UserLAnd is the easiest way to run Linux on Android. It is distributed through the Play Store. It does not require you to root your device. It is simple to use. It is also FOSS (Free and Open Source Software).

Command Line Power

Install packages. Compile executables. Play text based games. Whatever you want, you have the power to do it. Your phone is no longer a plaything, you can unlock the powerful computer that it is without rooting your device.

You don't have to stop at the command line. You can run a simple X session, or a full blown desktop. You can also install and use other graphical tools like, GIMP, gnuplot or firefox.

APP:
https://play.google.com/store/apps/details?id=tech.ula