Crontab for different ime zones

GT catch.all at
Mon May 25 02:21:17 UTC 2009

On Sun, 2009-05-24 at 18:45 -0600, Tim Judd wrote:

> How about a jail for America/NY, and a jail for AU/Sydney?  that might
> work.
> --TJ 

That's a good solution, but I am still somewhat puzzled by cron's
behaviour relative to what I expected from the man page.

>From the man page for crontab(8):

"In order to provide finer control over when  jobs  execute,  users  can
 also  set  the  environment  variables  CRON_TZ  and  CRON_WITHIN.  The
 CRON_TZ variable can be set to an  alternate  time  zone  in  order  to
 affect when the job is run.  Note that this only affects the scheduling
 of the job, not the time zone that the job perceives when  it  is  run.
 If  CRON_TZ  is defined but empty (CRON_TZ=""), jobs are scheduled with
 respect to the local time zone."

Problem is, CRON_TZ just doesn't work as written above. 

If you insert CRON_TZ=America/New_York and then write the * * * * * (schedule) 
to reflect NY time (say), but your server time is Chicago, cron ignores 
the CRON_TZ definition.  

Here's a CRONTAB extract which provides a good example of 
the odd behaviour:

# ----- sample crontab

# change tz and cron_tz to America/NY

# check that timezone change 'stuck' (commented out except for testing)
* * * * * printf "CRON_TZ is now "$CRON_TZ". Now doing US\n"  >> /home/targetdir/public_html/tmp/log.txt 2>&1
* * * * * date >> /home/targetdir/public_html/tmp/log.txt 2>&1

# Now, change both TZs to Australia/Sydney

# check that timezone change 'stuck' (commented out except for testing)
* * * * * printf "CRON_TZ is now "$CRON_TZ". Now doing Australia\n"  >> /home/targetdir/public_html/tmp/log.txt 2>&1
* * * * * date >> /home/targetdir/public_html/tmp/log.txt 2>&1

# ----- end sample crontab

The result of that will be 

Mon May 25 11:02:01 EST 2009
CRON_TZ is now Australia/Sydney. Now doing Australia
Sun May 24 21:02:01 EDT 2009
CRON_TZ is now America/New_York. Now doing US

'date' is right, and $CRON_TZ changes as expected, but has no effect in cron.

One other thing strikes me as odd: the output file is in reverse order 
to the crontab. If changes to CRON_TZ actually worked as written, then one would 
need to verify that the changes propagated 'downstream' from the variable declaration. 

It's not that output is being written to the top of the outfile though - output 
in subsequent minutes is APPENDED to the outfile.

And now for the big "DUH, I'm an idiot" moment - apparently FreeBSD, Fedora and Ubuntu don't have CRON_TZ support built in.

It's gotten to the stage where you have to be wary of what you find in external man pages...



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