As per the title:
after install completes is it possible to have system just halt instead of rebooting
I’d like to start the TW install, go away and come back and find the system shutdown after the install finishes instead of it rebooting like it normally does.
Is that possible?
During installation the system reboots, because it then switches from the installation medium to the fresh installed system. But the installation process as a whole is not complete then. Installation will be finished during that reboot and then it will be ready for usage. When you mean this reboot, then replacing it with a shutdown/power off, will leave you with an incomplete installed system, which is contrary to your explanation “system shutdown after the install finishes”.
What happens after a reboot depends on which installation type you do. For example, a server (GUI-less) installation doesn’t have anything else that it runs, and you just get a login prompt at the console.
For a GNOME installation, if you have autologin turned on, you get the screen:
So still not really any installation activities going on post-reboot. I could repeat with KDE, but I think I’ll find the same thing - the autologin has you signed in as a user rather than as root, so there’s not any system-level configuration that takes place in the foreground, and if it’s in the background, it doesn’t matter to the user if the system was rebooted or powered off, and then powered on later. Updates are about the only thing that would happen, and those are going to happen regardless.
For the OP:
It doesn’t appear that there is a way to do this - there are two parameters you can pass at boot time that control the reboot - one is kexec_reboot, which controls whether or not kexec is used for the reboot. The second is reboot_timeout, which changes the countdown timer’s duration. A 0 disables it.
Oh, there is also WaitReboot which has no description, but the name doesn’t suggest that it changes the reboot behavior. (Looking at the sources, it doesn’t seem to do anything, and there’s a comment suggesting that it’s been dropped or should be).
If you start the installation using reboot_timeout=0 on installer cmdline, then you can jump to vtty2 and issue command systemctl halt on reaching the “system will reboot” screen.