No ROS 'sourceing' anymore after installing rosdep2

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Is that a typo or did you install rosdep with the package python-rosdep2 ? From the API :

Experimental: the rosdep2 Python library is still unstable.

That might explain why there is an issue. If you just want rosdep I would uninstall python-rosdep2 and then you can simply follow the wiki :

sudo apt-get install python-rosdep

Thanks for the hint! I just uninstalled rosdep2 and installed rosdep, all without issues. Unofortunately, the situation didn't change.

Alright, so when you do source /opt/ros/meldoic/setub.bash is there an issue ? Can you show what env | grep ROS returns after sourcing ros ?

Doing source /opt/ros/melodic/setup.bash behaves as usual, giving no feedback, after hitting Enter, which seems to be normal.

Output of env | grep ROS is:

ROS_ETC_DIR=/opt/ros/melodic/etc/ros
ROS_ROOT=/opt/ros/melodic/share/ros
ROS_VERSION=1
ROS_PYTHON_VERSION=2
ROS_PACKAGE_PATH=/opt/ros/melodic/share
ROS_DISTRO=melodic

Everything seems fine though, do you still have the same error when trying roscore ? If you try locate roslaunch do you find it in /opt/ros/melodic ? I would try a sudo apt update and updgrade to check for missing dependencies.

Did all of that. Situation unchanged.

Though, it's funny: locate roslaunch shows me for instance /opt/ros/melodic/bin/roslaunch but I do not have such a file within /opt/ros/melodic/bin.

I will now try to remove my ROS installation and just reinstall it as a whole. I do not see any other option now :-(

Does apt complain about something ? The fact that there isn't the binary roslaunch is problematic so indeed reinstalling ros should work but your situation is odd. What happened after sudo apt install python-roslaunch ? Did it resolve other dependencies ?

Yes my situation is very odd!

apt did not complain about anything. Trying to use sudo apt install python-roslaunch lets me run into a chain of unresolvable dependency problems such as:

The following packages have unmet dependencies:
 python-roslaunch : Depends: python-roslib but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

After executing sudo apt install ros-melodic-roslib I am getting

The following packages have unmet dependencies:
 python-roslib : Depends: catkin but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

and so on and so forth,