- enable developer tools / USB debugging
- adb push supersu.zip /sdcard
- adb reboot bootloader
- fastboot oem unlock
- fastboot boot twrp-126.96.36.199-shieldtv-unofficial.img
- unplug USB cable, connect mouse via USB, install SuperSU
TWRP boot image is attached. Off to explore...
EDIT: by request, some more detailed instructions:
- On the Shield device, head to Settings -> About, and click "Build number" 7 times. This will enable Android's Developer options.
- Go to Settings -> Developer options, and enable USB debugging.
- Install the ADB/fastboot drivers and utilities if you don't already have them. When "adb devices" shows your Shield, you're good to go.
- Download and extract recovery.zip from this thread.
- Head to the SuperSU forum and download the latest version.
- Open a command prompt and navigate to the directory where you downloaded everything. (Google this for your OS if you don't know how).
- With the Shield still booted, run "adb push supersu.zip /sdcard" (change supersu.zip to appropriate file name). This will upload the SuperSU update zip to your device, which you'll install from TWRP.
- If that worked fine, run "adb reboot bootloader". Your device should shut down, and display the Fastboot screen after a few seconds.
- FOLLOW THE NEXT THREE INSTRUCTIONS AT YOUR OWN RISK. This WILL wipe all your user data, back it up with ADB first if you want to preserve it. DISCLAIMER #2: I have no idea if there are any DRM keys or anything that get wiped when this is done (this was the case on my Xperia Z3 Compact). I will say that Grid streaming still works fine after doing this.
- This step will unlock your bootloader to enable booting unsigned images.
- Run "fastboot devices" and make sure your Shield is visible.
- Run "fastboot oem unlock" and follow the prompts on the screen. Your bootloader is now unlocked.
- Now, boot the TWRP image with: "fastboot boot twrp-188.8.131.52-shieldtv-unofficial.img" - Note: I used this instead of "fastboot flash" so I could keep the stock recovery around, to grab an image of it after rooting.
- You should now see a TWRP recovery menu (upside down for now).
- Connect a keyboard or mouse via OTG cable, and select "Install", navigate to your SuperSU zip, select it, and follow the prompts.
- Click "reboot" and you should now be rooted.