Make persistent changes to init.rc

Question

I want to change the init.rc file of an android pad. But after I change it and reboot the system, the original init.rc comes back.

How can

Answer 1

Your root partition (where /init.rc lives) is a ramdisk which is unpacked from an initrd file and mounted every time your device boots. Any changes you make are to the ramdisk only, and will be lost on the next reboot.

If you can get the initrd file, you can mount it on your Linux host system, modify the files there, unmount it, and write it back to your Android.

The initrd file exists in its own partition on the device. If you can figure out which partition it is, you can grab it from the device onto your host, mount it, modify it, and write it back to the device. This is what tripler was talking about above.

In general, modifying boot.img is something that only system developers do. If you're building the entire Android system, you'll have access to the necessary source code. My workflow for this looks like this:

# Modify init.rc
m -j8 bootimage_signed
adb reboot bootloader
fastboot flash boot $OUT/boot.img
fastboot reboot

Answer 2

Please note that it may be easier for you to use an app like Scripter to run a script at boot time than modify this file.

Before following @tripler's instructions above you need a file called boot.img which can be extracted by (run on rooted Android device, untested without root):

dd if=/dev/block/platform/<someplatform>/by-name/boot of=/sdcard/boot.img

Then connect your Android to your computer and copy the boot.img file from there.

Script:

http://linuxclues.blogspot.ca/2012/11/split-bootimg-python-android.html

Here is a modified, easier to see version of tripler's instructions (assuming boot.img is in tmp):

cd /tmp
mkdir fs
# Now use the linked script above to split the boot.img file into ramdisk.gz and kernel
python split_boot_img.py -i boot.img -o parts
cd fs
gunzip -c ../parts/ramdisk.gz | cpio -id
# make changes to init.rc

At that point you will have to rebuild the boot.img back together before reflashing, which will be device-specific. Can't help you with that, sorry!

Answer 3

You have to edit/change the init.rc before building your Android pad file system. This is the preferred way, and always works.

Answer 4

Unpack the uramdisk using following command in host PC(Linux)

mkdir /tmp/initrc cd /tmp/initrd
sudo mount /dev/sdb1 /mnt          

sdb1 is partion where uramdisk/uInitrd resides.

dd bs=1 skip=64 if=/mnt/uInitrd of=initrd.gz
gunzip initrd.gz

At this point running the command file initrd should show:

mkdir fs
cd fs
cpio -id < ../initrd

Make changes to init.rc

Pack uramdisk using following commands:

find ./ | cpio -H newc -o > ../newinitrd
cd ..
gzip newinitrd
mkimage -A arm -O linux -C gzip -T ramdisk -n "My Android Ramdisk Image" -d newinitrd.gz uInitrd-new

Answer 5

A number of Android devices include code to prevent root modifications to the system files. The way this is done is by using the recovery partition. On reboot, they basically restore the system partition using the recovery image. If your system is doing that then you cannot make persistent changes - the best you could do would be to hook up something to run after reboot to re-apply your change. In CyanogenMod they had hooks in the init.rc to run sdcard scripts if found. Perhaps you can create an app or widget to then launch a script to make the mods required using a setuid root script from the data partition. Without building your own ROM you are quite restricted in this area.

Possibly you could fetch the recovery image and try unpacking that, making your changes and repacking and flashing it. But make sure you can recover with fastboot before you try this.

Answer 6

Try this site: http://bootloader.wikidot.com/linux:boot:android Read the section at the bottom: •The Android boot image: boot.img ◦Unpack, re-pack boot image: http://android-dls.com/wiki/index.php?title=HOWTO:_Unpack%2C_Edit%2C_and_Re-Pack_Boot_Images#Background