ROS

ROS is a software platform for robotics.

Concepts

• Package: defined by a directory containing a file package.xml (REP-0127)
  • msg/MyMessageType.msg defines messages for the package, published to topics
  • srv/MyServiceType.srv defines services for the package, supporting request/reply semantics
• Node: a named process that performs computation
  • A running node has a type and a graph resource name
  • A node is typically launched standalone using rosrun
  • If a node is started with the name of an existing node, the existing node is stopped
• Master: implemented by the rosmaster package, usually started via roscore
• Topic: a named bus on which messages of a single type are published
  • rostopic list will list active topics on a running rosmaster
• Service: a named procedure, implemented by some Node
  • Connections can be persistent according to the client's choice
• Parameter: a key in the global configuration dictionary
  • XMLRPC data types

Names

Names aka graph resource names are unique for each topic, parameter, node, and service in the ros graph. It is a shared namespace; a topic cannot share a name with a node. The ROS_NAMESPACE environment variable can be used to scope the ros graph namespace. Names can be:

• relative,
• /global, and
• ~private (the node's name replaces the tilde)

Names can furthermore be remapped when a node is started by e.g. rosrun.

• Special case: launch node into namespace with `__ns`
• Special case: change node name with variable `__name`
  • To launch barnode with the name barnode3: `roslaunch foopkg barnode __name:=barnode3`

Executables

• rosrun: launch an executable from a package
• roslaunch: starts collections of nodes specified in a launch XML file
  • roscore: roslaunch to start core ros functionality