expr EXPERSSION returns 1 when the
EXPERSSION is empty or 0. Thus, you cannot really use it to do arithmetic.
For example, you may find your script doing
expr 1 - 1 returns as error.
To be fair, man page
expr actually mentions this. However, instead of a dedicate section EXIT STATUSES, it is hidden in the second last paragraph of DESCRIPTION.
So do yourself a favour, use BASH arithmetic like
KDE Connect connects between your mobile and Linux, wirelessly.
You can copy photos, videos, or other files from mobile, or vise versa.
You can use your mobile as remote control of Linux media player, or even wireless mouse and keyboard. On the other hands, the clipboard of mobile and Linux are shared, so you can use your favorite desktop keyboard and input methods on mobile applications, like Clash-of-Clans and WeChat. The mobile and Linux should be at the same subnet, though.
Another interesting way to use KDE Connect is replacing Yubikey. I use the Yubikey almost everyday. Consequently, it become loose contact, so I need to wiggle it to get the contact. KDE Connect with Google authenticator or FreeOTP Authenticator might to the trick.
AirDroid is feature richer, however, you need to register to AirDroid to use Airdroid, but with KDE Connect you just need to pair it, and the connection is encrypted. Most importantly, AirDroid is not open source.
So far, KDE connect is available in Google play and most major Linux distribution, including Fedora. iOS version is not yet in app store though.