Field-by-Field Breakdown of `0 */12 * * *`
| Position | Field Name | Allowed Values | Value in `0 */12 * * *` | Evaluation Meaning |
|---|---|---|---|---|
| 1 | Minute | 0 – 59 | 0 | 0 (At minute 0) |
| 2 | Hour | 0 – 23 | */12 | */12 (Hours 0 and 12) |
| 3 | Day of Month | 1 – 31 | * | * (Every day of the month: 1-31) |
| 4 | Month | 1 – 12 | * | * (Every month: 1-12) |
| 5 | Day of Week | 0 – 6 (Sun-Sat) | * | * (Every day of the week: 0-6) |
Semi-Daily Operational Rhythm
A 12-hour cadence (twice daily) is standard for operations that need fresh data twice in a 24-hour cycle without placing heavy continuous load on servers. Typical applications include daytime and nighttime database dumps, clearing temporary staging buckets, and updating inventory reconciliations.
Both 0 */12 * * * and 0 0,12 * * * evaluate identically in all standard cron daemons. The step */12 in the hour position matches hours 0 (midnight) and 12 (noon).
If running resource-intensive tasks, consider offsetting from noon (12:00) to avoid peak traffic hours. For instance, 0 4,16 * * * runs at 4:00 AM and 4:00 PM, avoiding the lunch peak.
Common Production Scenarios for `0 */12 * * *`
Twice-Daily Full Application Backup
Creates comprehensive snapshots of application databases and file uploads, compressing archives for off-site cold storage.
/usr/local/bin/backup_app_full.sh >> /var/log/backup.log 2>&1Stale Cache & Session Garbage Collection
Prunes expired Redis keys, orphaned session files, and abandoned temporary uploads from application storage disks.
/usr/bin/php /var/www/artisan sessions:gcE-Commerce Inventory Sync Across Warehouses
Reconciles online store inventory counts against physical warehouse management systems (WMS) twice daily.
/usr/bin/python3 /opt/inventory/reconcile_stock.pyProduction Gotchas When Running `0 */12 * * *`
- Noon Peak Overload: Executing heavy database dumps at 12:00 PM noon can cause database query latency for real users. Prefer off-peak hours like 0 3,15 * * *.
- Missing Runs on Laptop/Workstation: Cron will not execute missed runs if your development machine is asleep at noon or midnight.
- Job Timeouts: At 12 hours between invocations, scripts should have strict timeout wrappers to avoid lingering zombie processes.
Frequently Asked Questions About `0 */12 * * *`
Is 0 */12 * * * the same as 0 0,12 * * *?
Yes, both syntaxes execute at midnight (00:00) and noon (12:00) every day.
How do I run at 6 AM and 6 PM instead of midnight and noon?
Specify the hours explicitly in the hour field: 0 6,18 * * * /path/to/script.sh.
How many times does 0 */12 * * * run in a month?
Twice per day, so 60 to 62 times depending on the number of days in the month.