console

Then run the HDFS Shell command to generate the DEBUG logs.

If you want to print INFO logs again, run the following command:

export HADOOP_ROOT_LOGGER=INFO,console
How do I delete HDFS files permanently?
HDFS provides a recycle bin mechanism. Typically, after an HDFS file is deleted, the file is moved to the recycle bin of HDFS. If the file is no longer needed and the storage space needs to be released, clear the corresponding recycle bin directory, for example, hdfs://hacluster/user/xxx/.Trash/Current/xxx.