What init system was used by Debian before systemd?

Before systemd, Debian used SysV init, also known as sysvinit.

SysV init follows the System V Unix startup model. The init process becomes process ID 1 after the kernel starts, reads configuration such as `/etc/inittab`, and launches services through shell scripts organized around runlevels. Debian traditionally used the `sysv-rc` package to manage those scripts.

Debian 8, code-named Jessie, made systemd the default init system in 2015. Earlier Debian releases used sysvinit as their standard supported choice; Debian’s own documentation describes it as the only supported init system in Debian Squeeze and earlier.

The alternatives in this question are easy to confuse with Debian’s history. Upstart was used prominently by Ubuntu, while OpenRC and runit are alternative init systems adopted by other distributions. Systemd can also run many legacy SysV-style service scripts, which is why old `/etc/init.d/` files may still appear on a modern Debian installation.