Running a script as Grub boot option

Question

I'm assembling some DR stuff for my Linux boxes. One item that would be really handy would be a GRUB option that would run a script to change my boxes from static IPs to dynamic.

I know we could do a single user mode, run a script, reboot, etc but it would greatly simply things for the other admins (mostly with a Windows background) to select one item and have it complete the process automatically.

I would swear that I've seen configuration settings provided to let Grub do this at one point or another but it's been a while. Not sure what to google for.

EDIT:

I'm aware of the rc.local method, however I don't want this to run on every boot. I want there to be an option listed (line item in grub.conf, probably) which allows for a specific script to be run only when that boot option is selected.

Answer 1

Grub itself can't do this. Its filesystem drivers only support reading.

You can make an entry that loads a Linux kernel (and initrd or initramfs as appropriate) and runs a specific command by passing init=/path/to/script on the kernel command line. That script would do all the requisite mounting, user prompting, file modifications, and finally reboot.

You can also make separate entries that pass a custom argument on the kernel command line. Unknown arguments are ignored, the assumption being that it may be used by some driver which is perhaps loaded as a module. So make Grub entries that pass a kernel argument local.network_interfaces=…, and in /etc/rc.local, look up the value of that argument (</proc/cmdline tr ' ' '\n' | sed -n 's/^local.network_interfaces=//p').

Answer 2

Let the kernel setup network

set netcfg='ip=none'
menuentry 'Switch DHCP on/off' {
   if [ $netcfg = 'ip=dhcp' ]; then
      set netcfg='ip=off'
   else
      set netcfg='ip=dhcp'
   fi
   echo Setting $netcfg
   sleep 1
}
menuentry 'Linux' {
   linux vmlinuz $netcfg
}