A tool to facilitate working with relative dates in cronjobs such as "third Monday of the month", "tenth Wednesday of the year", etc.
This tool was born of a need I had to run a cron job on the third Monday of every month. Crontab syntax does not allow for this, hence the creation of a utility that I could include in the crontab to only run a desired command if third-monday returned an exit code of 0, based on a provided date specification.
The following screenshot shows the tool being used to check the current date against two different date specifications: (1), 1#5, "the first Friday of the month", and (2) 1#2, "the first Tuesday of the month".
Note that the second date specification returned a non-zero exit code since Friday, March 4, 2022 is not the first Tuesday of the month.
The date specification consists of two parts separated by a "#" symbol. The first part indicates "first", "second", "third", etc. and corresponds to ordinals returned by the time#Time.Day() function (or, if the --year flag is specified, by the time#Time.YearDay() function modulo 7). The second part indicates the day of the week: "Monday", "Tuesday", "Wednesday", etc. and corresponds to ordinals returned by the time#Time.Weekday() function.
Each part of the date specification may possess more than one ordinal. A list of ordinals consists of ordinals separated by commas. Examples:
1,3#0- "the first and third Sundays of the month"2#2,5- "the second Tuesday and the second Friday of the month"1,2#1,6"the first and second Mondays and Saturdays of the month"
An additional flag, --year allows you specify days in the year such as 14#2 or "the fourteenth Tuesday of the year.
Note: I should add that this tool currently lacks any checks for absurd specifications like "the fifteenth Tuesday of the week" or "the five-hundreth Friday of the year". So YMMV.
Suppose, for example, I want to run a cron job that executes a command called helloworld on the second Tuesday and second Thursday of each month at 9:30am, the corresponding crontab item would be:
30 9 * * * third-monday check "2#2,4" && helloworld >/dev/null 2>&1
Essentially, the cron will fire every day at 9:30am but helloworld will only be executed on the second Tuesday and second Thursday of each month.