Run a script at random times within a daily window

Standard cron cannot choose 20 random times or keep them inside a daily window; it only matches clock minutes. The practical pattern is to give cron a set of fixed base slots that stay inside the window even after a random delay, then let a small wrapper script add the delay and decide whether this slot runs at all.

# 26 base slots, 09:00 through 21:30, every day
0,30 9-21 * * *

# the crontab line calls a wrapper, not the script itself
0,30 9-21 * * * /path/to/jittered.sh

#!/bin/bash
# jittered.sh: wait 0-5399 seconds (up to 89 min 59 s), then run
sleep $((RANDOM % 5400))
/path/to/script.sh

Read 0,30 9-21 * * * from left to right: 0,30 selects minutes 0 and 30; 9-21 selects every hour from 09:00 through 21:59; the three wildcards allow every day of the month, every month, and every day of the week. The latest slot is 21:30, and a delay of at most 5,399 seconds ends by 22:59:59, so every run lands between 9am and 11pm. A random skip such as [ $((RANDOM % 26)) -lt 20 ] || exit 0 at the top of the wrapper averages 20 runs per day but does not guarantee that count. For exactly 20, pick 20 of the 26 slots once a day, write them to a file, and have each slot check whether it is on the list.