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How to execute a shell script via a cronjob every 45 days?

If you don't need exactly 45 days, but "one and a half months" will do, then a straightforward method would be to run at the beginning of the month every three months, and at the middle of the next month after each of those:

0 12 1 1,4,7,10 * /path/to/script
0 12 16 2,5,8,11 * /path/to/script

For general arbitrary intervals, the other answers are obviously better, but 45 days sounds like it's based on the length of a month anyway. Human users might also be more used to something happening in the beginning or the middle of a month, instead of seeing the exact date drift a day or two each time.

Since cron doesn't support a simple, direct translation of that requirement, I would approach it with a timestamp file and a daily cron job:

0 0 * * * /path/to/wrapper/script/below.sh

the wrapper script:

#!/bin/bash
timestampfile=/var/run/this-job/timetampfile

# exit now if it hasn't been 45 days
(( 
 $(date +%s) - $(stat -c %Z "$timestampfile") 
 < 
 (45 * 24 * 60 * 60) 
)) && exit 0

touch "$timestampfile"
: run real script here

This runs several risks:

• someone deleting the timestamp file


• someone touching the timestamp file (for better or for worse)


• leaving it non-obvious from the cron job entry that the actual script runs every 45 days

Based on this answer on stack overflow, you could accomplish this with the following syntax:

00 12 * * * test $(( $(date +%s)/24/60/60%45 )) = 20 && your_script

This job will run every day at noon. It then calculates the modulo of the number of days from epoch and tests if it is equal to the modulo you specify. In this case I have specified 20 which would make it evaluate to true for today (2018-05-09 in between 00:00 GMT and 23:59 GMT), then 45 days from today, etc. You can calculate/test with the following:

$ echo $(( $(date +%s)/24/60/60%45 ))
20
$ echo $(( $(date -d '+45 days' +%s)/24/60/60%45 ))
20
$ echo $(( $(date -d '+44 days' +%s)/24/60/60%45 ))
19

(beware that if you're not in a UTC timezone, you may have to take the hour of the day into account).

Alternative solution: systemd is eating all parts of the Linux system. With this in mind, it now has a timer unit that can be related to a service unit of the same name to create a schedule and mimic anacron. Example:

Service. We will call it foo and it will run your script /home/user/foo.sh. When working with timers your service unit does not need to have the [Install] section:

/etc/systemd/system/foo.service

[Unit]
Description=My foo script

[Service]
Type=simple
ExecStart=/home/user/foo.sh

Timer. This is the special unit that will handle time schedule:

/etc/systemd/system/foo.timer

[Unit]
Description=Run foo every 45 days and on boot
Persistent=true

[Timer]
OnBootSec=15min
OnUnitActiveSec=45d 

[Install]
WantedBy=multi-user.target

Taking a look at the [Timer] section, your script will run 15 minutes after boot(you can remove this), and each 45 days. The Persistent= parameter means that even after rebooting the schedule will persist to disk and not be restarted.

And then, enable both units:

systemctl enable foo.service
systemctl enable foo.timer

You may not like systemd, but this is a quick solution to problems with schedule and cron.

Further Reading:

• systemd - Timer units that mimic anacron behaviour


• systemd/Timers - Arch Linux Wiki