Cron Command Practice
cron runs commands on a schedule defined by a five-field expression (minute hour day month weekday). This track drills the expression syntax — step, range, and day-of-week selectors — plus the crontab management commands you type to edit, list, and remove schedules without a GUI.
Command Reference
schedule
| Command | Description |
|---|---|
0 * * * * <command> | Run a command at the start of every hour (minute 0). |
*/15 * * * * <command> | Run a command every 15 minutes. |
15 2 * * * <command> | Run a command at 2:15 AM every day. |
* * * * * <command> | Run a command every minute. |
0 */2 * * * <command> | Run a command every 2 hours at the top of the hour. |
10 9 * * 5 <command> | Run a command at 9:10 AM every Friday (day-of-week 5). |
0 0 * * 0 <command> | Run a command at midnight every Sunday (day-of-week 0). |
0 0 1 * * <command> | Run a command at midnight on the 1st day of every month. |
0 0 1 1 * <command> | Run a command at midnight on January 1st each year. |
0 9 * * 1-5 <command> | Run a command at 9 AM every weekday (Monday through Friday). |
15 2 * * 1L <command> | Run a command at 2:15 AM on the last Monday of every month. |
15 0 * * 4#2 <command> | Run a command at 00:15 AM on the second Thursday of every month. |
manage
| Command | Description |
|---|---|
crontab -e | Open the current user's crontab file in the default editor, creating it if it doesn't exist. |
crontab -r | Remove the current user's entire crontab file. |
crontab -l | Print the current user's crontab entries to stdout. |
crontab -v | Display the timestamp of the last time the current user's crontab was edited. |
sudo crontab -e -u <user> | Open a specific user's crontab file for editing (requires root). |
crontab <file> | Replace the current user's crontab with the contents of a file. |
special
| Command | Description |
|---|---|
@yearly <command> | Run a command once per year on Jan 1st at midnight — equivalent to '0 0 1 1 *'. |
@reboot <command> | Run a command once at system startup. |
@hourly <command> | Run a command once every hour — equivalent to '0 * * * *'. |
@daily <command> | Run a command once per day at midnight — equivalent to '0 0 * * *'. |
@weekly <command> | Run a command once per week on Sunday at midnight — equivalent to '0 0 * * 0'. |
@monthly <command> | Run a command once per month on the 1st at midnight — equivalent to '0 0 1 * *'. |
sudo crontab -r -u <user> | Remove another user's crontab file (requires root). |
sudo crontab -l -u <user> | List the crontab entries for a specific user (requires root). |
Key Use Cases
- Run a backup script at 2:15AM every night
- Trigger a health check every 15 minutes
- Schedule a job for the last Monday of each month
- Edit or list the current user's crontab
- Use @daily or @reboot instead of writing the full five fields
- Edit another user's crontab with sudo
Frequently Asked Questions
What does */15 mean in a cron field?
The slash is a step. */15 in the minute field means 'every 15 minutes starting at 0' — i.e., at :00, :15, :30, :45. Without the *, writing 15 alone would only match minute 15.
What is the difference between @daily and @midnight?
They are identical — both expand to '0 0 * * *'. @midnight is an alias kept for backward compatibility. Either is accepted here.
Does cron use 0 or 7 for Sunday?
Both 0 and 7 are accepted for Sunday in the day-of-week field on most cron implementations. This track uses 0 to stay consistent with the POSIX definition.
How do I stop a cron job from running?
crontab -r removes your entire crontab. To remove a single job, use crontab -e, delete the line, and save — there is no flag to remove a single entry non-interactively.
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