You can run any command or script any time or repeatedly with the help of linux utility
cron. To add a cron, just run command
crontab -e, this will open a file with crontab entries if any.
The format of a cron line is like below:
MIN HOUR DOM MON DOW CMD
MIN - Minute field, which can have values between 0 - 59
HOUR - Hour field, which can have values between 0 - 23
DOM - Day of the Month field, values 1 - 31
MON - Month field, values 1 - 12
DOW - Day of Week field, values 0 - 6
CMD - Command field, here you can mention the command which you want to schedule.
For example, if you want to run
/home/chetna/gather_stats.sh on 27th January at 2.30 pm. Then you can add a cron entry like below:
30 14 27 01 * /home/chetna/gather_stats.sh
So, how do you write a cron to run on last day of month? The problem is, we don’t have a number to put in DOM field, as it could be 28, 30, 31 days in month, sometimes 29. For eg.
59 23 28-31 * * /home/chetna/gather_stats.sh
This will run the script on 28,29,30 and 31st of every month at 11.59 pm. But in our case, if a month has 31 days, say January, we don’t want to run our script on other 3 days.It should only run on 31 January.
But in all the months, the next day will be 1. So we can use
to check if next day is 1, and if yes, run my script.
This is how I achieved the task:
59 23 28-31 * * [‘$(date +%d -d tomorrow)’ == ’01’ ] && /home/chetna/gather_stats.sh
date +%d -d tomorrow
will give tomorrows date as two character string. So we can check it, if it matches “01”