/lib/partman/init.d/01early_command is in ubiquity 2.18.7.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 | #! /bin/sh
set -e
if [ -f /var/lib/partman/early_command ]; then
exit 0
fi
preseed_command partman/early_command
mkdir -p /var/lib/partman
touch /var/lib/partman/early_command
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