Please also refer to http://www.debian.org/releases/buster/releasenotes and use your brain! If you can’t figure out what one of the commands below does, this is not for you. Expert mode only :)

# Crossgrading ?!?
[ "$(dpkg --print-architecture)" == "i386" ] && echo "How about crossgrading to amd64 as described in https://stbuehler.de/blog/article/2017/06/28/debian_stretch__upgrade_32-bit_to_64-bit.html?"

# upgrade to UTF-8 locales (http://www.debian.org/releases/buster/amd64/release-notes/ap-old-stuff.en.html#switch-utf8)
dpkg-reconfigure locales

# remove unused config file
rm -rf /etc/network/options /etc/environment

# are there 3rd party packages installed? (https://www.debian.org/releases/buster/amd64/release-notes/ch-upgrading.de.html#system-status)
aptitude search '~i(!~ODebian)'

# check for ftp protocol in sources lists (https://www.debian.org/releases/buster/amd64/release-notes/ch-information.en.html#deprecation-of-ftp-apt-mirrors)
rgrep --color "deb ftp" /etc/apt/sources.list*

# Transition and remove entries from older releases
sed -i /lenny/d /etc/apt/sources.list*
sed -i /sarge/d /etc/apt/sources.list*
sed -i /squeeze/d /etc/apt/sources.list*
sed -i /wheezy/d /etc/apt/sources.list*
sed -i /jessie/d /etc/apt/sources.list*
sed -i /volatile/d /etc/apt/sources.list*
sed -i /proposed-updates/d /etc/apt/sources.list*
# change distro (please move 3rd party sources to /etc/apt/sources.list.d/), maybe look into http://ftp.cyconet.org/debian/sources.list.d/
sed -i s/stretch/buster/g /etc/apt/sources.list*
sed -i "s/ stable/ buster/g" /etc/apt/sources.list*
sed -i s/stretch/buster/g /etc/apt/preferences*
sed -i s/stretch/buster/g /etc/apt/sources.list.d/*stretch*
rename s/stretch/buster/g /etc/apt/sources.list.d/*stretch*
rgrep --color stretch /etc/apt/sources.list*
apt-get update

# check package status
dpkg --audit
aptitude search "~ahold" | grep "^.h"
dpkg --get-selections | grep hold
 
# unmark packages auto
aptitude unmarkauto vim net-tools && \
aptitude unmarkauto libapache2-mpm-itk && \
aptitude unmarkauto $(dpkg-query -W 'linux-image-4.9.0*' | cut -f1)
 
# have a look into required and free disk space
apt-get -o APT::Get::Trivial-Only=true dist-upgrade || df -h

# record session
script -t 2>~/upgrade-buster.time -a ~/upgrade-buster.script

# install our preseed so libc doesn't whine
cat > /tmp/stretch.preseed <<EOF
libc6 glibc/upgrade boolean true
libc6 glibc/restart-services string
libc6 libraries/restart-without-asking boolean true
EOF
/usr/bin/debconf-set-selections /tmp/stretch.preseed

# update aptitude first
[ "$(which aptitude)" = "/usr/bin/aptitude" ] && aptitude install aptitude && \
[ "$(which apt)" = "/usr/bin/apt" ] && apt install apt

# minimal system upgrade
aptitude upgrade

# randomize crontab
if [ -f /etc/crontab.dpkg-new ]; then CFG=/etc/crontab.dpkg-new; else CFG=/etc/crontab; fi
sed -i 's#root    cd#root    perl -e "sleep int(rand(300))" \&\& cd#' $CFG
sed -i 's#root\ttest#root\tperl -e "sleep int(rand(3600))" \&\& test#' $CFG

# chrony update, modify the new config to our needs and place it where it is expected.
# Keep LOCAL config if asked when upgrading 
if [ -f /etc/chrony/chrony.conf.new ]; then CFG=/etc/chrony/chrony.conf.new; else CFG=/etc/chrony/chrony.conf; fi
sed s/2.debian.pool/0.de.pool/g /usr/share/chrony/chrony.conf > $CFG

