How can I set cron to run certain commands every one and a half hours?

Question

How can I set cron to run certain commands every one and a half hours?

Answer 1

You can achieve any frequency if you count the minutes(, hours, days, or weeks) since Epoch, add a condition to the top of your script, and set the script to run every minute on your crontab:

#!/bin/bash

minutesSinceEpoch=$(($(date +'%s / 60')))

# every 90 minutes (one and a half hours)
if [[ $(($minutesSinceEpoch % 90)) -ne 0 ]]; then
    exit 0
fi

date(1) returns current date, we format it as seconds since Epoch (%s) and then we do basic maths:

# .---------------------- bash command substitution
# |.--------------------- bash arithmetic expansion
# || .------------------- bash command substitution
# || |  .---------------- date command
# || |  |   .------------ FORMAT argument
# || |  |   |      .----- formula to calculate minutes/hours/days/etc is included into the format string passed to date command
# || |  |   |      |
# ** *  *   *      * 
  $(($(date +'%s / 60')))
# * *  ---------------
# | |        | 
# | |        ·----------- date should result in something like "1438390397 / 60"
# | ·-------------------- it gets evaluated as an expression. (the maths)
# ·---------------------- and we can store it

And you may use this approach with hourly, daily, or monthly cron jobs:

#!/bin/bash
# We can get the

minutes=$(($(date +'%s / 60')))
hours=$(($(date +'%s / 60 / 60')))
days=$(($(date +'%s / 60 / 60 / 24')))
weeks=$(($(date +'%s / 60 / 60 / 24 / 7')))

# or even

moons=$(($(date +'%s / 60 / 60 / 24 / 656')))

# passed since Epoch and define a frequency
# let's say, every 7 hours

if [[ $(($hours % 7)) -ne 0 ]]; then
    exit 0
fi

# and your actual script starts here