tty doesn't list ttyUSB0 so arduino doesn't list serial ports Okay so lsusb gives 
Bus 002 Device 019: ID 1a86:7523 QinHeng Electronics HL-340 USB-Serial adapter

dmesg|grep 'ttyUSB' gives

[63723.589107] usb 2-1: ch341-uart converter now attached to ttyUSB0

but tty lists only /dev/pts/0 although in /dev there is a ttyUSB0 file (or whatever it is because vim says its not a file)

I can use cu -l /dev/ttyUSB0 -s 9600 and transmit to arduino nano as the rx LED blinks. On starting arduino I get the "java.lang.NullPointerException thrown while loading gnu.io.RXTXCommDriver" error.

System: Linux Mint 19.1 Tessa (64 bit)
Sorry, I was using the software center to get arduino IDE and that didn't work well. Downloading the official IDE from arduino.cc worked without hassle. Lesson learnt: Not all linux distros have package repos as up-to-date as Arch. :P