Up until this week the only way to experience Ubuntu on a mobile phone or tablet was to flash it yourself, a fairly non-trivial process—or so Ubuntu would have you believe. But thanks to a particularly clever person over at XDA, Nexus and some other lucky Android users have a much easier way to install and use Ubuntu, all without affecting the primary ROM on their sidearm of choice.
It’s an app called MultiROM, and just like the name suggests, it allows you to run multiple ROMs on a single Android device. I used it this morning to install Ubuntu on my Nexus 5; here’s how it went…
Since MultiROM is going futz with the partitions on your device, it’s going to need root access. But no need to worry; should you ever decide to uninstall the app it will restore everything to the way it was before.
There are three components to a MultiROM install: (1) the app itself, (2) a modified recovery image and optionally (3) a patched kernel. The existing kernel on my Nexus 5 was apparently okay—it has kexec-hardboot patch, whatever that is…
The next step is for the app to download the modified TWRP recovery. If I were to do this again I’d download the image on my desktop computer and flash it manually from there, only because downloading on the phone took forever!
Next step is to reboot, watch a script execute and then be brought to a screen I unfortunately couldn’t grab, basically a boot manager. Since at this point I still only had my primary ROM installed, I had no other option than to boot that.
After you’ve rebooted you can fire up the app again, where you’ll see the option to install Ubuntu front and centre on the main screen. Not only that, but you can choose which version to install. Pretty slick.
And then, another long download… Just remember how easy this all is compared to the Ubuntu way.
Another reboot—which makes sense, right? Ubuntu has been downloaded but not yet installed.
And after one more recovery script and reboot, look at that—I can now reboot into Ubuntu! Not only that, but so long as I have enough disk space I can install any other Android ROM without affecting my main ROM or Ubuntu. Jolla’s Sailfish and Firefox OS are other available options—but I’ll likely pass on Firefox, for obvious reasons.
Below is a quick list of supported devices for the app, with links to their respective discussion threads on XDA. I’m not 100% sure if this will work with Android 5.0 as your base ROM, or if it’s a good idea to mix Android 4.x and 5.x ROMs. What I do know is that it’s yet another reason why you might want to consider harnessing the full power of your Android device with root. 
MultiROM Official Builds:
Nexus 7 (2013)
Nexus 7 (2012)
Nexus 5
Nexus 4
MultiROM Unofficial Builds:
HTC One M8
HTC One M7
Motorola Moto G
OnePlus One
Samsung Galaxy S4 i9505
Sony Xperia Z3
Sony Xperia Z2
Sony Xperia Z1
Sony Xperia Z1 Compact
Sony Xperia Z
Sony Xperia Z Ultra
Sony Xperia ZL