Interactive pre-/post-install scripts in RedHat KickStart

RedHat Enterprise Linux, Fedora, CentOS and similar systems can be auto-installed using KickStart scripts. Unfortunately the KickStart concept is not designed to conveniently interact with the operator. One of the most Frequently Asked Questions about KickStart is “How do I enter a hostname and IP address during installation?” That’s indeed a common and valid question but there’s no well known answer.

If you’re in position to install dozens of RedHat machines, perhaps a classroom full of workstations, you will probably look at using a customised DVD with a kickstart file tuned to your needs. In such a case you want all the machines be exactly the same, except for their hostname and IP address. Sure, you can use DHCP to set both, but in some cases that’s not possible. It may be better to ask for the hostname and IP during installation. How to do that?

KickStart supports custom pre-install and post-install scripts, but normally doesn’t let the user see the output or enter any input. But it is possible to do that:

# Install in text mode.
install
text
[... all the other kickstart settings ...]

%pre
# Pre-install script — beware that at this point
# the system is not yet installed and the target
# filesystem may not yet be created. That means
# you can't yet do any changes to it!

# This is the trick — automatically switch to 6th console
# and redirect all input/output
exec < /dev/tty6 > /dev/tty6 2> /dev/tty6
chvt 6

# We can break into a shell prompt
/bin/sh

# Or even run a python script
python << __EOF__
# Any other Python code can come here
print "This is pre-install shell"
__EOF__

# Then switch back to Anaconda on the first console
chvt 1
exec < /dev/tty1 > /dev/tty1 2> /dev/tty1

%post
# Same chvt/exec magic as above
# Post-install by default runs chrooted in the just installed system,
# feel free to ask for hostname and IP address and update the system files ;-) 

Just bear in mind that at the moment the pre-install script runs the target filesystem is not yet available and you therefore can’t customise it. Wait for post-install to do that ;-)

That’s about it. Once your pre/post script gains control over the 6th console you can do whatever you like, ask some questions, or at least see the output of your commands. Even that is a huge convenience improvement!

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