postinst is in ssh-import-id 3.21-0ubuntu1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | #!/bin/sh
set -e
# Migrate from shell syntax configuration file to JSON to
# smooth migration from ssh-import-id < 3.0. Though this was
# never in Debian, let's still provide this incase some had
# installed the package from Ubuntu.
sed -i -e "/^#/d" -e "s/^URL=\"\(.*\)\"/{\"URL\": \"\1\"}/" /etc/ssh/ssh_import_id || true
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