# Fix our ssh pub key package configuration
# Accept MAINTAINERS version (and run this snippet afterwards)
[ -x /var/lib/dpkg/info/config-openssh-server-authorizedkeys-core.postinst ] && \
  /var/lib/dpkg/info/config-openssh-server-authorizedkeys-core.postinst configure

# migrate unattended-upgrades config, modify the new config to our needs and place it where it is expected.
# Keep LOCAL config if asked when upgrading (and run this snippet afterwards, when dpkg is not blocked anymore)
if [ -f /etc/apt/apt.conf.d/50unattended-upgrades.ucf-old ]; then CFG=/etc/apt/apt.conf.d/50unattended-upgrades.ucf-old; else CFG=/etc/apt/apt.conf.d/50unattended-upgrades; fi && \
cp /usr/share/unattended-upgrades/50unattended-upgrades /tmp/ && \
MAIL=$(grep ^Unattended-Upgrade::Mail $CFG | awk -F\" '{print $2}'); sed -i 's#//Unattended-Upgrade::Mail ".*";#Unattended-Upgrade::Mail "'${MAIL}'";#g' /tmp/50unattended-upgrades && \
TIME=$(grep ^Unattended-Upgrade::Automatic-Reboot-Time $CFG | awk -F\" '{print $2}'); if [ "${TIME}" != "" ]; then sed -i 's#//Unattended-Upgrade::Automatic-Reboot-Time "02:00"#Unattended-Upgrade::Automatic-Reboot-Time "'${TIME}'"#' /tmp/50unattended-upgrades; fi
sed -i 's#//      "origin=Debian,codename=${distro_codename}-updates"#        "origin=Debian,codename=${distro_codename}-updates"#' /tmp/50unattended-upgrades && \
sed -i 's#//Unattended-Upgrade::Remove-Unused-Dependencies "false"#Unattended-Upgrade::Remove-Unused-Dependencies "true"#' /tmp/50unattended-upgrades && \
sed -i 's#//Unattended-Upgrade::Automatic-Reboot "false"#Unattended-Upgrade::Automatic-Reboot "true"#' /tmp/50unattended-upgrades && \
sed -i '/codename=..distro_codename.-updates/ s#^//#  #' /tmp/50unattended-upgrades && \
/bin/bash /usr/bin/ucf --three-way --debconf-ok /tmp/50unattended-upgrades /etc/apt/apt.conf.d/50unattended-upgrades && \
[ "$CFG" == "/etc/apt/apt.conf.d/50unattended-upgrades.ucf-old" ] && mv $CFG /etc/apt/apt.conf.d/50unattended-upgrades.ucf-save

## phpmyadmin
if [ -f /etc/phpmyadmin/config.inc.php.dpkg-new ]; then CFG=/etc/phpmyadmin/config.inc.php.dpkg-new; \
   else CFG=/etc/phpmyadmin/config.inc.php; fi
sed -i "s/\['auth_type'\] = 'cookie'/\['auth_type'\] = 'http'/" $CFG
sed -i "s#//\$cfg\['Servers'\]\[\$i\]\['auth_type'\] = 'http';#\$cfg['Servers'][\$i]['auth_type'] = 'http';#" $CFG

# full-upgrade
apt-get dist-upgrade

# Migrate (webserver) from php7.0 to php7.3
apt install $(dpkg -l |grep php7.0 | awk '/^i/ { print $2 }' |grep -v ^php7.0-opcache |sed s/php7.0/php/)
a2dismod php7.0; a2enmod php7.3 && systemctl restart apache2; ls -la /etc/php/7.0/*/conf.d/
# php-fpm
tail -10 /etc/php/7.0/fpm/pool.d/www.conf
vi /etc/php/7.3/fpm/pool.d/www.conf 
systemctl disable php7.0-fpm && systemctl stop php7.0-fpm && systemctl restart php7.3-fpm
# nginx
rename s/php70/php73/g /etc/nginx/conf.d/*php70*.conf
sed -i s/php7.0-fpm/php7.3-fpm/g /etc/nginx/conf.d/*.conf /etc/nginx/snippets/*.conf /etc/nginx/sites-available/*
systemctl restart nginx

