Where / How to find the source code of ROSRUN command ?

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As you can see at wiki/rosrun:

rosrun is part of the rosbash suite. Documentation moved to rosbash/rosrun.

At wiki/rosbash, we see:

Clicking on the URL shown at the Source bullet (ie: https://github.com/ros/ros), we get to the ros/ros repository, which has the sources of rosrun at tools/rosbash/scripts/rosrun.

And a note (also for future readers): please do not install ROS packages by cloning the Source URLs into your Catkin workspace.

Always prefer to use binary packages provided by the buildfarm and install using apt (on Ubuntu and Debian) or whatever package manager your system uses. Refer to #q320046 for some pointers on how to figure out a package has been released and can be installed using apt/the package manager.