Ubuntu – What happens during a package upgrade

With a debian/preinst file you can perform actions on install but not upgrade.

#!/bin/sh
set -e

case "$1" in
    install)
        # do some magic
        ;;

    upgrade|abort-upgrade)
        ;;

    *)
        echo "postinst called with unknown argument \`$1'" >&2
        exit 0
        ;;
esac

#DEBHELPER#

exit 0

Though as the name implies, this is run before your package is installed. So you might not be able to do what you need here. Most packages simply test in the configure stage of the postinst if the user has been created already. Here's colord

$ cat  /var/lib/dpkg/info/colord.postinst
#!/bin/sh

set -e

case "$1" in
    configure)

# create colord group if it isn't already there
    if ! getent group colord >/dev/null; then
            addgroup --quiet --system colord
    fi

# create the scanner group if it isn't already there
    if ! getent group scanner >/dev/null; then
        addgroup --quiet --system scanner
    fi

# create colord user if it isn't already there
    if ! getent passwd colord >/dev/null; then
            adduser --system --ingroup colord --home /var/lib/colord colord \
        --gecos "colord colour management daemon"
        # Add colord user to scanner group
        adduser --quiet colord scanner
    fi

# ensure /var/lib/colord has appropriate permissions
    chown -R colord:colord /var/lib/colord

    ;;
esac    



exit 0

I would continue to use the pre/post inst scripts,

preinst - This script executes before that package will be unpacked from its Debian archive (".deb") file. Many 'preinst' scripts stop services for packages which are being upgraded until their installation or upgrade is completed (following the successful execution of the 'postinst' script).

postinst - This script typically completes any required configuration of the package foo once foo has been unpacked from its Debian archive (".deb") file. Often, 'postinst' scripts ask the user for input, and/or warn the user that if he accepts default values, he should remember to go back and re-configure that package as the situation warrants. Many 'postinst' scripts then execute any commands necessary to start or restart a service once a new package has been installed or upgraded.

see - https://www.debian.org/doc/manuals/debian-faq/ch-pkg_basics.en.html

The syntax of invoking start|stop|restart is written as a conditional, see https://www.debian.org/doc/debian-policy/ch-opersys.html section 9.3.3.2 Running initscripts

if which invoke-rc.d >/dev/null 2>&1; then

invoke-rc.d package

else

/etc/init.d/package

fi

so ...

if which service >/dev/null 2>&1; then
        service package <action>
elif which invoke-rc.d >/dev/null 2>&1; then
        invoke-rc.d package <action>
else
        /etc/init.d/package <action>
fi

and add another conditional for systemd when needed ;)

So yes, the proper way to start|stop|restart a service is with the appropriate wrapper script (invoke-rc.d / system), when possible, rather then calling the init script (/etc/init.d/package) and falling back to the /etc/init.d script when no wrapper is available.