How to read and insert new values into the debconf database

I have already described how to copy answers to the configuration questions for Debian packages, but there is an easier way to read and insert new values into the debconf database.

You will need debconf-set-selections on a target system on which you want to insert new values into the debconf database. It is provided by debconf package, so it is installed by default.

$ dpkg-query -S /usr/bin/debconf-set-selections

debconf: /usr/bin/debconf-set-selections

You will need debconf-get-selections on a source system on which you want to read values from the debconf database. It is provided by debconf-utils package, so you need to install it manually.

$ dpkg-query -S /usr/bin/debconf-get-selections

debconf-utils: /usr/bin/debconf-get-selections

Install debconf-utils package.

$ sudo apt-get install debconf-utils 

Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following NEW packages will be installed:
  debconf-utils
0 upgraded, 1 newly installed, 0 to remove and 1 not upgraded.
Need to get 56.6 kB of archives.
After this operation, 108 kB of additional disk space will be used.
Get:1 http://ftp.task.gda.pl/debian stretch/main amd64 debconf-utils all 1.5.61 [56.6 kB]
Fetched 56.6 kB in 0s (146 kB/s)   
Selecting previously unselected package debconf-utils.
(Reading database ... 28000 files and directories currently installed.)
Preparing to unpack .../debconf-utils_1.5.61_all.deb ...
Unpacking debconf-utils (1.5.61) ...
Setting up debconf-utils (1.5.61) ...
Processing triggers for man-db (2.7.6.1-2) ...

Read values from the debconf database which applies to the iptables-persistent package.

$ sudo debconf-get-selections | grep ^iptables-persistent

iptables-persistent	iptables-persistent/autosave_v6	boolean	true
iptables-persistent	iptables-persistent/autosave_v4	boolean	true

Insert new values into the debconf database.

$ cat << EOF | sudo debconf-set-selections
iptables-persistent iptables-persistent/autosave_v6 boolean true
iptables-persistent iptables-persistent/autosave_v4 boolean false
EOF

Verify contents of the debconf database.

$ sudo debconf-get-selections | grep ^iptables-persistent
iptables-persistent	iptables-persistent/autosave_v6	boolean	true
iptables-persistent	iptables-persistent/autosave_v4	boolean	false

It is as simple as that.