HADOOP-12807 S3AFileSystem should read AWS credentials from environment variables. Contributed by Tobin Baker.

hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/S3AFileSystem.java

@@ -31,6 +31,7 @@ import java.util.Map;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.TimeUnit;
 
+import com.amazonaws.auth.EnvironmentVariableCredentialsProvider;
 import com.amazonaws.AmazonClientException;
 import com.amazonaws.AmazonServiceException;
 import com.amazonaws.ClientConfiguration;
@@ -464,6 +465,7 @@ public class S3AFileSystem extends FileSystem {
           new BasicAWSCredentialsProvider(
               creds.getAccessKey(), creds.getAccessSecret()),
           new InstanceProfileCredentialsProvider(),
+          new EnvironmentVariableCredentialsProvider(),
           new AnonymousAWSCredentialsProvider()
       );
 

hadoop-tools/hadoop-aws/src/site/markdown/tools/hadoop-aws/index.md

@@ -202,6 +202,25 @@ credentials in S3AFileSystem.
 For additional reading on the credential provider API see:
 [Credential Provider API](../../../hadoop-project-dist/hadoop-common/CredentialProviderAPI.html).
 
+#### Authenticating via environment variables
+
+S3A supports configuration via [the standard AWS environment variables](http://docs.aws.amazon.com/cli/latest/userguide/cli-chap-getting-started.html#cli-environment).
+
+The core environment variables are for the access key and associated secret:
+
+```
+export AWS_ACCESS_KEY_ID=my.aws.key
+export AWS_SECRET_ACCESS_KEY=my.secret.key
+```
+
+These environment variables can be used to set the authentication credentials
+instead of properties in the Hadoop configuration. *Important:* these
+environment variables are not propagated from client to server when
+YARN applications are launched. That is: having the AWS environment variables
+set when an application is launched will not permit the launched application
+to access S3 resources. The environment variables must (somehow) be set
+on the hosts/processes where the work is executed.
+
 ##### End to End Steps for Distcp and S3 with Credential Providers
 
 ###### provision