Couldn't acquire root after running superboot
Because that way (superboot) is old and doesn't work on the current software releases, TBH I am surprised it still boots at all after that, probably because you just just manually booted it and didn't flash it. 

I will assume this is the Titan (non-LTE) version. The proper way to do this if you are unlocked is simple... Download the latest TWRP for your device and SuperSU and place the later on your device.

Start the device in bootloader mode, and in the terminal enter

sudo fastboot boot twrp.img &lt;- Replace filenames with appropriate ones for your installation

Once TWRP is loaded, Install (flash) SuperSU ZIP you downloaded from the link above.

Reboot, enjoy root on your device. 

BTW, you didn't mention your Linux distro or fastboot version, you may need to get the current releases from the Android Dev package if your repos do not have current ones.