I want to install TWRP on my phone that uses the A/B partition schema. The official TWRP installer ZIP by default installs to both boot_a and boot_b. This always breaks the system in the inactive slot because I install Magisk after flashing/OTA.
Can I install TWRP to only one slot, or an offline boot.img file (like Magisk Manager supports)?
So this way, I ended up creating a script on my own.
Lucky enough, installing TWRP to boot.img isn't hard at all. The install script in TWRP just replaces ramdisk.cpio with the one ramdisk-twrp.cpio provided in the ZIP.
The effective part of the script is as following, root is of course required.
ln -s /system/bin/linker64 /sbin/linker
cp <path-to-magiskboot> /sbin/magiskboot
mkdir /data/local/tmp/twrp
cd /data/local/tmp/twrp
/sbin/magiskboot --unpack <path-to-boot.img>
cat <path-to-[ramdisk-twrp.cpio]> > ramdisk.cpio
/sbin/magiskboot --repack boot.img
cp new-boot.img /sdcard/new-boot.img
Now the TWRP-installed new-boot.img is located at /sdcard/.
To install to a single slot, dd the boot image out and apply the same process as shown above and dd the new image back.