ROS Melodic 1.6 Environment setup No such file error

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Did you not see errors while running sudo apt install ros-melodic-desktop or similar commands?

"Ubuntu One" should not be needed here. A regular sudo apt-get update && sudo apt-get install <pkg> should suffice.

Correct. Except it takes me too long to download a fresh .iso from ubuntu.com every time one of my poorly recreated Gazebo Frankensteins blows one or more of my test box' OS away. To save time, I use an 18.04.2 amd64 downloaded a couple of months ago. It is probably true if you don't simulate new features every month, or do a fresh ubuntu download, then you do not need to update ubuntu in order to download ros Melodic. I save around a day's worth of waiting by reusing a 2-month-old shelf copy of ubuntu .iso. I, unlike others who load a fresh .iso, have to update ubuntu in order to avoid broken ROS installs. Thank you for your comment and I hope my short comment helps the whole world to continue building robots with ros.

I'm not saying you shouldn't update your Ubuntu installation.

The first thing you'd do after installing Ubuntu would be sudo apt update and then a sudo apt upgrade.

After that you can continue installing ROS.

Depending on your internet connection this should not take a day, but if it's very slow that could of course happen.

My confusion was with the remark about "Ubuntu One".

Except it takes me too long to download a fresh .iso from ubuntu.com every time one of my poorly recreated Gazebo Frankensteins blows one or more of my test box' OS away.

I still don't understand how user-level code in your home directory is able to "blow away" an OS install, but if you prefer to reinstall everything that is of course fine.