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Crontab Calculator

Parse cron expressions and calculate next execution times for Linux scheduled tasks

Tool Overview

The Crontab Calculator is a free online tool that helps you parse and understand cron expressions, calculate next execution times, and learn about Linux crontab scheduled tasks. It supports standard 5-field cron syntax with special characters like *, -, /, and comma.

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Crontab Expression Syntax

A cron expression consists of 5 fields separated by spaces:

minute hour day-of-month month day-of-week

Field Ranges

Field Allowed Values Description
minute 0-59 Minute of the hour
hour 0-23 Hour of the day (24-hour format)
day-of-month 1-31 Day of the month
month 1-12 or JAN-DEC Month of the year
day-of-week 0-7 or SUN-SAT Day of the week (0 and 7 both represent Sunday)

Special Characters

  • * - Wildcard, matches any value
  • - - Range, matches values from start to end
  • , - List, matches any value in the comma-separated list
  • / - Step, matches every Nth value within a range
  • L - Last, matches the last day of the month or last weekday
  • W - Weekday, matches the nearest weekday to the specified date

Common Examples

Expression Description
*/5 * * * * Every 5 minutes
0 * * * * Every hour at minute 0
0 0 * * * Every day at midnight
0 9 * * 1 Every Monday at 9 AM
0 8 * * 1-5 Every weekday at 8 AM
*/15 9-17 * * 1-5 Every 15 minutes during business hours (9 AM - 5 PM), Monday to Friday
0 0 1 * * First day of every month at midnight
0 0 L * * Last day of every month at midnight
0 12 * * 1,3,5 Every Monday, Wednesday, and Friday at noon
0 0 * * 0 Every Sunday at midnight

Linux Crontab Scheduled Tasks

Crontab is a time-based job scheduler in Linux and Unix-like operating systems. It allows users to schedule jobs (commands or scripts) to run periodically at fixed times, dates, or intervals.

How Crontab Works

  1. Cron daemon (crond) runs in the background and checks the crontab files every minute
  2. Each user has their own crontab file located at /var/spool/cron/crontabs/<username>
  3. System-wide cron jobs are stored in /etc/cron.d/ and /etc/crontab
  4. When a cron expression matches the current time, the associated command is executed

Crontab File Format

A standard crontab entry has the following format:

minute hour day month weekday command

Example:

0 2 * * * /usr/bin/backup.sh > /var/log/backup.log 2>&1

Features

  • Parse standard 5-field cron expressions
  • Calculate next 10 execution times
  • Support for wildcards (*), ranges (-), lists (,), and steps (/)
  • Common example buttons for quick reference
  • Real-time calculation as you type
  • Error handling for invalid expressions
  • Offline support - works without internet connection

FAQ

What is a cron expression?

A cron expression is a string consisting of 5 time-related fields that define when a scheduled task should run. The fields are: minute (0-59), hour (0-23), day-of-month (1-31), month (1-12), and day-of-week (0-7).

What does * mean in a cron expression?

The asterisk (*) is a wildcard that matches any value for that field. For example, * * * * * means "every minute of every hour of every day."

What is the difference between day-of-month and day-of-week?

The day-of-month field specifies which days of the month to run (1-31), while day-of-week specifies which days of the week (0-7, where 0 and 7 are Sunday). Both conditions must be satisfied for the job to run, except when one is set to *.

Why isn't my cron job running?

Common reasons include: incorrect cron syntax, wrong file permissions, missing PATH environment variable, the cron daemon not running, or the command producing errors that are not logged. Check /var/log/syslog or /var/log/cron for error messages.

How do I test a cron expression?

Use this Crontab Calculator to verify your expression and see the next execution times. You can also use the date command to test individual time components, and check syslog for actual execution logs.