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

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]

Two lines in the crontab. Along the lines of:

0 0,3,6,9,12,15,18,21 * * * /usr/bin/foo 
30 1,4,7,10,13,16,19,22 * * * /usr/bin/foo
 

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>
 

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