Tutorial: Revive Android device booted into wrong partition slot

Background

I use a rooted Android device to enjoy several upsides in device capabilities. I occasionally run into trouble with OS upgrades though.

Procedure for installing upgrades

1. Download and install the System Updates
2. Press “Install” in Magisk
3. Choose “Install to Inactive Slot (After OTA)”
4. “Reboot”

Problem

Occasionally I perform the Magisk steps too soon after the System Updates only finish downloading and aren’t installed completely. The device obliges and boots into the inactive partition slot, and enters a bootloop or asks you to wipe the data.

Solution

Connect Android device to Linux system via USB cable and run following commands in the terminal

sudo apt install android-tools-adb
sudo apt install android-tools-fastboot
fastboot devices
fastboot getvar current-slot
fastboot --set-active=b
# If current-slot is "b", set active slot to be "a"