Best way to programm on Ubuntu Server / ROS2 Foxy / RPi4

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In your case, I see 3 possibilities:

1. Simply develop on your PC and compile on it to run tests, for example. And then synchronize the sources (with git, for example) to your Raspberry Pi and rebuild locally. This is not the best workflow, but this is very easy to start with.
2. You could cross-compile on your PC for your Raspberry Pi (your target). This is a little more complicated, but should be the more efficient. You could use https://github.com/ros-tooling/cross_... that is the best solution to me. And the repository have a nice "Usage" section that describe how to use it. Cross-compiling is basically: compile code for another target (than the one you are developing on). Once you compiled for your Raspberry Pi, you send the binaries to your Raspberry Pi (rsync is a good option).
3. You could also develop on the remote Raspberry Pi. Some IDEs have a remote developing capability. For example, Visual Studio Code (it's free and also has some ROS plugins) allows you to develop on a remote machine through ssh, https://code.visualstudio.com/docs/re... . This should be easy to setup but in this case you need to be connected all the time to your Raspberry.

I hope it helps