Modifying init.rc can be tricky because init.rc is read only once at system boot up, and it’s restored from the recovery image when rebooted.

Therefore, to modify init.rc, you need to modify the recovery image.

I’m writing this post based on Nexus7, but editing init.rc should be similar with other Android devices.

Steps to edit init.rc

1. Download boot.img from Android device
2. Unpack the boot.img
3. Edit init.rc
4. Repack the boot.img
5. Replace the boot.img with new boot image

1. Download boot.img from Android device

First go into Android shell using

adb root
adb shell

, and check out the partitions using this command

cat /proc/partitions

For my device, the command prints out

major minor #blocks name

179 0 15466496 mmcblk0
179 1 12288 mmcblk0p1
179 2 8192 mmcblk0p2
179 3 665600 mmcblk0p3
179 4 453632 mmcblk0p4
179 5 512 mmcblk0p5
179 6 10240 mmcblk0p6
179 7 5120 mmcblk0p7
179 8 512 mmcblk0p8
179 9 14302208 mmcblk0p9
179 32 2048 mmcblk0boot1
179 16 2048 mmcblk0boot0

From the above list of partitions, we need to find out which one is boot partition

In order to find boot partition, run

ls -l /dev/block/platform/sdhci-tegra.3/by-name/

And the command will give the name of each partition like shown below

lrwxrwxrwx root root 2000-01-02 21:22 APP -> /dev/block/mmcblk0p3
lrwxrwxrwx root root 2000-01-02 21:22 CAC -> /dev/block/mmcblk0p4
lrwxrwxrwx root root 2000-01-02 21:22 LNX -> /dev/block/mmcblk0p2
lrwxrwxrwx root root 2000-01-02 21:22 MDA -> /dev/block/mmcblk0p8
lrwxrwxrwx root root 2000-01-02 21:22 MSC -> /dev/block/mmcblk0p5
lrwxrwxrwx root root 2000-01-02 21:22 PER -> /dev/block/mmcblk0p7
lrwxrwxrwx root root 2000-01-02 21:22 SOS -> /dev/block/mmcblk0p1
lrwxrwxrwx root root 2000-01-02 21:22 UDA -> /dev/block/mmcblk0p9
lrwxrwxrwx root root 2000-01-02 21:22 USP -> /dev/block/mmcblk0p6

LNX -> boot.img

From the output above, we know LNX -> /dev/block/mmcblk0p2 is the boot.img

Using fdisk -l /dev/block/mmcblk0p2 to get more information

Copy the image from boot partitions to /data/local/tmp/ directory using this command

cat /dev/block/mmcblk0p2 > /data/local/tmp/boot.img

Now we need to bring the boot.img from android device to our computer using

adb pull /data/lcoal/tmp/boot.img .

This command will copy boot.img to your current directory executing the command

2. Unpack the boot.img

The boot and recover images are not proper filesystems, so we are going to need to unpack the boot image to see the contents in there.

We can extract the image using a script, which you can download from here : Extract-Tool

./unpack-bootimg.pl boot.img

Will extract files such as

boot.img-ramdisk.cpio.gz
boot.img-kernel.gz
boot.img-ramdisk/

3. Edit init.rc

init.rc and other files are in boot.img-ramdisk/ directory, so you can change the bootimage as you want.

4. Repack the boot.img

Run this command to repack boot image

./repack-bootimg.pl boot.img-kernel.gz boot.img-ramdisk newboot.img

repack-bootimg.pl and mkbootimg should be in same directory.

5. Replace the boot.img with new boot image

adb push newboot.img /data/local/tmp/

adb shell

cat /data/local/tmp/newboot.img > /dev/block/mmcblk0p2

Resource

http://droidcore.blogspot.com/2012/12/how-to-edit-initrc-in-android.html

http://android-dls.com/wiki/index.php?title=HOWTO:_Unpack%2C_Edit%2C_and_Re-Pack_Boot_Images#Background

https://forum.xda-developers.com/showthread.php?t=1739119