(debian-policy.info)Prompting in maintainer scripts

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3.9.1 Prompting in maintainer scripts
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Package maintainer scripts may prompt the user if necessary.  Prompting
must be done by communicating through a program, such as ‘debconf’,
which conforms to the Debian Configuration Management Specification,
version 2 or higher.

Packages which are essential, or which are dependencies of essential
packages, may fall back on another prompting method if no such interface
is available when they are executed.

The Debian Configuration Management Specification is included in the
‘debconf_specification’ files in the debian-policy package.  It is also
available from the Debian web mirrors at
‘https://www.debian.org/doc/packaging-manuals/debconf_specification.html’.

Packages which use the Debian Configuration Management Specification may
contain the additional control information files ‘config’ and
‘templates’.  ‘config’ is an additional maintainer script used for
package configuration, and ‘templates’ contains templates used for user
prompting.  The ‘config’ script might be run before the ‘preinst’ script
and before the package is unpacked or any of its dependencies or
pre-dependencies are satisfied.  Therefore it must work using only the
tools present in `essential' packages.  (1)

Packages which use the Debian Configuration Management Specification
must allow for translation of their user-visible messages by using a
gettext-based system such as the one provided by the po-debconf package.

Packages should try to minimize the amount of prompting they need to do,
and they should ensure that the user will only ever be asked each
question once.  This means that packages should try to use appropriate
shared configuration files (such as ‘/etc/papersize’ and
‘/etc/news/server’), and shared debconf variables rather than each
prompting for their own list of required pieces of information.

It also means that an upgrade should not ask the same questions again,
unless the user has used ‘dpkg --purge’ to remove the package’s
configuration.  The answers to configuration questions should be stored
in an appropriate place in ‘/etc’ so that the user can modify them, and
how this has been done should be documented.

If a package has a vitally important piece of information to pass to the
user (such as “don’t run me as I am, you must edit the following
configuration files first or you risk your system emitting
badly-formatted messages”), it should display this in the ‘config’ or
‘postinst’ script and prompt the user to hit return to acknowledge the
message.  Copyright messages do not count as vitally important (they
belong in ‘/usr/share/doc/package/copyright’); neither do instructions
on how to use a program (these should be in on-line documentation, where
all the users can see them).

Any necessary prompting should almost always be confined to the ‘config’
or ‘postinst’ script.  If it is done in the ‘postinst’, it should be
protected with a conditional so that unnecessary prompting doesn’t
happen if a package’s installation fails and the ‘postinst’ is called
with ‘abort-upgrade’, ‘abort-remove’ or ‘abort-deconfigure’.

   ---------- Footnotes ----------

   (1) Debconf or another tool that implements the Debian Configuration
Management Specification will also be installed, and any versioned
dependencies on it will be satisfied before preconfiguration begins.