Going Back to Android

Going Back to Android

In one sentence: Android never leaves the device; sudo mu300-next-boot android and a reboot take you back, su -c mu300-linux in Android takes you to Linux again, and ./uninstall.sh removes Linux completely.

The simple version: the device is a house with two rooms. Linux is the room you walk into by default. If the Linux room's door jams a few times in a row, the device walks into the Android room by itself. You can also just walk over whenever you like.

How the automatic fallback works

flowchart TD
    P[Power on] --> LK[Bootloader picks the Linux slot<br/>as a trial]
    LK --> S{Does Linux finish starting?<br/>USB network and SSH up,<br/>then mu300-boot-ok after ~30 s}
    S -->|yes| OK[Boot confirmed<br/>failed-boot count resets]
    S -->|no: power cut, hang, crash| C[One try used up]
    C --> N{N failed boots in a row?<br/>N = 1-6, default 5}
    N -->|not yet| P
    N -->|yes| A[Android starts<br/>and stays the default]
    L[Locked Linux] -.->|never counts down| OK

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• The count is chosen at install (1-6, default 5). Change it with sudo mu300-next-boot attempts N.
• A boot that never reaches mu300-boot-ok (about a minute after power-on) counts as failed. One good boot resets the count.
• If Linux does not come up at all, the device also returns to Android, unless Linux is locked.

Switching by hand

I want to…	Do this
go to Android from Linux	sudo mu300-next-boot android, then sudo reboot (panel: Switch to Android)
go to Linux from Android	su -c mu300-linux on the device, or the Action button of the "MU300 Linux" Magisk module
see which system is on which slot (Android)	su -c 'mu300-linux status'
go to Linux from Android, from a computer	boot/android-boot-linux.sh work/boot-linux-slotb.img
make Linux the default again	sudo mu300-next-boot linux
check the boot state	sudo mu300-next-boot status

The installer puts the Magisk module in Android for you. It writes only the same 32 bytes of misc that the installer does. On the panel, "Switch to Android" warns when band or cell locks are still set (those live in the modem and also apply under Android) and offers to reset them first.

No password, no computer

If you forgot the password and Linux boots by default: unplug the device about ten seconds after it powers on, plug it in again, and repeat as many times in a row as your failed-boot count (5 by default). The device then sees unfinished boots and falls back to Android. A new password can be set from a computer with tools/reset-password.sh without booting Linux (see Troubleshooting). This does not work while Linux is locked.

Locked Linux

sudo mu300-next-boot lock      # or "Lock Linux" on the panel's home page
sudo mu300-next-boot unlock    # back to the failed-boot attempts

Locked, the device never goes to Android by itself: not after failed boots, and not when the system does not start. In that case it waits on USB with a telnet shell at the device's address, where sh /run/to-android goes to Android. Android is then only by hand: mu300-next-boot android or "Switch to Android" on the panel.

The one exception is a new kernel: mu300-update gives a new boot image its first boots on the usual attempts, so a kernel that does not start still falls back, and the first good boot locks again.

Uninstalling

With the device back in Android:

./uninstall.sh                # macOS / Linux
.\uninstall.ps1               # Windows (PowerShell)
uninstall.cmd                 # Windows (cmd)

It makes Android the boot system again, restores the second boot partition and erases the Linux filesystem. Your Android data is left alone. Like the installer, it offers to reboot the device from Linux into Android first.

Erase choice	What happens
secure (default)	overwrites the whole Linux region, then checks it is empty; takes a few minutes
quick	erases only the filesystem headers; files stay readable on the flash until reused
keep	leaves the Linux filesystem; it simply never boots again

On an SD card: erase (default) overwrites the first 64 MiB, which is quick (the files stay readable until the space is reused); keep leaves the card. A card with any other filesystem is never touched.

If you shrank userdata on the 32 GB variant, the uninstaller offers to grow it back. That is off by default and asks twice, because it erases Android's data again (and on the 64 GB device would hand Android the free area it has always had).