Ever wanted to run a full linux command line on Android? Then you have been looking for Termux, Termux is a GPL licensed terminal emulator for android that allows you to run most GNU/Linux command line utilites on your Android phone (or tablet) without root.
Okay, but how does that help me become productive? you might ask, well think about it this way, you can add a physical keyboard and even a monitor if you want and suddenly you just got a powerful GNU/Linux machine in your pocket. You can use this with tmux to have multiple terminal sessions open, you could also ssh into your homeserver and do whatever you like, or if you don't have a server you could do android and C devolopment (and a lot of other programming languages).
I personally use termux with a bluetooth keyboard when i sit on a bus, that's where i write most of my "articles", i also use it to edit other personal things on my server, and to read man pages an learn new "unix" stuff.
I wouldn't say this is a replacemnt for a GNU/Linux computer but rather a nice portable way to work on the go, where it might not be convinient to carry a laptop. There still is a few packages not yet compatible, you most likely won't be able to run any propertary programs (not including drivers and host system), and suprisingly enough you will actually be able to run a full fletched GUI just follow this guide.
It also has some nice aditions like being able to list all your contacts with all their information and grep through them or export them to a text file, you can also send sms text messages (provided your phone has that ability) all from the command line.
You also have acces to your android filesystem, which means you would be able to use something like youtube-dl to download videos from almost all websites (which includes, YouTube, Vimeo, Facebook etc.) and store them in your regular filesytem to be viewed by an android media applciation like vlc. You can also open external programs akka. native android apps from termux, so what i do when i use termux i use something like ranger to view all my in this example video files, find the one i'm looking for and launch the vlc application installed on my android phone.
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Check out Termux's website for more information
Note: I do not endorse nor recommend the use of Android devices or any other celluar device!