Let's say I want to install mysql from a script without being asked any configuration questions like what root password I want to set by apt. I would then preset the debconf variables:

echo mysql-server-5.5 mysql-server/root_password password xyzzy | debconf-set-selections
echo mysql-server-5.5 mysql-server/root_password_again password xyzzy | debconf-set-selections

I got this from a tutorial. What is unclear to me: How did the guy find out the variable names? How did he knew that he had to set mysql-server-5.5 mysql-server/root_password password and mysql-server-5.5 mysql-server/root_password_again respectively?

I know I could extract the .deb package by issuing dpkg-deb -R package.deb EXTRACTDIR/ - but I don't see where those variables are stored.

How would I find out the debconf variables for any other package?


You can inspect what gets stored in debconf using debconf-get-selections. This is useful if you have actually done the installation already.

Alternately, these settings are used in the package maintainer scripts. With the dpkg-deb command you have run, these are in the DEBIAN subdirectory of EXTRACTDIR.

As an example, from lightdm:

$ grep db_ lightdm/DEBIAN -R
lightdm/DEBIAN/postrm:  db_purge
lightdm/DEBIAN/prerm:    db_unregister shared/default-x-display-manager
lightdm/DEBIAN/prerm:    if db_get shared/default-x-display-manager; then
lightdm/DEBIAN/prerm:      db_metaget shared/default-x-display-manager owners
lightdm/DEBIAN/prerm:      db_subst shared/default-x-display-manager choices "$RET"
lightdm/DEBIAN/prerm:      db_get shared/default-x-display-manager
lightdm/DEBIAN/prerm:     if db_get "$RET"/daemon_name; then
lightdm/DEBIAN/prerm:        db_fset shared/default-x-display-manager seen false
lightdm/DEBIAN/prerm:        db_input critical shared/default-x-display-manager || true
lightdm/DEBIAN/prerm:        db_go
lightdm/DEBIAN/prerm:          db_get shared/default-x-display-manager
lightdm/DEBIAN/prerm:          db_get "$RET"/daemon_name
lightdm/DEBIAN/postinst:  if db_get shared/default-x-display-manager; then
lightdm/DEBIAN/postinst:    if db_get "$DEFAULT_DISPLAY_MANAGER"/daemon_name; then
lightdm/DEBIAN/config:if db_metaget shared/default-x-display-manager owners; then
lightdm/DEBIAN/config:if db_metaget shared/default-x-display-manager choices; then
lightdm/DEBIAN/config:  db_subst shared/default-x-display-manager choices "$OWNERS" || :
lightdm/DEBIAN/config:  db_fset shared/default-x-display-manager seen false || :
lightdm/DEBIAN/config:    db_set shared/default-x-display-manager "$CURRENT_DEFAULT"
lightdm/DEBIAN/config:  if db_get shared/default-x-display-manager; then
lightdm/DEBIAN/config:    db_set shared/default-x-display-manager lightdm
lightdm/DEBIAN/config:    db_fset shared/default-x-display-manager seen true
lightdm/DEBIAN/config:    db_input high shared/default-x-display-manager || :
lightdm/DEBIAN/config:    db_go || :
lightdm/DEBIAN/config:if db_get shared/default-x-display-manager; then

The various db_* functions are helper functions for handling debconf, obtained from /usr/share/debconf/confmodule.

So, in the case of lightdm, shared/default-x-display-manager is an important debconf key.


You can get the variables for a specific installed package using debconf-show packagename


$ sudo debconf-show mysql-server-5.7
* mysql-server/root_password: (password omitted)
* mysql-server/root_password_again: (password omitted)
  mysql-server-5.7/start_on_boot: true
  mysql-server-5.7/really_downgrade: false
  mysql-server-5.7/postrm_remove_databases: false

You can get a list of all the installed packages that have variables in the database using debconf-show --listowners, so if you're not sure what the package name is you could do something like

# debconf-show --listowners | grep mysql | xargs debconf-show
* mysql-server/root_password: (password omitted)
* mysql-server/root_password_again: (password omitted)
  mysql-server-5.7/postrm_remove_databases: false
  mysql-server-5.7/start_on_boot: true
  mysql-server-5.7/really_downgrade: false

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