How can I run a script 20 times a day at random times, between 9am and 11pm?

Cron only matches clock-aligned minutes, so “truly random” times within a window need a wrapper. The trick above schedules deterministic base times (every hour from 8 AM to 9 PM, at :15 and :45 — 28 runs) and adds a random delay before launching the actual script. ${RANDOM:0:2} yields a 1-2 digit number (0-99); sleep ...m waits that many minutes. Combined, each run fires somewhere in a ~99-minute jitter window after the scheduled minute.

The “20 times a day” ask is hard to satisfy exactly without a workflow engine. The pragmatic version: schedule slightly more than you need (e.g. 28 base slots) and have the script self-skip — keep a counter file, increment it, exit early once you've hit 20 runs. Or use a stochastic gate: [ $((RANDOM % 28)) -lt 20 ] || exit 0 gives ~20 runs per day on average.

Jenkins users have a cleaner option: the H hash modifier automatically jitters the minute deterministically per job. See the Jenkins cron guide for that pattern.