Cron is most suitable for scheduling repetitive tasks. Scheduling one-time tasks can be accomplished using the associated at utility.
Min Hour Day Mon Weekday
* * * * * command to be executed
┬ ┬ ┬ ┬ ┬
│ │ │ │ └─ Day of Week (0=Sun .. 6=Sat)
│ │ │ └────── Month (1..12)
│ │ └─────────── Day of Month (1..31)
│ └──────────────── Hour (0..23)
└───────────────────── Minute (0..59)
| Field | Range | Special characters |
|---|---|---|
| Minute | 0 - 59 | , - * / |
| Hour | 0 - 23 | , - * / |
| Day of Month | 1 - 31 | , - * ? / L W |
| Month | 1 - 12 | , - * / |
| Day of Week | 0 - 6 | , - * ? / L # |
| Example | Description |
|---|---|
*/15 * * * * |
Every 15 mins |
0 * * * * |
Every hour |
0 */2 * * * |
Every 2 hours |
15 2 * * * |
At 2:15AM of every day |
15 2 * * ? |
At 2:15AM of every day |
10 9 * * 5 |
At 9:10AM of every Friday |
0 0 * * 0 |
At midnight of every Sunday |
15 2 * * 1L |
At 2:15am on the last monday of every month |
15 0 * * 4#2 |
At 00:15am on the second thursday of every month |
0 0 1 * * |
Every 1st of month (monthly) |
0 0 1 1 * |
Every 1st of january (yearly) |
@reboot |
Every reboot (non-standard) |
| Special String | Meaning |
|---|---|
| @reboot | Run once, at system startup (non-standard) |
| @yearly | Run once every year, "0 0 1 1 *" (non-standard) |
| @annually | (same as @yearly) (non-standard) |
| @monthly | Run once every month, "0 0 1 * *" (non-standard) |
| @weekly | Run once every week, "0 0 * * 0" (non-standard) |
| @daily | Run once each day, "0 0 * * *" (non-standard) |
| @midnight | (same as @daily) (non-standard) |
| @hourly | Run once an hour, "0 * * * *" (non-standard) |
| - | - |
|---|---|
crontab -e |
Edit or create a crontab file if doesn’t already exist. |
crontab -l |
Display the crontab file. |
crontab -r |
Remove the crontab file. |
crontab -v |
Display the last time you edited your crontab file. (non-standard) |
| Special Character | Description |
|---|---|
Asterik(*) |
Matches all values in the field or any possible value. |
Hyphen(-) |
Used to define a range.Ex: 1-5 in 5th field(Day Of Week) Every Weekday i.e., Monday to Friday |
Slash (/) |
1st field(Minute) /15 meaning every fifteen minute or increment of range. |
Comma (,) |
Used to separate items.Ex: 2,6,8 in 2nd fields(Hour) executes at 2am,6am and 8am |
L |
It is allowed only for Day of Month or Day Of Week field, 2L in Day of week indicates Last tuesday of every month |
Hash (#) |
It is allowed only for Day Of Week field, which must be followed within range of 1 to 5. For example, 4#1 means "The first Thursday" of given month. |
Question mark (?) |
Can be instead of '*' and allowed for Day of Month and Day Of Week. Usage is restricted to either Day of Month or Day Of Week in a cron expression. |