Each machine in a Hadoop cluster has a configuration script for environment settings.
Edit the hadoop-env.sh Bash script on each machine or have a mechanism for sharing environment settings; e.g., rsync .
Values for many environment variables can be identical for all machines in the cluster. Not all machines will have the same hardware profile, though. Configure each machine’s Hadoop environment so that it best uses its resources.
This file defines which machines will run datanodes and/or tasktrackers
Note: We don’t need to specify which machine(s) will run a NameNode and/or a JobTracker. The Hadoop control scripts are responsible for NamNode and JobTracker nodes when they are run on a given machine.