Using postinstall scripts

While running genimage to generate diskless or statelite osimage, you may want to customize the root image after the package installation step. The postinstall attribute of the osimage definition provides a hook to run user specified script(s), in non-chroot mode, against the directory specified by rootimgdir attribute.

xCAT ships a default postinstall script for the diskless/statelite osimages that must be executed to ensure a successful provisioning of the OS:

lsdef -t osimage -o rhels7.3-ppc64le-netboot-compute -i postinstall
Object name: rhels7.3-ppc64le-netboot-compute
postinstall=/opt/xcat/share/xcat/netboot/rh/compute.rhels7.ppc64le.postinstall

Customizing the postinstall script, can be done by either one of the methods below:

  • Append your own postinstall scripts

    chdef -t osimage -o <osimage> -p postinstall=/install/custom/postinstall/rh7/mypostscript

  • Create your own postinstall script based on the default postinstall script

    cp /opt/xcat/share/xcat/netboot/rh/compute.rhels7.ppc64le.postinstall /install/custom/postinstall/rh7/mypostscript
# edit /install/custom/postinstall/rh7/mypostscript
chdef -t osimage -o <osimage> postinstall=/install/custom/postinstall/rh7/mypostscript

Common questions about the usage of postinstall scripts:

When do postinstall scripts run?

High level flow of genimage process:

  1. install the packages specified by pkglist into rootimgdir directory
  2. cumstomize the rootimgdir directory
  3. generate the initrd based on the rootimgdir directory

The postinstall scripts are executed in step b).

Do postinstall scripts execute in chroot mode under rootimgdir directory?

No. Unlike postscripts and postbootscripts, the postinstall scripts are run in non-chroot environment, directly on the management node. In the postinstall scripts, all the paths of the directories and files are based on / of the managememnt node. To reference inside the rootimgdir, use the $IMG_ROOTIMGDIR environment variable, exported by genimage.

What are some of the environment variables available to my customized postinstall scripts?

Environment variables, available to be used in the postinstall scripts are listed in postinstall attribute section of linuximage