How to get IMEI Number From adb Command
Given the tag 7.0-nougat, I'm assuming you have or are targeting Android Nougat. Android Nougat comes with toybox which acts as an alternative to busybox and has some useful utilities. I managed to use those tools to print IMEI like this:

adb shell
service call iphonesubinfo 1 | toybox cut -d "'" -f2 | toybox grep -Eo '[0-9]' | toybox xargs | toybox sed 's/\ //g'


Since you wouldn't be getting an interactive shell when using a script or an app, you can pass the commands like this:

adb shell "service call iphonesubinfo 1 | toybox cut -d \"'\" -f2 | toybox grep -Eo '[0-9]' | toybox xargs | toybox sed 's/\ //g'"