了解ROS Nodes（节点/结点）

1.相关概念

Nodes:A node is an executable that uses ROS to communicate with other nodes.
Messages: ROS data type used when subscribing or publishing to a topic.
Topics: Nodes canpublishmessagesto a topic as well assubscribetoa topic to receive messages.
Master: Name service for ROS (i.e. helps nodes find each other)
rosout: ROS equivalent of stdout/stderr
roscore: Master + rosout + parameter server

2.Nodes

    A node really isn't much more than an executable file within a ROS package. ROS nodes use a ROS client library to communicate with other nodes. Nodes can publish or subscribe to a Topic. Nodes can also provide or use a Service.

3.客户端库

   根据编程语言不同有：

    rospy = python client library
    roscpp = c++ client library

4.roscore

    当运行ROS时第一件事就是运行roscore命令。

5.使用rosnode

    由于执行了roscore命令后Terminal界面就被占用了，此时需要再打开一个新Terminal再进行如下操作才能有相应结果（即操作依赖于roscore的运行）：

    rosnode list

    此时会看到：

    /rosout

    当前只有rosout这个node在运行，运行roscore就一直存在，用于调试或打印输出信息。

    rosnode info /rosout

    该命令可以看到rosout这个node的更多信息。

6.使用rosrun

    运行ROS自带的turtle例子：

    rosrun turtlesim turtlesim_node

    其中，turtlesim是包名，turtlesim_node是节点名。

    命令运行后有如下输出：

    [ INFO] [1495245056.523181709]: Startingturtlesim with node name /turtlesim
    [ INFO] [1495245056.543466285]: Spawningturtle [turtle1] at x=[5.544445], y=[5.544445], theta=[0.000000]

    对应的运行界面如下：

turtle

    此时执行rosnode list命令，会有如下输出：

    /rosout
    /turtlesim

    多了一个turtlesim结点，对于结点名，我们也可以指定，如下命令：

    rosrun turtlesim turtlesim_node __name:=slam_turtle

    即在使用rosrun时通过__name参数来设定，执行该命令后再执行rosnode

list，会有如下返回值：

    /rosout
    /slam_turtle

    接下来使用rosnode ping slam_turtle命令可以测试该结点的运行状况，命令执行返回值：

    rosnode: node is [/slam_turtle]
    pinging /slam_turtle with a timeout of 3.0s
    xmlrpc reply from http://slam:46576/    time=3.997087ms
    xmlrpc reply from http://slam:46576/    time=1.501083ms
    xmlrpc reply from http://slam:46576/    time=1.564980ms
    xmlrpc reply from http://slam:46576/    time=1.533031ms
    xmlrpc reply from http://slam:46576/    time=1.444101ms
    ^Cping average: 2.008057ms

7.回顾

roscore = ros+core : master (provides name service for ROS) + rosout (stdout/stderr) +parameter server
rosnode = ros+node : ROS tool toget information about a node.
rosrun = ros+run : runs a node from a given package.

参考网址：http://wiki.ros.org/ROS/Tutorials/UnderstandingNodes