Problem: Running Cron Jobs Biweekly
Cron jobs are tasks that run at set times. Setting up a cron job to run every two weeks can be hard, as cron's usual options don't support this timing. You need a custom method to run jobs every two weeks.
Solution: Using Advanced Cron Expressions
Method 1: Using Date Calculations
The date calculation method uses the Unix timestamp to determine if the current week is odd or even. This method lets you run a job every second Tuesday.
Here's the cron expression using the date command:
0 6 * * Tue expr `date +\%s` / 604800 \% 2 >/dev/null || /scripts/fortnightly.shHere's what this expression does:
0 6 * * Tue: Runs the command every Tuesday at 6:00 AM.expr \date +\%s` / 604800 \% 2`: Calculates if the current week is odd or even.date +\%s: Gets the current Unix timestamp./604800: Divides by the number of seconds in a week.\% 2: Checks if the result is odd or even.
>/dev/null: Hides the output of the expression.|| /scripts/fortnightly.sh: Runs the script if the expression returns false (0).
Tip: Testing Your Cron Expression
Before implementing your cron job, test the expression to make sure it works as expected. You can use the date command with the -d option to simulate different dates:
date -d "2023-05-02" +%s | xargs -I{} expr {} / 604800 % 2
date -d "2023-05-09" +%s | xargs -I{} expr {} / 604800 % 2Run these commands for several consecutive Tuesdays to verify the alternating pattern.
Method 2: Using Step Values
Step values in cron let you skip intervals. While not perfect for biweekly schedules, they can be used for approximate solutions.
A cron expression using step values might look like this:
0 6 1-7,15-21 * TueThis expression runs the job on Tuesdays in the first and third weeks of each month.
Good points of this method:
- Easy to set up
- Works with standard cron syntax
Bad points of this method:
- Not exactly biweekly
- May run three times in some months
- Skips some Tuesdays in longer months
