How to connect iPad to Macbook I have a macbook air running Mavericks, and I want to transfer files to and from an ipad. When I connect them via usb, the ipad does not appear in the filesystem anywhere, and I cannot mount it via Disk Utility either. 

Some things I have seen recommended elsewhere: 


Upgrade the Macbook's operating system to Sierra, then upgrade iTunes, then transfer individual files via iTunes. I want to avoid this if possible, and I have deleted iTunes because it was causing problems. Also, I want to be able to transfer files which are not compatible with iTunes. 
Jailbreak the iPad. I also wish to avoid this; the iPad is not mine and it would void the owner's warranty. 


I use this for accessing the filesystem on my android device via my macbook and it works fine. Is there something like "adb" for the ipad so I can connect via the command line?
In short, no. 

Android does not sandbox apps like iOS does. There are many apps that expose the filesystem without jailbreaking, but every iOS app doesn’t look outside it’s own sandbox for files. 

Workarounds:


use a filer app like Dropbox, box, documents (I believe by readdle) and iTunes to transfer all your files to that app sandbox. 
wait for iOS 11 when general file management arrives
use any of the dozens of cloud sync options. (Some of these even have the ability to set up a local cache so the Mac uploads to a server on your network and the iPad downloads from that local server the content but almost all end up,putting a copy of your file in the cloud somewhere outside your local network)


As to your points. 


iTunes will write just about any file to the container apps. If the iOS app tells iTunes to restrict files to a pattern, that’s more on the iOS app you choose to contain the files than iTunes which will transfer any file to a general storage app. 
Not a good idea as jailbreak aren’t widely shared anymore and also, you can get official apps that do most of what jailbrreaking used to provide.
most apps on iOS just play by the rules and use document share scheme and the upcoming filer API with iOS 11. Since iOS implemented legitimate doc sharing, all the apps that used to do this (sugar sync, cubby, all manner of WebDAV and filer apps ) have withered or died.