⏲️ Cron式生成ツール

What is a cron expression?

A cron expression is a string consisting of five or six fields separated by white space. It represents a set of times for scheduling tasks, typically used in Unix-like operating systems and various software applications. The fields represent, in order: minutes (0-59), hours (0-23), day of the month (1-31), month (1-12 or names), and day of the week (0-7 or names, where 0 and 7 are Sunday). An optional sixth field can represent seconds. For example, the expression "0 2 * * *" would schedule a task to run at 2:00 AM every day.

What do the asterisk (*) and question mark (?) mean in a cron expression?

The asterisk (*) is a wildcard that means "every" for the given field. For instance, an asterisk in the "hours" field means the task runs every hour. The question mark (?) is used only in the "day of month" and "day of week" fields. It means "no specific value" and is used when you need to ignore one of these two fields. Since the day of the month and day of the week are interdependent, you typically specify a value in one and use a ? in the other to avoid confusion.

How do I schedule a job to run on weekdays only?

To schedule a task to run Monday through Friday (weekdays), you use the "day of week" field. The common expression is "0 9 * * 1-5". This breaks down to: minute 0, hour 9 (9 AM), every day of the month, every month, and days 1 through 5 (Monday to Friday). Note that in some systems, days may be numbered 0-6 (Sunday-Saturday), so 1-5 would then represent Monday-Friday. Always check your specific cron implementation's documentation.

What is the difference between slash (/) and comma (,) in cron syntax?

The slash (/) is used for specifying increments or steps. For example, "*/15" in the minutes field means "every 15 minutes" (i.e., at 0, 15, 30, and 45 minutes past the hour). The comma (,) is used to specify a list of values. For instance, "0 8,12,17 * * *" would run a task at 8 AM, 12 PM, and 5 PM every day. You can combine them: "0 0,12 */2 * *" would run at midnight and noon every two days.

Can I use month or day names in a cron expression?

Yes, many cron implementations allow you to use three-letter abbreviations for months and days of the week. For example, you can write "0 0 1 JAN,APR,JUL,OCT *" to run a task on the first day of January, April, July, and October. Similarly, "0 9 * * MON-FRI" is equivalent to "0 9 * * 1-5" for weekdays. This can make expressions more readable. However, always verify that your specific system (like your server's cron daemon or your programming library) supports this non-numeric format.

How do I schedule a task for the last day of the month?

Scheduling for the last day of the month requires a special notation, as the day number varies (28, 29, 30, or 31). The most reliable method is to use the "L" character in the "day of month" field. The expression "0 0 L * *" would run at midnight on the last day of every month. Some advanced cron systems also support "W" for the nearest weekday and "#" for specifying instances like "the second Friday" (e.g., "0 0 * * 5#2"). Check your cron version for support of these special characters.

Where can I test and validate my cron expressions?

You can test and validate cron expressions using various online tools and libraries. Many websites offer cron expression generators and validators where you can input your expression and see a human-readable description of the schedule (e.g., "At 05:30 on every day-of-month from 1 through 7."). For development, programming languages like Python, JavaScript (Node.js with libraries like `node-cron`), and Java (with `Quartz` scheduler) also have built-in or library-based validators to parse expressions and calculate the next execution times.