• Map: Android device setup

    Anton Yaky7 days ago 0 comments

    To display maps, I need a device that has:

    • GNSS/GPS receiver
    • Offline maps
    • Video output

    My trusty and crusty old Samsung Galaxy S5 has all that functionality, even video output to HDMI via MHL.

    What I want the device to do is:

    • Boot as soon as I start the car (i.e. device is connected to power).
    • Launch a navigation app after booting.
    • Shut down if the car is off for some time (i.e. device is not connected to power but has some battery left)

    For the map/navigation application, I chose Organic Maps, as it is lightweight, has a great interface, and works offline. I have used it for many years now.
    Other options I considered:

    • OsmAnd - more customizable, the OG of OpenStreetMap navigation
    • Osmin - similar to Organic Maps, and works great on Linux (Raspberry Pi, PinePhone etc.)

    Boot on plug

    The recommended method is to enable this function through fastboot. This does not work on Samsung devices, though, but I will leave it here for reference.
    Boot to fastboot mode (aka download mode) on the device.
    Install fastboot on main machine and run:

    fastboot oem off-mode-charge 0
    

    Since my device is a Samsung, I rooted it with Magisk, and then downloaded and installed Magisk Autoboot module. Although their documentation states it works for Android 7, I could not get it to work with custom Lineage 14 (Android 7 equivalent) for some reason. On a clean Lineage 15, it works, and S5 takes about a minute to boot after connecting power.

    Launch app after boot

    I installed Termux and Termux:Boot via F-Droid, and ran Termux:Boot once to initialize it.

    Per instructions, I created a startup script:

    mkdir -p ~/.termux/boot
    nano ~/.termux/boot/start-maps.sh
    

     And set the script to start Organic Maps:

    #!/data/data/com.termux/files/usr/bin/sh
    am start app.organicmaps/.SplashActivity

    This should also be possible to implement using Tasker, but I have not tried this. Might be easier if you don't like the terminal.

    According to a StackExchange post:

    • Create a Profile Event / System / Device Boot
    • Create a Task with Action: App / Launch App / (select the app)
    • Tasker also recommends adding Task - Wait for around 5 seconds first before doing other tasks to ensure that the system is ready after booting.
    • Link the Profile to the Task

    Shutdown on condition

    I implemented this using Termux, Termux:API and crontab.
    This requires root, because shutdown is a system function.

    To check the battery status, I used Termux and Termux:API extension:

    termux-battery-status

    This returns JSON, which can be parsed using jq.

    pkg install jq

    Now I can extract the values I need, for example:

    batt_pct=`termux-battery-status | jq -r '.percentage'`
    batt_status=`termux-battery-status |jq -r '.status'`

    To shut down Android:

    su -c "reboot -p"     

    Note: There are multiple ways to shut down an Android device, and many of those commands work fine in user's Termux session, but are very slow and buggy when they are run through crontab.

    Putting it all together: (with an extra spoken message using another API)

    batt_pct=`termux-battery-status | jq -r '.percentage'`
    batt_status=`termux-battery-status |jq -r '.status'`
    if [ $batt_pct -lt 60 ] && [ "$batt_status" = 'DISCHARGING' ]
    then
        termux-tts-speak 'Low power. Shutting down.'
        su -c "reboot -p"
    fi
    

    I put this in a file at

    /data/data/com.termux/files/usr/bin/yaky-auto-shutdown

    This directory is in $PATH, so there is no need to specify the full path in the future.

    To automate this script, I used crontab:

    pkg install cronie
    crontab -e

    Added an entry to run the auto-shutdown script:

    * * * * * yaky-auto-shutdown

    To run the cron daemon, I added another script to execute at boot:

    nano ~/.termux/boot/start-cron.sh

    And the script is:

    #!/data/data/com.termux/files/usr/bin/sh
    termux-wake-lock
    crond
    

    This sets a wakelock (to prevent Android from killing the long-running crond process), and then launches crond.
    Every minute, cron runs the script, which checks the battery percentage and status, and shuts down the device if the specified conditions are met. Although shutdown command requires root, root privileges...

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