Run a script via cron every other week

I’ve done quite a bit of research in how to do this, and I see there’s no direct way in cron to run a job, say, every other Thursday.

Right now, I’m leaning toward making a script that will just run every week, and will touch a “flag” file when it runs, and if it runs and the file is already there, to remove the file (and not perform the bi-weekly action).

My question is, is there any other more elegant or simpler way to accomplish this goal of a bash script of actions running every other week automatically?


Asked By: dmkmills


One way might be to use the at utility if you’re running Linux. You could put this at the end of your script:

at X + 2 weeks -f myscript

where X is the time you want to run your script at. To initialize just call at for the time you want to run it the first time and then the above statement keeps renewing your scheduled call.

Answered By: gvkv

I’m not sure if there is a more elegant way to define the every other week, but this may work for you.

00 06 1-7,15-21 * * test `date +%a` = Thu && /path/to/script

This will launch the script at 6:00am on the first and third Thursdays of the month.

Answered By: pferate
0 0 * * Thu bash -c '(($(date +%s) / 86400 % 14))' && your-script

I used bash to do my math because I’m lazy; switch that to whatever you like. I take advantage of January 1, 1970 being a Thursday; for other days of the week you’d have to apply an offset. Cron needs the percent signs escaped.

Quick check:

function check {
  when=$(date --date="$1 $(($RANDOM % 24)):$(($RANDOM % 60))" --utc)
  echo -n "$when: "
  (($(date +%s --date="$when") / 86400 % 14)) && echo run || echo skip

for start in "2010-12-02" "2011-12-01"; do
  for x in $(seq 0 12); do
    check "$start + $(($x * 7)) days"

Note I’ve chosen random times to show this will work if run anytime on Thursday, and chosen dates which cross year boundaries plus include months with both 4 and 5 Thursdays.


Thu Dec  2 06:19:00 UTC 2010: run
Thu Dec  9 23:04:00 UTC 2010: skip
Thu Dec 16 05:37:00 UTC 2010: run
Thu Dec 23 12:49:00 UTC 2010: skip
Thu Dec 30 03:59:00 UTC 2010: run
Thu Jan  6 11:29:00 UTC 2011: skip
Thu Jan 13 13:23:00 UTC 2011: run
Thu Jan 20 20:33:00 UTC 2011: skip
Thu Jan 27 16:48:00 UTC 2011: run
Thu Feb  3 17:43:00 UTC 2011: skip
Thu Feb 10 05:49:00 UTC 2011: run
Thu Feb 17 08:46:00 UTC 2011: skip
Thu Feb 24 06:50:00 UTC 2011: run

Thu Dec  1 21:40:00 UTC 2011: run
Thu Dec  8 23:24:00 UTC 2011: skip
Thu Dec 15 22:27:00 UTC 2011: run
Thu Dec 22 02:47:00 UTC 2011: skip
Thu Dec 29 12:44:00 UTC 2011: run
Thu Jan  5 17:59:00 UTC 2012: skip
Thu Jan 12 18:31:00 UTC 2012: run
Thu Jan 19 04:51:00 UTC 2012: skip
Thu Jan 26 08:02:00 UTC 2012: run
Thu Feb  2 17:37:00 UTC 2012: skip
Thu Feb  9 14:08:00 UTC 2012: run
Thu Feb 16 18:50:00 UTC 2012: skip
Thu Feb 23 15:52:00 UTC 2012: run
Answered By: Roger Pate

If you can use anacron on the system, things will be much simpler. To use anacron you must have it installed and also you must have root access. It doesn’t work on older systems (i.e. RHEL 5.x), the older versions of ancron ONLY run on boot up. It does work with newer systems (i.e. RHEL 6.x).

With anacron, one can schedule jobs in a more flexible way, e.g. run X job once a week. Also anacron runs jobs when computer becomes available, i.e. you don’t have to consider when the system is up or down.

To run a script every other week, you have to add a line similar to following to the /etc/anacrontab:

14 5 myScript

Take a look at the man page for details.

Answered By: memin

I would run it every week, and have the script check to see if the week number is an odd number: if yes exit.

This would need some more thought to work from Dec to Jan.

Perhaps have the script set a “last successful run” flag file. When the script launches, check the flag file was last modified 14 days ago.

Answered By: glenn jackman
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