Upgrading Debian to the next major version

8 -> 9 (jessie -> stretch)

cat /etc/issue
apt-get update
apt-get upgrade
apt-get dist-upgrade
sed -i 's/jessie/stretch/g' /etc/apt/sources.list
apt-get update
apt-get upgrade
apt-get dist-upgrade
apt-get -y autoremove
shutdown -r now
cat /etc/issue

9 -> 10 (stretch -> buster)

cat /etc/issue
apt-get update
apt-get upgrade
apt-get dist-upgrade
sed -i 's/stretch/buster/g' /etc/apt/sources.list
apt-get update
apt-get upgrade
apt-get dist-upgrade
apt-get -y autoremove
shutdown -r now
cat /etc/issue

10 -> 11 (buster -> bullseye)

cat /etc/issue
apt-get update
apt-get upgrade
apt-get dist-upgrade
sed -i 's/buster/bullseye/g' /etc/apt/sources.list
sed -i 's/bullseye\/updates/bullseye-security/g' /etc/apt/sources.list
apt-get update
apt-get upgrade
apt-get dist-upgrade
apt-get -y autoremove
shutdown -r now
cat /etc/issue

or something shorter but with less control:

apt-get update && apt-get -y upgrade && apt-get -y dist-upgrade
sed -i 's/buster/bullseye/g' /etc/apt/sources.list
sed -i 's/bullseye\/updates/bullseye-security/g' /etc/apt/sources.list
apt-get update && apt-get -y upgrade && apt-get -y dist-upgrade && apt-get -y autoremove
shutdown -r now
cat /etc/issue