Moto G OTA update error ~ Less is more

If you get an OTA (over-the-air) update for your Moto G and it fails, here's what you can do. First, a few requirements:

A Moto G. This may work with the Moto E or X as well. YMMV.
Retail Android. This goes without saying, but you won't get an OTA update unless you're running one of the stock/retail ROMs.
Your phone has been rooted or otherwise customized (custom boot logo, custom recovery, etc). This isn't a strict requirement, but it's probably the reason the update is failing in the first place.
These instructions were written for Linux, but they should run with minor adjustments on Mac and a few not-so-minor adjustments on Windows.

There are two main ways to go about resolving this:

Method 1: recommended

This method is the easiest and probably the fastest:

Get the model number for your phone
Settings → About phone → Hardware SKU

Or:

Download the retail firmware for your phone corresponding to the phone's model and build numbers. You can download them from here:
http://www.filefactory.com/folder/c6cdedc45a775d27

For me, I had the Moto G XT1034 with build number KXB21.14-L1.61, so I downloaded this file:
AWSRETAIL_XT1034_4.4.4_KXB21.14-L1.61_cid9_CFC.xml.zip

Here's the easiest way to do this in Ubuntu:
sudo apt-get -y install android-tools-fastboot

For other operating systems, follow the instructions here:
https://developer.android.com/sdk/index.html#Other

Put your phone into fastboot mode. To do so, power off the phone, press and hold the volume down button, press power button, hold both for about 3 seconds and then let go

If the update fails again, you have one of two options. If you don't mind wiping your phone and starting from scratch (which will erase everything), the easiest thing to do is to do fully restore the stock firmware. You'll put your phone into fastboot mode as before, but this time you're going to overwrite everything:

sudo fastboot flash partition gpt.bin sudo fastboot flash motoboot motoboot.img sudo fastboot flash logo logo.bin sudo fastboot flash boot boot.img sudo fastboot flash bootloader bootloader.img sudo fastboot flash recovery recovery.img for file in system.img_sparsechunk.*; do sudo fastboot flash system $file; done sudo fastboot flash modem NON-HLOS.bin sudo fastboot erase modemst1 sudo fastboot erase modemst2 sudo fastboot flash fsg fsg.mbn sudo fastboot erase cache sudo fastboot erase userdata sudo fastboot reboot

If, however, you don't want to completely wipe your phone and you don't mind getting your hands dirty, read on.

Method 2: advanced

At a minimum, you will need to make sure the stock recovery is flashed. Follow the instructions above, flashing only the recovery:

sudo fastboot flash recovery recovery.img sudo fastboot reboot

Now try to apply the OTA update. If it fails, now the fun begins! First of all, if your phone isn't rooted, you'll need to root it to get access to the update log files. If you flash a custom recovery to root, then flash the stock recovery back (the OTA will fail with a custom recovery).

Once your phone is rooted, connect to it using adb and look at the update log, which will be located in /cache/recovery/last_log

adb shell su more last_log

Look particularly at the end of the log file to see what went wrong. This is what mine said:

script aborted: "/system/etc/install-recovery.sh" has unexpected contents. "/system/etc/install-recovery.sh" has unexpected contents. E:Error in /cache/Blur_Version.210.12.61.falcon_umts.AWSRetail.en.US.zip (Status 7) Installation aborted.

You can see that apparently the file /system/etc/install-recovery.sh had been modified, probably from when I rooted the phone.

If the file causing errors is in this list:
- /system/bin/app_process32
- /system/bin/app_process64
- /system/bin/install-recovery.sh
- /system/etc/install-recovery.sh

If another file has been modified, you need to replace the original file. This will take some work. First, you'll need a special utility called simg2img. You can probably find a pre-built binary for your system. I just compiled it myself:

sudo apt-get -y install build-essential git zlib1g-dev git clone https://android.googlesource.com/platform/system/core cd core/libsparse gcc -o simg2img -Iinclude simg2img.c sparse_crc32.c backed_block.c output_file.c sparse.c sparse_err.c sparse_read.c -lz sudo cp simg2img /usr/local/bin

Go to the folder where you extracted the retail firmware, and you should have one or more files starting with system.img_sparsechunk. The next step is to combine them using simg2img:

simg2img system.img_sparsechunk* system.raw.img.tmp

Now we're going to extract the ext4 partition from the image file we made:
offset=`LANG=C grep -aobP -m1 '\x53\xEF' system.raw.img.tmp | head -1 | awk '{print $1 - 1080}'` dd if=system.raw.img.tmp of=system.raw.img ibs=$offset skip=1

Getting close! Now, we'll mount the image we just created:

sudo mkdir /mnt/system sudo mount system.raw.img /mnt/system

Now we should have access to all of the original files from the retail firmware. The next step is to copy them to the device. Here's what I did to copy /system/etc/install-recovery.sh:

adb push /mnt/system/etc/install-recovery.sh /sdcard/

Now connect to the device and get the mount point for the /system partition:

adb shell mount | grep system

Finally, remount /system as read-write, copy the file, and then remount /system as read-only. Don't forget to replace the mountpoints in my example (/dev/block/platform/msm_sdcc.1/by-name/system) with the mountpoint you got from the previous step:

mount -o rw,remount /dev/block/platform/msm_sdcc.1/by-name/system /system cp /sdcard/install-recovery.sh /system/etc/install-recovery.sh mount -o ro,remount /dev/block/platform/msm_sdcc.1/by-name/system /system

If you've made it this far, congratulations! Try the update again. Hopefully, as in my case, it will now work. If not, you'll have to look at the update log file again and see if there's anything else you missed. Thankfully it'll go much quicker now that you've already done all the prep work.

Less is more