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Up to this point, we have been running all of our rosters nodes independently, with each node needing

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its own dedicated terminal to run.

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We have also had to open extra terminals to do actions such as setting parameters.

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Launch files are a feature in ROS which allow you to launch multiple nodes and configure them all with

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a single command.

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In this lecture we will create a launch file to launch our RPM publisher node.

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So the first thing we will do is open up V's code and create a new folder in our package by right clicking

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and clicking a new folder and we will call this folder launch within our newly created launch folder.

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We'll create a new file which we will call RPM underscore node dot launch dot py.

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Now I know this may be a bit confusing to have two file extensions in a file name, but in ROS two launch

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files are essentially made from Python code.

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That's why we edit it with the dot py extension.

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But essentially the launch portion of it is a tribute to ROS one launch files which used to use XML

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syntax rather than python code.

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Just know that it's generally accepted to have the word launch near the end of the launch file.

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So if you wanted to, you could have used an underscore instead of a period before the word launch for

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this python file, we are going to be importing functionality from the launch module.

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Specifically, we will be including launch description from launch and node from launch roster actions.

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Note that this is not the same node object from the ROS client library we have been using in our code

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so far.

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In order for the ROS to launch command to interpret this python file, we will need to have a function

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called Generate Launch description in it, which returns a launch description object.

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This launch description object will contain a list of things we want to configure or run in this launch

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file.

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In this case, we just want to launch our RPM node, so I will create a node object.

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So on the screen, I'm showing you the source code for this node class, which contains all the optional

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parameters you can include with launching a node.

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For this case, I will specify the package, which is our Udemy ras to package our executable, which

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is RPM, pub node pie and the name of the node which I will just call RPM pub node.

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No.

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And I forgot a comma here.

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Here we go.

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If I wanted to add more nodes, I could simply add them to the list and then I could configure this

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one just as I did the one above.

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But for now, I'm just to keep the simple and just launch our RPM node.

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Now we can save this file and head over to our make list text file.

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So here we're going to have to configure our make list X file to be able to run launch files.

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The only thing we need to do is to create another install function, which I will put after the one

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we currently have to set up our Python scripts.

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So the only thing we need to do is create another install function, which I will just put after the

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one we currently have to set up our Python scripts.

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Within this install function we will use the directory keyword and the directory we will be including

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is our launch folder.

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Then we will point the destination location to the share folder followed by the project name.

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Ock with that saved the file.

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And now we can run our newly created launch file by going ahead and rebuilding our workspace directory,

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which you can do by either going to terminal run, build task or control shift B which will run our

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can build Simulink install command.

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Oh, and it seems I made a mistake here.

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I accidentally placed our launch folder within the workspace.

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I meant to place it within the package.

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So I'll go ahead and move this.

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Into our package.

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All right, so now we're in our sauce raw stew package, and then we have our launch folder, RPM,

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long shot pie.

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And just to show you what this looks like inside the workspace directory.

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So here we have our package, so we have our scripts folder with all our Python files and our launch

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folder with our launch file.

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All right.

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Let's go ahead and try to rebuild that.

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All right.

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And that time we build successfully.

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So let's go ahead and open up a terminal and we'll go ahead and source it.

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And now we can run our launch file using the Ross to launch command, which takes in the name of the

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package and the name of the launch file.

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In this case, we will launch our raw package's RPM node launch file.

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Oh, and it seems I made a typo within my launch file.

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So our package name is Ros two package, so I only forgot the two there so we can go ahead and save

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this.

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And all the changes should take effect because we're using Simulink install.

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So if I try to rerun this one more time,

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we see process started with PID and the corresponding number and I can double check that this is running

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by running across to node list and we see our RPM publisher node is running just as it would have been

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with Ros to run.

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So congratulations.

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You have successfully launched your own node using a launch file.

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Now it's worth noting the launch files can be used to run external system commands as well.

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We can do this with the execute process object, which we will have to import from the launch actions

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module.

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Then within our launch subscription list, we will add an execute process to run a command.

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In this case, I will run the ROS to topic list command.

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This command will be the value of the CMD argument.

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We split this command up word for word in a list like so.

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Then I will set the output argument to screen, which means the output of this command will show up

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in our terminal.

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With that.

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Let's save the file.

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And again, it should automatically take effect because we're using Simulink install.

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So now if I go ahead and rerun this launch file.

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We see here.

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We got a subset of information printing to the screen where it says Ross 2-2 because this is the second

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command that's running within our launch file and it starts with Ross two and we see it prints out the

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corresponding topics if we had run Ross to topic list.

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So we see the same information here within our launch file.

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So that's just one of the ways which you can configure your launch file to utilize terminal actions.

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One thing I briefly want to mention before we head over to the next lecture is if you wanted to configure

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parameters within your node object, you can also do that using the parameters argument.

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And this takes in a list of dictionaries, which in this case each dictionary will have the value of

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your particular parameter.

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So you'd put in your parameter name with the colon and you'd specify the parameter value.

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So I'm going to put this back to the way it was, but just know that this is the syntax in order to

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set parameters of your node.

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If they have been declared already, which we will be taking a look at in the next lecture.

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Great.

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You have learned how to use launch files to automatically start up ROS nodes and run terminal commands.

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Let's review what we learned.

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In this lecture, we learned how we could create launch files so that we could run multiple nodes as

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well as other functionality with a single ROS to launch terminal Command.

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We created our launch files in Python by importing the launch description node and execute process objects

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from the different launch modules and then utilize them within our launch files.

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Generate Launch Description function.

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In our lunch description, we were able to run our Ross nodes we created using the node object and specifying

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the package and executable name.

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We can then also pass in a node name which will override the default node name specified in the code.

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You can even specify values of particular parameters of that node.

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We also took a look at the execute process instance in which we can run terminal commands utilizing

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our launch file and we specify output to screen so that the output of these commands can be seen on

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the terminal.

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We run the launch file in.