Launch Files¶

Hopefully by this point you've realized that starting every node individually is not always the best option. Doing so not only takes time, but it also results in a plethora of command terminals. 'roslaunch' is a tool in ROS that allows you to start multiple nodes (which are defined in a launch file) with only one command.

Prerequisites¶

In order to complete this tutorial we need a package in which to place our launch file in. Go ahead and complete these two tutorials from the official ROS tutorials.

Using roslaunch¶

roslaunch starts nodes as defined in a launch file.

Usage:

 roslaunch [package] [filename.launch]

First go to the beginner_tutorials package we created and built earlier:

 roscd beginner_tutorials

If roscd says something similar to roscd: No such package/stack 'beginner_tutorials this simply means that ROS can't find your package. Check your ~/.rosrc file in your home directory (which runs every time you open a new terminal) for the command:

 source ~/your_workspace/devel/setup.bash

Then let's make a launch directory:

 roscd beginner_tutorials
 mkdir launch
 cd launch

NOTE: The directory to store launch files don't necessarily have to be named as launch. In fact you don't even need to store them in a directory. roslaunch command automatically looks into the passed package and detects available launch files. However, it is good practice to do so.

The Launch File¶

Now let's create a launch file called turtlemimic.launch and paste the following:

<launch>

  <group ns="turtlesim1">
    <node pkg="turtlesim" name="sim" type="turtlesim_node"/>
  </group>

  <group ns="turtlesim2">
    <node pkg="turtlesim" name="sim" type="turtlesim_node"/>
  </group>

  <node pkg="turtlesim" name="mimic" type="mimic">
    <remap from="input" to="turtlesim1/turtle1"/>
    <remap from="output" to="turtlesim2/turtle1"/>
  </node>

</launch>

The Launch File Explained¶

Now, let's break the launch xml down.

     <launch>

Here we start the launch file with the launch tag, so that the file is identified as a launch file.

  <group ns="turtlesim1">
    <node pkg="turtlesim" name="sim" type="turtlesim_node"/>
  </group>

  <group ns="turtlesim2">
    <node pkg="turtlesim" name="sim" type="turtlesim_node"/>
  </group>

Here we start two groups with a namespace tag of turtlesim1 and turtlesim2 with a turtlesim node with a name of sim. This allows us to start two simulators without having name conflicts.

<node pkg="turtlesim" name="mimic" type="mimic">
    <remap from="input" to="turtlesim1/turtle1"/>
    <remap from="output" to="turtlesim2/turtle1"/>
  </node>

Here we start the mimic node with the topics input and output renamed to turtlesim1 and turtlesim2. This renaming will cause turtlesim2 to mimic turtlesim1.

</launch>

This closes the xml tag for the launch file.

Additional Renaming Methods¶

In addition to using group ns as described above, there are two other methods for renaming nodes

<remap from="/sim" to="george"/>
<node pkg="turtlesim" name="sim" type="turtlesim_node"/>

<node pkg="turtlesim" name="sim" type="turtlesim_node" ns="george"/>

roslaunching¶

Now let's roslaunch the launch file:

 roslaunch beginner_tutorials turtlemimic.launch

Two turtlesims will start and in a '''new terminal''' send the rostopic command:

  rostopic pub /turtlesim1/turtle1/cmd_vel geometry_msgs/Twist -r 1 -- '[2.0, 0.0, 0.0]' '[0.0, 0.0, -1.8]'

You will see the two turtlesims start moving even though the publish command is only being sent to turtlesim1.

figures/mimic

NOTE: Don't worry too much about understanding the rostopic pub command, we will return to it later. Basically, this usage of the command continuously publishes commands to turtlesim which tells the turtle to travel in a circle.

We can also use rqt_graph to better understand what our launch file did. Run rqt's main window and select rqt_graph:

rqt

Or simply:

 rqt_graph

In one command from the command line we were able to start 4 nodes! This is obviously a much cleaner and more efficient approach to starting multiple nodes than starting them individually.