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cron – daemon to execute scheduled commands (ISC Cron V4.1)
USAGE
cron [-l load_avg] [-n] [-p]

DESCRIPTION
Cron should be started from /etc/rc or /etc/rc.local. It will return
immediately, so you don’t need to start it [...]

crontab – maintain crontab files for individual users (ISC Cron V4.1)
USAGE
crontab [-u user] file
crontab [-u user] [-l | -r | -e][-i]

DESCRIPTION
Crontab is the program used to install, deinstall or list the [...]

atd – run jobs queued for later execution
USAGE
atd [-l load_avg] [-b batch_interval] [-d] [-s]

DESCRIPTION
atd runs jobs queued by at(1).

OPTIONS
-l Specifies a limiting load factor, over which batch jobs [...]

at – execute commands at a later time
USAGE at [-V] [-q queue] [-f file] [-mldbv] TIME at [-V] [-q queue] [-f file] [-mldbv] -t time_arg at -c job [job...] atq [-V] [...]