Android - How can I run a script on boot?

While looking around my Android filesystem, I found that it did, in fact have a /etc/init.d/ directory. After peeking around in there, I found /etc/init.d/20userinit with the following lines:

if [ -e /data/local/userinit.sh ];
then 
    log -p -i -t userinit "Executing /data/local/userinit.sh";
    busybux chmod +x /data/local/userinit.sh;
    logwrapper /system/bin/sh /data/local/userinit.sh;
    setprop cm.userinit.active 1;
fi;

This being, of course, exactly what I needed, I wrote the following script on my computer then pushed it to my device:

#!/system/bin/sh 

dropbear -s -g 

(pushed to device via scp userinit.sh phone:/data/local/userinit.sh, mind you :] )

Rebooted the device, then ran ps | grep "[d]ropbear", and sure enough, it's running. Coolness!

/data/init.sh runs at boot, if you have root you can edit it as you like. Be careful ;)

Edit: Apparently you might need to shoehorn the edited script into the boot image as well. Info on how to do that here: http://forum.xda-developers.com/showthread.php?t=443994

Look to /etc/ directory. Usually it is placed in /system/ partition which you can mount as RW:

$ ls -l /etc
lrwxrwxrwx 1 root root 11 Jan 1  2009 /etc -> /system/etc
$ su
$ mount -o remount,rw /system
$ chmod o+w /system/etc  # for "adb push"

Some above steps may be replaces with:

$ adb root
$ adb remount

and later remount RO:

$ chmod o-w /system/etc
$ mount -o remount,ro /system

Now your task to find executable or *rc file which you modify to achieve your goal:

$ find /etc -type f -perm +110
$ find /etc -name "*rc"
$ find /etc -name "init*"
$ grep -R /data /etc
$ grep -R /system /etc

Google about each candidate to get know how this file was used.

Good candidate for including custom scripts are lines from:

$ grep service /init*.rc

As each device unique you may need to do own guess about search criteria...

For example I found /etc/mkshrc which used by Korn shell. I update this file to extend PATH env var and now each time I do adb shell I have Busybox symlinks in my PATH!

See also hard way (if you have no luck with finding magic file): https://stackoverflow.com/questions/9768103/make-persistent-changes-to-init-rc