It already exists:
https://lists.debian.org/debian-user/2017/04/msg00097.html
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You can just append:
preseed/late_command="in-target apt-get install -y sysvinit-core"
to the installer command line.
Or you can roll your own install media with its own syslinux.cfg which
adds that or something more complicated in a preseed file.
You don't need to fork the installer, or submit any patches upstream.
If you want something more complicated, like not installing systemd at
all, you'll have to pass --include and --exclude options to debootstrap
using the base-installer/includes and base-installer/excludes preseed
options; something like:
base-installer/includes=sysvinit-core base-installer/excludes=systemd-sysv
but that's totally untested.
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