What are the 5 fields in a cron expression?

A cron expression has 5 fields from left to right: Minute (0-59), Hour (0-23), Day of month (1-31), Month (1-12), and Day of week (0-6, where 0 is Sunday). Each field accepts specific values, ranges (1-5), lists (1,3,5), steps (*/5), or the wildcard (*) for 'every'.

What does */5 * * * * mean in cron?

The expression */5 * * * * means 'every 5 minutes'. The /5 is a step value applied to the minute field. The asterisks in the remaining fields mean 'every hour, every day of the month, every month, every day of the week'.

What are the cron special strings like @daily and @reboot?

Cron supports shorthand strings: @reboot (run once at startup), @yearly or @annually (0 0 1 1 *), @monthly (0 0 1 * *), @weekly (0 0 * * 0), @daily or @midnight (0 0 * * *), and @hourly (0 * * * *). These are supported by most cron implementations but not all.

Why is my cron job running at the wrong time?

The most common causes are: timezone mismatch (cron uses the server's timezone, not yours), forgetting that day-of-month and day-of-week are OR'd together (not AND), using * in the minute field which runs every minute of that hour, and not using full paths for commands since cron runs with a minimal PATH environment variable.

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Cron syntax

Field	Values	Special chars
Minute	0-59	* , - /
Hour	0-23	* , - /
Day of month	1-31	* , - /
Month	1-12	* , - /
Day of week	0-6 (Sun=0)	* , - /

Special characters

*

Any value. * * * * * runs every minute.

,

Value list. 0 0 1,15 * * runs on the 1st and 15th.

-

Range. 0 9-17 * * * runs every hour from 9am to 5pm.

/

Step. */15 * * * * runs every 15 minutes.

Common examples

Expression	Description
/5 * * *	Every 5 minutes
0 /2 * *	Every 2 hours
0 0 * * *	Every day at midnight
0 9 * * 1-5	Weekdays at 9am
0 0 1 * *	First day of every month
0 0 * * 0	Every Sunday at midnight
30 4 1,15 * *	4:30am on the 1st and 15th

Crontab tips

Minute zero matters. If you want something to run once per hour, use 0 * * * * not * * * * *. The latter runs 60 times per hour.

Day-of-month and day-of-week are OR'd. 0 0 15 * 5 runs on the 15th AND every Friday, not just Fridays that fall on the 15th.

Cron uses server timezone. If your server is in UTC but you want 9am EST, you need 0 14 * * *. Or set TZ in your crontab.

Redirect output. Cron emails output by default. Use >/dev/null 2>&1 to suppress, or redirect to a log file to debug failures.

Use full paths. Cron runs with a minimal PATH. Use absolute paths for commands and scripts: /usr/bin/python3 /home/user/script.py

Cron Expression Syntax Explained

A cron expression is five fields separated by spaces. Each field controls when the job runs, read from left to right: minute, hour, day of month, month, and day of week.

# ┌───── minute (0-59)

# │ ┌───── hour (0-23)

# │ │ ┌───── day of month (1-31)

# │ │ │ ┌───── month (1-12)

# │ │ │ │ ┌───── day of week (0-6, Sun=0)

# │ │ │ │ │

0 9 * * 1-5 ← "At 9:00 AM, Monday through Friday"

Each field supports four types of values. A specific number (30) matches that exact value. An asterisk (*) matches every possible value. A range (1-5) matches all values from start to end inclusive. A step (*/10) matches every Nth value. You can combine these with commas: 0,15,30,45 fires at those exact minutes.

The interaction between day-of-month and day-of-week is the biggest gotcha. When both are set to something other than *, they're combined with OR logic, not AND. The expression 0 0 15 * 5 runs on the 15th of every month AND every Friday, not just Fridays that fall on the 15th.

Cron Special Strings and Shortcuts

Most cron implementations support shorthand strings that are easier to read than raw expressions:

Shorthand	Equivalent	Meaning
@reboot	N/A	Run once at system startup
@hourly	0 * * * *	Start of every hour
@daily	0 0 * * *	Midnight every day
@weekly	0 0 * * 0	Midnight on Sunday
@monthly	0 0 1 * *	Midnight on the 1st
@yearly	0 0 1 1 *	Midnight on January 1st

@reboot is particularly useful for starting services or daemons that should run whenever the server boots. Note that these shortcuts aren't supported by every cron implementation. Vixie cron and cronie support them. If you're using a cron library in a programming language, check its documentation.

Common Cron Schedule Mistakes

Running every minute instead of once per hour. * * * * * fires 60 times per hour. If you want "once per hour," use 0 * * * *. The minute field must be a specific value, not *.

Timezone assumptions. Cron uses the server's system timezone. If your server runs UTC and you schedule 0 9 * * * thinking it's 9 AM local time, it won't be. Set the TZ variable in your crontab or convert times manually.

Overlapping executions. If your job takes 10 minutes but runs every 5, you'll have multiple copies running simultaneously. Use flock to prevent overlap: flock -n /tmp/job.lock /path/to/script.sh

Silent failures. Cron emails stdout/stderr by default, but most servers don't have local mail configured. Your job might be failing for weeks without anyone knowing. Redirect output to a log file, or better yet, monitor your cron jobs and get alerted when they miss a scheduled run.

Related Resources

Cron Job Monitoring -Get alerted when your cron jobs fail, run late, or take too long
Load Average Interpreter -Check if overlapping cron jobs are overloading your server
Disk Space Calculator -Plan storage for cron job logs and output files
Server Alerting & Notifications -Monitor server health alongside your scheduled tasks

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