Cron Expression: Cron Every Other Day

This cron expression represents: cron every other day

Cron Expression

0 0 */2 * *

What This Expression Does

Runs at midnight on every odd-numbered day of the month — the 1st, 3rd, 5th, and so on. The */2 step in the day-of-month field starts counting from day 1, so it selects days 1, 3, 5, …, 29, 31.

The Month-Boundary Caveat

*/2 resets every month, so it is not strict “every 48 hours.” After a 31-day month, the job fires on the 31st and then again on the 1st of the next month — two consecutive days. February in a leap year does the opposite (runs on the 29th, then a one-day gap to the 1st). For most maintenance jobs this once-a-month wobble is harmless.

If exact alternation matters, run daily and let a date check decide:

0 0 * * * [ $(( $(date +\%s) / 86400 \% 2 )) -eq 0 ] && /path/to/script.sh

This runs the script only on even days since the Unix epoch, giving a true every-second-day rhythm across month boundaries. (The % signs are escaped because % is special in crontab lines.)

Use Cases

This cron schedule is commonly used for:

• Backups where daily is overkill but weekly is too sparse
• Rotating or vacuuming medium-sized database tables
• Syncing datasets that change slowly
• Certificate or dependency freshness checks

How to Use

1. Copy the cron expression above
2. Open your crontab with crontab -e
3. Add a new line with: 0 0 */2 * * /path/to/your/script.sh
4. Save and exit

Alternative Formats

• Every other day at 6am instead of midnight: 0 6 */2 * *
• Odd days explicitly: 0 0 1-31/2 * * (same behavior as */2)
• Even days of the month: 0 0 2-30/2 * *