ROS study notes 5- appreciated node (Node)

In this article comes from: http://wiki.ros.org/ROS/Tutorials/UnderstandingNodes

1. FIG. (Graph) Conceptual Overview
   • Nodes ( Nodes ): ROS following a node is an executable program, used to communicate with other nodes ROS.
   • Message ( the Messages ): subscribe or publish the data structure when using a data type to the subject matter.
   • Topics ( Topics ): nodes can publish messages to a topic, theme or subscribe to receive messages, in order to achieve data exchange with other nodes.
   • Host node ( Master ): host node provides naming services to help find the node to other nodes.
   • ROS output ( rosout ): In the ROS is equivalent to stdout or stderr (standard output in ROS)
   • ROS core ( roscore ): host node + + standard output parameter server.
2. Core ROS (roscore)
   roscore is to use a command ros run, you need to run roscore before other nodes running.
   

   $ roscore
   ... logging to /home/shao/.ros/log/93b4e9f2-d146-11e9-a3fd-69f079ca327f/roslaunch-UAVlab1-13621.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://UAVlab1:40901/
   ros_comm version 1.12.14
   
   
   SUMMARY
   ========
   
   PARAMETERS
    * /rosdistro: kinetic
    * /rosversion: 1.12.14
   
   NODES
   
   auto-starting new master
   process[master]: started with pid [13631]
   ROS_MASTER_URI=http://UAVlab1:11311/
   
   setting /run_id to 93b4e9f2-d146-11e9-a3fd-69f079ca327f
   process[rosout-1]: started with pid [13644]
   started core service [/rosout]
   

   After successful operation as shown in FIG.

3. rosnode command
   using rosnode command can display information ros node. For example: rosnode list command lists ros nodes running.
   Opening a new terminal, run rosnode list, as follows:
   

   $ rosnode list
   /rosout
   

   As can be seen, there is a rosout node, start to roscore command.
   Use rosnode info command to view the information ros node:

   $ rosnode info /rosout
   --------------------------------------------------------------------------------
   Node [/rosout]
   Publications: 
    * /rosout_agg [rosgraph_msgs/Log]
   
   Subscriptions: 
    * /rosout [unknown type]
   
   Services: 
    * /rosout/get_loggers
    * /rosout/set_logger_level
   
   
   contacting node http://UAVlab1:42313/ ...
   Pid: 13644
   

    

4.  rosrun run node

   rosrun ros node can run the command syntax is as follows:
   

   $ rosrun [package_name] [node_name]
   

   The official website of the document in question, the right should be:

   rosrun <package> <executable>
   

   That second argument is an executable program name, and node name is not necessarily the same.
   Open a new terminal and run:

   $ rosrun turtlesim turtlesim_node
   

    

    

    Small turtle appeared myself.

   Then use rosnode can see:
   

   $ rosnode list
   /rosout
   /turtlesim
   

   You can see more than a node of a turtlesim.
   Opens new terminal, then remapping parameters can be renamed to the node name.

   $ rosrun turtlesim turtlesim_node __name:=my_turtle
   

    Then run rosnode list

   $ rosnode list
   /my_turtle
   /rosout
   /turtlesim
   

   You can see more than one node my_turtle.
   Use rosnode ping command to check the communication between a node and, similar to the ping command.

   $ rosnode ping /my_turtle
   rosnode: node is [/my_turtle]
   pinging /my_turtle with a timeout of 3.0s
   xmlrpc reply from http://UAVlab1:43947/	time=0.296116ms
   xmlrpc reply from http://UAVlab1:43947/	time=0.949144ms
   xmlrpc reply from http://UAVlab1:43947/	time=0.707150ms
   xmlrpc reply from http://UAVlab1:43947/	time=0.704050ms
   xmlrpc reply from http://UAVlab1:43947/	time=1.071930ms