Editing Cron (section)

=== Early versions ===
The cron in [[Version 7 Unix]] was a system service (later called a [[daemon (computing)|daemon]]) invoked from <code>/etc/rc</code> when the operating system entered multi-user mode.<ref>{{cite web | title=V7/etc/rc | website=Minnie's Home Page | url=http://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/etc/rc | access-date=2020-09-12 }}</ref> Its [[algorithm]] was straightforward:
# Read <code>/usr/lib/crontab</code><ref>{{cite web | title=V7/usr/src/cmd/cron.c | website=Minnie's Home Page | url=http://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/usr/src/cmd/cron.c | access-date=2020-09-12 }}</ref>
# Determine if any commands must run at the current date and time, and if so, run them as the [[superuser]], root.
# Sleep for one minute
# Repeat from step 1.

This version of cron was basic and robust but it also consumed resources whether it found any work to do or not. In an experiment at [[Purdue University]] in the late 1970s to extend cron's service to all 100 users on a time-shared [[VAX]], it was found to place too much load on the system.