Is it possible to schedule a cron job to run fortnightly?

Question

Is there a way to schedule a cron job to run every fortnight?

(One way I can think of, within crontab, would be to add two entries for "date-of-month"...)

Answer 1

No, cron only knows about the day of the week, the day of the month and the month.

Running a command twice a month on fixed days (e.g. the 1st and the 16th) is easy:

42 4 1,16 * *  do_stuff

Running a command every other week is another matter. The best you can do is to run a command every week, and make it do nothing every other week. On Linux, you can divide the number of seconds since the epoch (date +%s) by the number of seconds in a week to get a number that flips parity every week. Note that in a crontab, % needs to be escaped (cron turns % into newlines before executing the command).

42 4 * * 1  case $(($(date +\%s) / (60*60*24*7))) in *[02468]) do_stuff;; esac

Answer 2

You cannot directly have cron run a job fortnightly (every two weeks). However, it is reasonably straightforward to ensure that the main part of the job runs only every other week. @Gilles has offered one solution; here's another:

42 4 * * 1    test 1 -eq $(($(date +\%g) & 1)) && do_stuff...

The date +%g command returns the current week number (of the year), and this is bitwise ANDed to return either 1 or 0 before being used to determine whether the real job can be run.

The same caveat on the percent symbol %: in a crontab entry it must be escaped to prevent cron treating it specially.

Answer 3

I know I'm late to the party but, everyone wants a solution or three to any problem...

I found this post while wanting to do the same thing as the OP, except for a Sunday.

@user140906 has a valid concern, which can be observed when running this snippet;

for n in $(seq 17 33); do ncal -A1 -bw dec 20${n}; done

Which initially seems to repeat every 6 years - I didn't check them all! :-)

I then found this link which offers two solutions, one similar to @gilles-so-stop-being-evil, that should work, the most obvious one is below;

#!/bin/bash

# a file marking the state on disk
mark_file=$HOME/.job-run-marker-1

# check whether the job run last time it is invoked
if [ -e $mark_file ] ; then
  rm -f $mark_file
else
  touch $mark_file
  exit 0
fi

# job command is here

Coupled with a crontab entry like;

0 2 * * 2 my_script

Easily adapted to be used for multiple crontab entries (pass a value for mark_file or an ID instead of hard-coding it), or expand the same logic with some grep / awk / sed magic, to add/remove/append lines in a file that represent the ID and status/date of each job execution.

Either way, hopefully that helps someone, perhaps even a future me!