If you are not familiar with wsadmin.sh (Linux) / wsadmin.bat (Windows), check out my article Getting Started with wsadmin.

Here is an example of how to list nodes using wsadmin.

~]$ /opt/WebSphere/AppServer/bin/wsadmin.sh -lang jython -c "AdminTask.listNodes()"
WASX7209I: Connected to process "dmgr" on node Manager01 using SOAP connector;  The type of process is: DeploymentManager
u'Node01\nNode02'

Better yet, using print so that the stdout is a bit more pretty.

~]$ /opt/WebSphere/AppServer/bin/wsadmin.sh -lang jython -c "AdminTask.listNodes()"
WASX7209I: Connected to process "dmgr" on node Manager01 using SOAP connector;  The type of process is: DeploymentManager
Node01
Node02

Be aware that a node is different than a node agent.

~]$ /opt/WebSphere/AppServer/bin/wsadmin.sh -lang jython -c "AdminTask.listServers('[-serverType NODE_AGENT]')"
WASX7209I: Connected to process "dmgr" on node Manager01 using SOAP connector;  The type of process is: DeploymentManager
u'nodeagent(cells/Cell01/nodes/Node01/servers/nodeagent|server.xml)'