Crontab Validator

Validate, explain, lint, and calculate next run times for cron expressions. Use when asked to check cron syntax, explain a crontab entry, find next scheduled runs, or lint cron expressions for common mistakes. Triggers on "crontab", "cron expression", "cron schedule", "cron syntax", "cron explain", "cron next run", "*/5 * * * *".

Crontab Validator & Explainer

Validate cron syntax, get human-readable explanations, calculate next run times, and lint for common mistakes.

Validate

bash

# Single expression
python3 scripts/cron_check.py validate "*/15 * * * *"

# Multiple expressions with lint
python3 scripts/cron_check.py validate --lint "0 2 * * *" "* * * * *" "0 0 31 2 *"

Explain in Detail

bash

python3 scripts/cron_check.py explain "30 4 1,15 * 1-5"

Next Run Times

bash

# Next 5 runs (default)
python3 scripts/cron_check.py next "0 9 * * 1-5"

# Next 10 runs
python3 scripts/cron_check.py next "0 */6 * * *" --count 10

# From specific time
python3 scripts/cron_check.py next "0 9 * * *" --from-time 2026-01-01T00:00:00

Lint

bash

# Check for common mistakes
python3 scripts/cron_check.py lint "* * * * *" "0 0 31 2 *" "0 0 29 2 *"

# Strict mode (exit 1 on warnings)
python3 scripts/cron_check.py lint --strict "0 0 31 4 *"

Output Formats

bash

python3 scripts/cron_check.py -f json explain "0 9 * * 1-5"
python3 scripts/cron_check.py -f markdown validate --lint "*/5 * * * *"

Supported Syntax

Feature	Example	Description
Wildcard	*	Every value
Specific	5	Exact value
Range	1-5	Values 1 through 5
List	1,3,5	Values 1, 3, and 5
Step	*/15	Every 15th value
Range+Step	1-30/2	Odd values 1-30
Names	mon-fri	Day/month names
Shortcuts	@daily	Predefined schedules

Shortcuts

Shortcut	Equivalent	Meaning
@yearly	0 0 1 1 *	Once a year
@monthly	0 0 1 * *	First of month
@weekly	0 0 * * 0	Every Sunday
@daily	0 0 * * *	Every midnight
@hourly	0 * * * *	Every hour

Lint Checks

Check	Level	Description
Every-minute	Warning	* * * * * runs 1440 times/day
Day 31 in short months	Warning	Apr, Jun, Sep, Nov have 30 days
Feb 29-31	Warning	Only runs in leap years (29) or never
DOM + DOW conflict	Info	Both specified = OR logic
High frequency	Info	More than 288 runs/day