What do the 5 fields of a standard cron expression represent?

Standard Unix crontab expressions consist of 5 space-delimited fields: 1. Minute (0-59), 2. Hour (0-23 in 24-hour format), 3. Day of the Month (1-31), 4. Month of the Year (1-12 or JAN-DEC), and 5. Day of the Week (0-6 or SUN-SAT, where 0 and 7 represent Sunday).

How does day of month and day of week interaction work in cron?

In standard POSIX cron, if both the Day of Month and Day of Week fields are specified with values other than asterisk (*), the job executes when EITHER field matches (an OR relationship, not an AND relationship). For example, '0 0 15 * 1' runs on the 15th of the month AND on every Monday.

What is the difference between asterisk (*) and slash (/)?

An asterisk (*) means every possible value in that field. A slash (/) specifies step increments. For example, */15 in the minute field means 'every 15 minutes starting from 0' (0, 15, 30, 45). Similarly, 10-30/5 means 'every 5 minutes between minute 10 and 30'.

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Translate complex crontab schedules into plain English, inspect the next 10 trigger dates, and generate 1-click cron daemon commands.

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Live Cron Syntax

Presets:

Minute 0 - 59

Hour 0 - 23

Day of Month 1 - 31

Month 1 - 12 (JAN-DEC)

Day of Week 0 - 6 (SUN-SAT)

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Human-Readable Meaning

“At 12:00 AM (midnight), every day”

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Executed Command or Script Path

Standard & Error Output Log Redirect

Ready to paste into crontab -e:

0 0 * * * /usr/bin/python3 /opt/scripts/backup.py >> /var/log/backup.log 2>&1

Cron Daemon Best Practices

Always use absolute paths: The cron environment has a minimal PATH. Explicitly define binaries like /usr/bin/node.
Capture stderr: Always append 2>&1 to capture silent runtime crashes into your log file.
Timezone Caveats: The crond daemon triggers jobs according to the server's system timezone (often UTC).

Mastering Linux Crontab Expressions

1. Special Character Syntax

* (Asterisk): Wildcard representing every valid integer in that field range.
, (Comma): Value list separator (e.g. 1,15,30).
- (Hyphen): Inclusive integer range (e.g. 9-17 represents hours 9 through 17).
/ (Slash): Step intervals (e.g. */10 in minutes means every 10 minutes).

2. Special Nicknames in Vixie Cron

Many modern cron daemons accept shorthand string shortcuts:

@yearly / @annually ➔ 0 0 1 1 *

@monthly ➔ 0 0 1 * *

@weekly ➔ 0 0 * * 0

@daily / @midnight ➔ 0 0 * * *

@hourly ➔ 0 * * * *

@reboot ➔ Run once at startup

Frequently Asked Questions

Why did my cron job fail to run at the scheduled time? ▾

The most common cause is missing environment variables or relative paths. When cron executes a script, it does not load your personal shell profile (~/.bashrc or ~/.zshrc), so environment variables like PATH, HOME, or virtualenvs may not exist. Always use absolute paths and set full environment variables inside your script.

What happens during Daylight Saving Time (DST) changes? ▾

During the spring transition (clocks jump forward 1 hour), jobs scheduled between 02:00 and 02:59 may be skipped entirely. During the fall transition (clocks repeat 1 hour), jobs may execute twice. For mission-critical background jobs, servers should always have their system clock configured to UTC (Coordinated Universal Time), which has no daylight saving shifts.

How do I prevent overlapping cron job executions? ▾

If a job takes longer than its scheduling interval (for example, a 5-minute task taking 7 minutes), multiple instances will run concurrently and may cause database lock contention. Wrap your crontab command with flock: 0 * * * * flock -n /var/lock/myjob.lock /opt/scripts/job.sh. The -n flag ensures new instances exit cleanly if an existing run is still active.