My phone keeps rebooting. I want to know what dmesg says at the moment it reboots, so I can diagnose.

Unfortunately, the reboots don't happen predictably, and only when I'm using it. I can't keep my phone connected to my laptop for days on end. I have it connected via adb right now, and of course the little varlet isn't rebooting. There is no last_kmsg, either.

The version is LineageOS 14. I'm posting here because the answer to this question would presumably apply to all unix-based systems. If you don't like, I suppose punt it over to Android.

What I'm envisioning is it writing to the log constantly until it reboots. I realise that if the phone doesn't reboot itself, the log will eventually fill up the SD card. So I'm picturing a script like:

dmesg to log.1
if log.1 >1mb, delete it 
dmesg log.2
if log.2 >1mb, delete it 
dmesg to log.1

I don't have the skills to write it though. Can anyone help?


If you have the wherewithall to run a bash script this will maintain a file counter and use it to create an circular sequence of dmesg log files.

dir=/path/to/storage                             # Location of persistent storage

[[ -z "$dir/count" ]] && echo 0 >"$dir/count"    # Seed the counter if missing

while sleep 0.25
    count=$(( $(cat "$dir/count") +1 ))          # Read the counter and increment
    [[ $count -gt 600 ]] && count=1              # Reset so we can reuse diskspace

    echo $count >"$dir/count"                    # Save the new value
    dmesg >"$dir/dmesg.$count"                   # Write the data

You will need to review the resulting set of up to 600 log files in date-modified order (ls -t) because they will be written and rewritten like a circular buffer.

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