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Starting Nodes

Prerequisites

For this tutorial we will use a lightweight simulator, please install it using

sudo apt install ros-kinetic-ros-tutorials

roscore

roscore is the first thing you should run when using ROS. You must have a roscore running in order for ROS nodes to communicate.

Please run: 

roscore

You will see something similar to: 

... logging to~/.ros/log/9cf88ce4-b14d-11df-8a75-00251148e8cf/roslaunch-machine_name-13039.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://machine_name:33919/
ros_comm version 1.4.7

SUMMARY
========

PARAMETERS
* /rosversion
* /rosdistro

NODES

auto-starting new master
process[master]: started with pid [13054]
ROS_MASTER_URI=http://machine_name:11311/

setting /run_id to 9cf88ce4-b14d-11df-8a75-00251148e8cf
process[rosout-1]: started with pid [13067]
started core service [/rosout]

If 'roscore' does not initialize, you probably have a network configuration issue. See the official ROS tutorials on Setup - Single Machine Configuration

If roscore does not initialize and sends a message about lack of permissions, probably the ~/.ros folder is owned by root, change recursively the ownership of that folder with:

sudo chown -R <your_username> ~/.ros

Starting Nodes

A node isn't really much more than an executable file within a ROS package. We want to start the turtlesim_node in the turtlesim package. In order to do so we will use rosrun.

rosrun allows you to use the package name to directly run a node within a package (without having to know the package path).

Usage:

rosrun [package_name] [node_name]

Starting Turtlesim

We can now run the turtlesim_node in the turtlesim package.

Leave roscore running in one terminal and in a new terminal__ start turtlesim_node with:

rosrun turtlesim turtlesim_node

You will see the turtlesim window:

Turtlesim

Naming Nodes

Often we wish to run multiple instances of the same node. In a '''new terminal''' try running another turtlesim_node:

 rosrun turtlesim turtlesim_node

You'll notice ROS does not allow two nodes with the same name to be running at the same time. We can remedy this problem by reassigning the name of the second node when we start it:

rosrun turtlesim turtlesim_node __name:=leo

We can now start as many turtlesim nodes that we want as long as each has a unique name!

Review

What was covered:

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