How to Run Simulations in Batch Mode from the Command Line

Do you find yourself creating new models in the COMSOL Multiphysics® software faster than you can launch them interactively in the COMSOL Desktop® environment? If having to wait for your current model to finish solving before launching the next one does not sound appealing, it’s time to learn how to run your simulations in Batch mode from the command line. As it turns out, this is quite a simple process.

Why Run COMSOL Multiphysics® in Batch Mode from the Command Line?

If you have used simulation tools for any significant period of time, you may have found yourself creating new models faster than your computer can solve them. This is especially common if your models are quick to set up but take a fair amount of time to solve. Running multiple models at the same time on the same computer is not a good option, as they would compete for resources (RAM, in particular) and therefore take longer to run simultaneously than they would sequentially, or back to back.

So, what’s a modeler to do?

You could launch your first model in the graphical user interface (GUI) and wait for it to solve, launch the second model in the GUI and wait for it to solve, and so on. But who would want to return to the office after hours or on weekends just to launch their next model?

Fortunately, there is a solution: creating a shell script, or a batch file, which automatically launches your COMSOL Multiphysics simulations one after the other. I’ll explain how to do this step by step on a computer with the Windows® operating system, but these ideas also apply to the other supported platforms (macOS and the Linux® operating system).

How to Run a Single Model from the Command Line

Let’s start with a demonstration of how to run a single COMSOL Multiphysics model from the command line.

First, we create a model file in the COMSOL Multiphysics GUI, also known as the COMSOL Desktop. Since we’re going over how to use a new functionality, the smaller and less detailed the model, the better. This will allow us to understand the functionality and perform tests on it quickly. Once you are comfortable with this functionality, it can, of course, also be applied to sophisticated models that take a long time to solve.

At this stage, we check that the model is properly set up by running it with a relatively coarse mesh. This presents the additional benefit of generating a default data set and one or two default plots in the Study branch of the model tree. Now that we’ve ensured that the model is properly set up, we can refine the mesh and save the file under the name Model1.mph in our working folder. In this example, that’s C:/Users/jf.

At this point, we can close the COMSOL Desktop.

Next, we open a Command Prompt window and, at the command line, make our way to our working folder. We type the name of the working folder:

cd C:\Users\jf

Then, we press the Enter key.

We are just about to call the COMSOL® software using the comsolbatch command. Before we can do that, we need to make sure that the Windows® operating system knows where to find that command. This is where, if we have not done so before, we add the path to the COMSOL® software executables to the Windows® path environment variable. On a computer running Windows®, with a default installation, these executables are located in C:\Program Files\COMSOL\COMSOL52a\Multiphysics\bin\win64.

Now, drum roll, please!

Back at the command line, we type the following command and then press Enter:

comsolbatch -inputfile Model1.mph -outputfile Model1_solved.mph

This command instructs Windows® to launch COMSOL Multiphysics® in Batch mode, hence without a graphical user interface. As the syntax suggests, we use Model1.mph as an input and the file Model1_solved.mph is the file with the solution. If we were to omit the “-outputfile Model1_solved.mph” part in the command above, the solution would be stored in the input file, Model1.mph.

As the software runs, some progress information is displayed at the command line. After a few moments, the run is done and we can open the output file, Model1_solved.mph, in the GUI. We can see that the model has indeed been solved and that we can postprocess the results interactively, just as if we had computed the solution in the COMSOL Desktop.

Writing and Running a Shell Script

Now that we’ve figured out how to launch a COMSOL Multiphysics model from the command line, let’s see how to automate running two or more simulations in a row.

Let’s create a second model, check that it is properly set up, and save the file to our working folder under the name Model2.mph. With that done, we can close the COMSOL Desktop again.

Using a text editor like Notepad, we create a plain text file containing the following two lines:

comsolbatch -inputfile Model1.mph -outputfile Model1_solved.mph
comsolbatch -inputfile Model2.mph -outputfile Model2_solved.mph

We then save this in our working folder as a plain text file with the .bat extension. Here, we named the file Batch_Commands_For_COMSOL.bat.

At the command prompt, still in our working folder, we launch Batch_Commands_For_COMSOL.bat. At the command line, we type:

Batch_Commands_For_COMSOL

Then, we press the Enter key.

COMSOL Multiphysics will run without the GUI open and solve the problem defined in the file Model1.mph. The COMSOL® software will then do the same for the problem defined in the file Model2.mph. Once the runs are finished, we can inspect the files Model1_solved.mph and Model2_solved.mph in the COMSOL Desktop to see that they indeed contain the solutions of these two analyses. On the other hand, if we open the files Model1.mph and Model2.mph in the GUI, we see that they have not changed and still contain the problem definitions, but no solutions.

If we want to run more than two files sequentially, we can just modify the .bat file accordingly and add lines for each file that we wish to run.

By learning how to run your COMSOL Multiphysics simulations in Batch mode from the command line, you will be able to complete your projects more efficiently and with ease.

Further Resources

• Learn more about the modeling capabilities of COMSOL Multiphysics in these core functionality blog posts
• Watch this introductory tutorial video on how to set up and run a simulation

Microsoft and Windows are either registered trademarks or trademarks of Microsoft Corporation in the United States and/or other countries.

macOS is a trademark of Apple Inc., in the U.S. and other countries.

Linux is a registered trademark of Linus Torvalds in the U.S. and other countries.