Altering ramdisk.img

If you have started building the Android source tree yourself, you have undoubtedly wondered how to modify the image files.  In this post, we'll look specifically at ramdisk.img. This is based on the work found here.

Let's pretend our directories are:
~/work/android: directory where you did your git, envbuild, lunch/choosecombo from.
~/work/tmp: empty directory for us to play in

If you have already done a build, you will find out/target/product/generic/ramdisk.img.  This is a gzipped cpio file.
Extracting ramdisk.img

~/work/tmp$ cp ~/work/android/out/target/product/generic/ramdisk.img ./ramdisk.cpio.gz

~/work/tmp$ gzip -d ramdisk.cpio.gz

~/work/tmp$ mkdir tmp2

~/work/tmp$ cd tmp2

~/work/tmp/tmp2$ cpio -i -F ../ramdisk.cpio

((modify files... for example, init.rc or init.goldfish.rc))

Rebuilding ramdisk.img

~/work/tmp/tmp2$ cpio -i -t -F ../ramdisk.cpio | cpio -o -H newc -O ../ramdisk2.cpio

~/work/tmp/tmp2$ cd ..

~/work/tmp$ gzip -c ramdisk2.cpio > ramdisk.img

Now you should be able to use the new ramdisk.img with your emulator =)