CLI Apps: Tips and Tricks
 | CLI Apps: Tips and Tricks RidgeRun documentation is currently under development. |
Contents
This wiki page is a compendium of different tips and tricks that can be used to improve the user experience of CLI applications on embedded Linux systems.
Example App
Let's use a simple Python application as an example to build upon.
 | mgruner: It is not clear which script is which, I recommend adding a title indicating the name of the script (please remove this box when addressed) |
import os
def main():
print("These are the devices connected to the I2C-2 bus\n")
os.system("sudo i2cdetect -y -r 2")
if __name__ == "__main__":
main()
The application would simply execute a shell command to see all the I2C devices connected to bus 2. The app can be run with:
sudo python3 demo.py
Autorun the App
To run the application automatically we will use SystemD and profile.d. Every time a new shell is created, the scripts contained in this directory will be executed. If we add a script to run the Python app, then the app will run every time a new shell is opened.
- Install application in a known directory, for example:
sudo cp demo.py /usr/local/bin/
- Create a bash script with the command that executes the app.
cd /home/$USER
sudo python3 /usr/local/bin/demo.py
| mgruner: I don't understand why do we need two different scripts. Can't we run the i2cdetect directly here? (please remove this box when addressed) |
- Change the permissions on the script:
sudo chmod 755 autorun-app.sh
- Finally copy the script that runs the app into the
profile.ddirectory. This will effectively run the Python demo app every time a new shell is started.
| mgruner: Is this preferred over systemd scripts? (please remove this box when addressed) |
sudo cp autorun-app.sh /etc/profile.d/
- To test this, simply open a new terminal window.
Linux RunLevels
Embedded platforms are often used to run a single CLI app and only that. Having a full GUI with multiple applications and icons can be distracting and even undesired. To help with this, Linux has implemented "runlevels". Different runlevels add/remove functionality to the Linux installation, for example to remove networking capabilities or the GUI.
In this example case, the application is a CLI app that does not require a GUI nor a display manager. To simplify the end user experience, the runlevel can be changed very easily to one that just has a terminal.
- Change to multi-user runlevel (no GUI):
sudo systemctl set-default multi-user
- In case you want to go back to the default runlevel with GUI:
sudo systemctl set-default graphical
- Reboot the system for the changes to take effect.
- After logging in, the Python app should run automatically, as it has been configured with
profile.d.
Change Kernel Logging Level
You might have noticed from the last step that when the app runs in multi-user runlevel there appear a lot of kernel log messages. This happens because in multi-user runlevel, the printk driver redirects any kernel logs to the standard output/error. This could be very annoying if you intend to use the multi-user runlevel because any kernel event could flood the terminal you are using. To help against this, the kernel logging level can be changed to suppress most of the messages.
- Check the current kernel logging level and save it in case you want to restore it later.
nvidia@nvidia-desktop:~$ sudo sysctl kernel.printk kernel.printk = 6 6 1 7 4
- Minimize kernel logging level:
sudo sysctl -w kernel.printk="3 4 1 3"
Conclusions
After running all these steps you will have an embedded platform that boots into multi-user runlevel without a GUI, with minimal kernel logging level and that runs the Python demo application automatically after logging in. The demo application was used as a simple example, but more complex user applications can be implemented.
References
- Five Ways To Run a Program On Your Raspberry Pi At Startup
- Run a Script on Startup in Linux
- What do the scripts in /etc/profile.d do?
- Understanding Linux runlevels the right way
- How to stop kernel log messages from flooding my console
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