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I don't want to have to press a key when I reboot Knoppix S90knoppix-reboot, but I don't see what I've to remove.
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I'm answering my own question here. I was right. There is a knoppix specific /etc/init. I moddified /etc/init.d/knoppix-halt(knoppix-reboot is a symlink of knoppix-halt) to prevent the use of the knoppix specific /etc/init. I commented out a couple lines to look like the following:

# Note: This needs the KNOPPIX version of static "init" which contains
# the "halt" command plus rmmod/umount calls
FINALCMD="/sbin/$command"
#if [ -z "$INSTALLED" ]; then
[ -x /etc/init ] || cp -p /sbin/init /etc/
[ -x /etc/init ] && FINALCMD="/etc/init"
#fi

This prevents the "Press return" message. However, I don't know what the further ramifications of this are. I have not noticed anything amiss as a result, but there is more than "Press return" in the knoppix specific /etc/init. Maybe someone closer to the project could elaborate?