Why systemd Doesn't Run Applications from Your Bash Script

Discover why your `systemd` service may not start applications as expected from a Bash script and find efficient solutions to resolve the issue.
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This video is based on the question https://stackoverflow.com/q/66801436/ asked by the user 'folibis' ( https://stackoverflow.com/u/2981610/ ) and on the answer https://stackoverflow.com/a/66801506/ provided by the user 'Raman Sailopal' ( https://stackoverflow.com/u/7840440/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Challenge of Running Applications with systemd

If you've ever faced the challenge of running multiple applications in the background using a Bash script under systemd, you might have encountered situations where your applications just won't start as expected. In this post, we'll dive into a specific scenario where a systemd service was created to execute a Bash script that launches several applications but failed to do so. Let's break down the problem, understand what's going wrong, and explore how to solve it effectively.

The Problem

Imagine having a service set up to run a collection of applications when your system boots up. You wrote a Bash script called startup.sh to handle this task within your startup.service. The script seems to work perfectly when executed directly from the command line, but when systemd runs it on startup, the applications don't appear to launch.

Here’s a recap of the setup:

Service File (startup.service):

[[See Video to Reveal this Text or Code Snippet]]

Bash Script (startup.sh):

[[See Video to Reveal this Text or Code Snippet]]

When you check the status of the service, you notice that it executed without errors, but the applications did not launch.

Why Did This Happen?

The primary reason your applications did not start is that systemd may not be able to run the script as expected due to process handling and how it manages background tasks. Here are the potential issues:

Script Header: The script may not have the correct header to indicate it's a Bash script.

Process Management: By default, systemd expects a process to run until it exits. If your script forks processes and returns, the service will see it as complete and may terminate.

The Solution

To resolve the issue and ensure your applications are launched successfully on startup, you can make a few adjustments to both the script and the service file.

Step 1: Ensure the Script is Executable

First, ensure your script has a proper shebang line at the top. If it's missing, add this line to the top of startup.sh:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Modify the ExecStart Line

Alternatively, instead of relying on the shebang, you can modify the ExecStart line in the startup.service file to explicitly call the Bash interpreter:

[[See Video to Reveal this Text or Code Snippet]]

This tells systemd to execute the script using Bash, providing clarity on how to run it.

Step 3: Change the Service Type

To manage the spawned processes correctly, change the service type from Type=simple to Type=forking. This will let systemd know that the application is expected to fork itself and that it should wait for the service to be properly initialized.

Modify the service file as follows:

[[See Video to Reveal this Text or Code Snippet]]

Step 4: Reload systemd and Restart the Service

After making these changes, remember to reload the systemd configuration and restart your service to apply the changes:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By following these steps, you should be able to run your applications in the background from a Bash script effortlessly with systemd. Troubleshooting service behavior can sometimes be tricky, but with the right adjustments, starting multiple applications from a simple script can be a smooth and efficient process.

Now you're equipped with the knowledge to handle similar situations where systemd might not behave as expected. Happy coding!

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