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

That's not possible with a single expression in normal cron.

The best you could do without modifying the code is:

0 0,3,6,9,12,15,18,21 * * *  [cmd]
30 1,4,7,10,13,16,19,22 * * * [cmd]

These might be compressible, depending on the version of cron you have to:

0 */3 * * * [cmd]
30 1-23/3 * * * [cmd]

Answer 2

Is there a good reason why you can't use 1 hour or 2 hours? It would be simpler for sure.

I haven't tried this personally, but you can find some info here on getting cron to run every 90 minutes: http://keithdevens.com/weblog/archive/2004/May/05/cron

An excert from the above link:

0 0,3,6,9,12,15,18,21 * * * <commands>
30 1,4,7,10,13,16,19,22 * * * <commands>

Answer 3

You could also use fcron which also accepts more complex time specifications such as :

@ 01h30 my_cmd

Answer 4

added the following to my crontab and is working

15 */1   * * *   root    /usr/bin/some_script.sh >> /tmp/something.log

Answer 5

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

Answer 6

You could do it with two crontab entries. Each runs every three hours and they are offset by 90 minutes something like this:

0 0,3,6,9,12,15,18,21 * * *

30 1,4,7,10,13,16,19,22 * * *