To answer the question in the comment, I need to write an answer.
<launch>
<node name="tuck_arms" pkg="pr2_tuckarm" type="tuck_arms.py" args="-r t -l t"/>
</launch>
This launch file executed a node instead of a launch file, since the beginning is "node name..."
1.node name = "tuck_arms" means you'll see a node named "tuck_arms" in rxgraph.
2.type="tuck_arms.py" means this launch file will execute this python file.
3.pkg="pr2_tuckarm" means the executable file you are going to execute is in this package.
As for "What should you see if you execute these launch file?", the effect should be same as running
rosrun pr2_tuckarm tuck_arms.py -r t -l t
And for the output message, mine is
rosfuerte@rosfuerte-K53SM:~/project/ros/code_training$ roslaunch ./tuck_arm.launch
... logging to /home/rosfuerte/.ros/log/5cf83a42-3227-11e2-b51f-c860003a5122/roslaunch-rosfuerte-K53SM-15395.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.
started roslaunch server http://rosfuerte-K53SM:47533/
SUMMARY
========
PARAMETERS
* /rosdistro
* /rosversion
NODES
/
tuck_arms (pr2_tuckarm/tuck_arms.py)
ROS_MASTER_URI=http://localhost:11311
core service [/rosout] found
Exception AttributeError: AttributeError("'_DummyThread' object has no attribute '_Thread__block'",) in <module 'threading' from '/usr/lib/python2.7/threading.pyc'> ignored
process[tuck_arms-1]: started with pid [15415]
[WARN] [WallTime: 1353315583.144633] [15.934000] You've passed in true for auto_start to the python action server, you should always pass in false to avoid race conditions.