Kickstart %pre script examples in RHEL CentOS 7 8

We use Kickstart to perform automated installation with Red Hat/CentOS and Fedora (or any other OS using Red Hat's environment). You have the option of adding commands to run on the system before actual installation starts using %pre. So in this article we will learn about Kickstart's %pre script examples in RHEL/CentOS 7/8

What is Kickstart %pre script?

The %pre scripts are run on the system immediately after the Kickstart file has been loaded, but before it is completely parsed and installation begins
All the Kickstart %pre install script must start with %pre and end with %end line
Commands related to networking, storage, and file systems are available to use in the %pre script, in addition to most of the utilities in the installation environment /sbin and /bin directories.
The %pre script does not run in the chroot environment.

What is Kickstart %pre-install script?

The commands in the %pre-install script are run after the following tasks are complete:
- System is partitioned
- File systems are created and mounted under /mnt/sysimage
- Network has been configured according to any boot options and kickstart commands
Each of the %pre-install sections must start with %pre-install and end with %end.
The %pre-install scripts can be used to modify the installation, and to add users and groups with guaranteed IDs before package installation.
The %pre-install script does not run in chroot environment.

Kickstart %pre script examples

Below are some kickstart %pre script examples

Example 1: Store logs from kickstart %pre script

We can use --log and define the path of a log file (if not found, a new file will be created) to store the logs of %pre script execution. In this kickstart %pre script example I will checking systems stats and storing them in /tmp/kickstart_pre.log

HINT:

You can also define set -x in the %pre section to enable debug mode

%pre --log=/tmp/kickstart_pre.log

echo "Currently mounted partitions"
df -Th

echo "=============================="
echo "Available memory"
free -m
echo "=============================="
echo "Kickstart pre install script completed at: `date`"
echo "=============================="

%end

By default any log file you create at kickstart %pre section will not be available after the installation completes. So we must use post scripts to copy these log files once the installation completes.

Example 2: Execute scripts at Kickstart %pre section using --interpreter

--interpreter= allows you to specify a different scripting language, such as Python. Any scripting language available on the system can be used; in most cases, these are /usr/bin/sh, /usr/bin/bash, and /usr/libexec/platform-python.

In this Kickstart %pre script example I have written some dummy python and bash script and storing the output in separate log files

%pre --interpreter=/usr/libexec/platform-python --log=/tmp/kickstart_python_pre.log
print("This is a sample python script called at %pre stage")
%end

%pre --interpreter=/usr/bin/bash --log=/tmp/kickstart_bash_pre.log
echo "This is a sample bash script called at %pre stage"
%end

Assuming that you have copied the log file, verify kickstart_bash_pre.log and kickstart_python_pre.log on the client node, after the kickstart installation completes.

# cat /var/log/kickstart_python_pre.log
This is a sample python script called at %pre stage

# cat /var/log/kickstart_bash_pre.log
This is a sample bash script called at %pre stage

Example 3: Create pre-install script

In this kickstart %pre-install script example we will get our system environment details

%pre-install --log=/tmp/kickstart_pre_install.log

echo "Currently mounted partitions"
df -Th

echo "=============================="
echo "Available memory"
free -m
echo "=============================="
echo "Kickstart pre install script completed at: `date`"
echo "=============================="

%end

Assuming that you have copied the log file, verify kickstart_pre_install.log on the client node, after the kickstart installation completes. You can check the difference of partition layout between %pre and %pre-install section.

# cat /var/log/kickstart_pre_install.log
Currently mounted partitions
Filesystem           Type      Size  Used Avail Use% Mounted on
devtmpfs             devtmpfs  1.9G     0  1.9G   0% /dev
tmpfs                tmpfs     2.0G  4.0K  2.0G   1% /dev/shm
tmpfs                tmpfs     2.0G   25M  2.0G   2% /run
tmpfs                tmpfs     2.0G     0  2.0G   0% /sys/fs/cgroup
10.10.10.12:/images/ nfs        22G   15G  6.0G  71% /run/install/repo
/dev/mapper/live-rw  ext4      2.9G  2.0G  936M  69% /
tmpfs                tmpfs     2.0G   21M  2.0G   2% /tmp
/dev/sda3            ext4       17G   45M   16G   1% /mnt/sysimage
/dev/sda1            ext4      976M  2.6M  907M   1% /mnt/sysimage/boot
tmpfs                tmpfs     2.0G     0  2.0G   0% /mnt/sysimage/dev/shm
==============================
Available memory
              total        used        free      shared  buff/cache   available
Mem:           3940         487        2572         100         879        3121
Swap:          2047           0        2047
==============================
Kickstart pre install script completed at: Sun Apr 26 11:41:45 UTC 2020
==============================