Kickstart Pre- and Postinstall Scripts

Kickstart installs the base OS for you, but it’s rare enough. Typically you have to further customize the setup to meet your (or your organization) policies and procedures.

Typical additional steps include adding banners, create some canonical directory structures, install non-RPM’d software, or modify specific config files. Kickstart allow you to run some code before and after it does its job in pre- and postinstall scripts, respectively. Here will we discuss post install scripts exclusively.

Pre and post install scripts are special section of ks.cfg, labeled %pre and %post. (These sections get extracted into separate files and executed at build time.) For example:

## ... other Kickstart directives, such as the network config 
## and root password ... 

## postinstall script 
%post 
PATH=/sbin:/usr/sbin:/bin:/usr/bin 

## disable printing daemons 
chkconfig cups off

## create our special mount point area 
mkdir /mnt/oradb 

## disable firewall 
chkconfig iptables off
chkconfig  ip6tables off

By default both the %pre and %post sections run under a plain Bourne shell (/bin/sh) by default. Append the --interpreter flag to specify a different interpreter, such as /usr/bin/perl or /usr/bin/python. Your main limitation is the tools available at the time the scripts are run, though: a preinstall script executes in the realm of the boot media, which includes just a limited toolset for rescue operations.

The postinstall script runs chrooted inside the freshly installed OS. It therefore has access to any tools you installed on the target machine. If your OS build included Perl or Python, then, your postinstall script could run code in those languages. As an alternative, you can specify %post’s --nochroot flag to not run the postinstall script inside the chroot. In this case, your postinstall script will have the same limited toolset as the preinstall script, but you then have the option of performing some other activities and manually invoking chroot to work inside the chroot area.

Postinstall scripts can perform tasks you want to handle each and every time after the install but before the machine reboots into full-user mode. For example, you can call chkconfig to disable unneeded services:

for SERVICE in avahi-daemon avahi-dnsconfig cups  ; do 
   chkconfig ${SERVICE} off 
done 

But it is important to keep post-install script simple and flexible. If your pre- and postinstall scripts do more than disable services or create directory paths, ask yourself a few questions:

• Is this too complex? Because postinstall scripts run in the context of a Kickstart build, they are more difficult to test and generally should be very short.
• Can I do this better by hand? It’s a noble goal to offload work to Kickstart, but you’ll eventually reach a point of diminishing returns. And if you do just a few installs per quater that point might be close to zero postinstall scripts. Many things can be done just after reboot in a normal enviroment with the same scripts. Return on investment of putting them into kickstart is the greatest if you do mass installs on a regular basis. After that it quickly fades.
• Can I abstract some of this functionality into a separate tool that can be useful in other circumstances too? You can trim your debug time by moving most of the logic into a separate script that you can run and debug outside of Kickstart. In turn, your %pre and %post sections then become one-liners to invoke these external scripts.
• Should I be doing this here? Build process in not the right place for such tasks as creation of application-scecific accounts like oracle and dba. Those tasks belong elsewhere. Other tasks shouls be performed with a formal change-management enables

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Last modified: March, 12, 2019