# transition docker-ce to buster package
DOCKER_VER="$(apt-cache policy docker-ce | grep debian-buster | head -1 | awk '{print $1}')" && [ -n "${DOCKER_VER}" ] && apt install docker-ce=${DOCKER_VER} docker-ce-cli=${DOCKER_VER}

# transition icingaweb2 to buster package
ICINGAWEB2_VER="$(apt-cache policy icingaweb2 | grep "\.buster" | head -1 | awk '{print $1}')" && [ -n "${ICINGAWEB2_VER}" ] && apt install icingaweb2=${ICINGAWEB2_VER} icingaweb2-common=${ICINGAWEB2_VER} icingaweb2-module-monitoring=${ICINGAWEB2_VER} php-icinga=${ICINGAWEB2_VER} icingacli=${ICINGAWEB2_VER}

# provide /etc/dovecot/private/dovecot.key from default config to prevent failing to start dovecot
ln -s /etc/ssl/private/ssl-cert-snakeoil.key /etc/dovecot/private/dovecot.key

# Fixing systemd unit for chrooted unbound (https://bugs.debian.org/931583)
mkdir -p /etc/systemd/system/unbound.service.d/ && cat > /etc/systemd/system/unbound.service.d/override.conf <<EOF
[Service]
BindPaths=/run/systemd/notify:/var/lib/unbound/run/systemd/notify
EOF
systemctl daemon-reload && systemctl restart unbound

# remove old squeeze packages left around (keep eyes open!)
apt autoremove && \
apt purge $(aptitude search ?obsolete | grep -v -E 'linux-image|mailscanner|phpmyadmin' | awk '/^i *A/ { print $3 }') && \
apt purge $(aptitude search ?obsolete | grep -v -E 'linux-image|mailscanner|phpmyadmin' | awk '/^i/ { print $2 }') && \
apt purge $(dpkg -l | grep etch | grep -v xen | grep -v unbound | grep -v finch | awk '/^rc/ { print $2 }') && \
apt purge $(dpkg -l | grep lenny | grep -v xen | awk '/^rc/ { print $2 }') && \
apt purge $(dpkg -l | grep -E 'deb6|squeeze' | grep -v xen | awk '/^rc/ { print $2 }') && \
apt purge $(dpkg -l | grep -E 'deb7|wheezy' | grep -v xen | grep -v  -E 'linux-image|mailscanner|openswan|debian-security-support' | awk '/^rc/ { print $2 }') && \
apt purge $(dpkg -l | grep -E 'deb8|jessie' | grep -v xen | grep -v  -E 'linux-image|debian-security-support' | awk '{ print $2 }') && \
apt purge $(dpkg -l | grep -E 'deb9|stretch' | grep -v xen | grep -v  -E 'linux-image|debian-security-support|icinga2' | awk '{ print $2 }') && \
apt -y install deborphan && apt purge $(deborphan | grep -v xen | grep -v -E 'libpam-cracklib|libapache2-mpm-itk')
apt purge $(dpkg -l | awk '/^rc/ { print $2 }')

# for the brave YoloOps crowd
reboot && sleep 180; echo u > /proc/sysrq-trigger ; sleep 2 ; echo s > /proc/sysrq-trigger ; sleep 2 ; echo b > /proc/sysrq-trigger

### not needed until now
# Upgrade postgres
# See also https://www.debian.org/releases/stretch/amd64/release-notes/ch-information.de.html#plperl
if [ "$(dpkg -l | grep "postgresql-9.4" | awk {'print $2'})" = "postgresql-9.4" ]; then \
 aptitude install postgresql-9.6 && \
 pg_dropcluster --stop 9.6 main && \
 /etc/init.d/postgresql stop && \
 pg_upgradecluster -v 9.6 9.4 main && \
 sed -i "s/^manual/auto/g" /etc/postgresql/9.6/main/start.conf && \
 sed -i "s/^port = .*/port = 5432/" /etc/postgresql/9.6/main/postgresql.conf && \
 sed -i "s/^shared_buffers = .*/shared_buffers = 128MB/" /etc/postgresql/9.6/main/postgresql.conf && \
 /etc/init.d/postgresql restart; \
fi
pg_dropcluster 9.4 main