I use my Android device for a lot of things: from booking train tickets and checking weather forecasts, to... Well, pretty much everything else. But for some reason, I've never seriously considered it to be useful as a camera tool. While I was tinkering with Termux, it occurred to me that it can transform an Android device into a camera companion that can perform both photo backup and processing duties.
Unfortunately, Termux can't access USB devices connected to the Android devices, so the first step of transferring JPEG and RAW files from a camera to the device must be done manually. Fortunately, it's a pretty straightforward thing to do: connect the camera or USB card reader to the Android device, and the system automatically detects and mounts the storage. Then it's a matter of using the default transfer utility to copy data from the camera or storage card to the Android device.
Once the photos and RAW files are on the device, you can access and manipulate them from within Termux using shell scripts. What you want to do with the files is up to you. I cobbled together a handful of scripts that I use to back up the transferred files to a remote machine, remove EXIF metadata, resize JPEG photos, and apply Hald CLUT presets. The backup script replaced the FolderSync Pro app, while the Hald CLUT script lets me use my own presets. And since ImageMagick, ExifTool, and other useful tools are available in the Termux repository, you can script practically any processing task imaginable.
If you want to know more, the Linux Photography book provides an in-depth description of how to use Termux and the shell scripts to turn an Android device into a photo backup and processing camera companion.