"In the days of yore we had a System V (SysV) type init daemon
to manage Linux system startup, and it was good. It was configured
with simple text files easily understood by mortals, and it was a
friendly constant amid the roiling seas of change. Then came
systemd, and once again we Linux users were cast adrift in
uncharted waters. Why all this change? Can't Linux hold still for
just a minute?
"Linux has been contentedly using sysvinit (System V
initialization) to manage system startups for ever so many years
now, except for distributions like Slackware that use the BSD-style
init. SysV and BSD init are similar enough that it's easy to use
either one without a lot of fuss."