I recently purchased an external USB hard drive and wanted to use it as a portable boot drive. I installed Linux Mint 18.1 on it and got everything working.
Then I started to think about using that drive to install Linux on other machines. I assumed that whatever a live boot USB does should be possible from a full-blown Linux installation. I looked around and the only option I found was from Ubuntu: Installation/From Linux. Their solution is to create a partition, fill it with the ISO contents and then boot from that to launch the installer. I did follow those instructions and got it working as expected, however, I still feel there must be a way to install Linux from Linux without booting into an ISO.
I just found a related question: Installing without booting. There is an answer there that suggests there is some sequence of operations that could be run to install Linux on another partition, but I would need more detail than provided there. Is that process documented somewhere?
Honestly, I would be more comfortable if I could just run the installers that are included in the live boot images of each distro. Or some kind of semi-authoritative script that would do the same thing. Is there a package in the repos that would provide such a thing (eg. a Linux Mint installer package that could be installed using