Crontab expression conventions

Contract

Write a five-field crontab expression to the standard form: the fields in the fixed order minute, hour, day-of-month, month, day-of-week; * for anything unconstrained; the */n step form for a regular interval; only one of the two day fields restricted unless a union is genuinely wanted (cron ORs them); 0 and 7 both meaning Sunday; and a leading comment naming the schedule in words. Apply when producing or reviewing a crontab line; not for systemd timers, for parsing a cron string, or for non-cron scheduling.

Rules

1. Five fields, fixed order. Exactly five space-separated fields, in this order:

┌ minute (0–59) │ ┌ hour (0–23) │ │ ┌ day-of-month (1–31) │ │ │ ┌ month (1–12) │ │ │ │ ┌ day-of-week (0–7)

The minute is first and the day-of-week is last — not the other way around.

1. * means every. A field you are not constraining is * — every minute, every day, every

month. Do not enumerate all values (1,2,3,…,12) to mean "every".

1. Operators. , for a list (0,30), - for an inclusive range (1-5), / for a step.

A step over the whole field is */n (*/15); a step over a range is start-end/n (9-17/2).

1. Regular intervals use */n. "Every 15 minutes" is */15, NOT the list 0,15,30,45 and

NOT a range like 0-59/15 or 0-45/15. Reach for the step form whenever the interval is regular.

1. The day-of-month / day-of-week trap. If BOTH the day-of-month field and the day-of-week

field are restricted (neither is *), cron runs the job when EITHER matches — a union, not an intersection. So constrain only ONE of them and leave the other *, unless you deliberately want the union. "First of the month at 2am" is 0 2 1 * * (day-of-week *), never 0 2 1 * 1.

1. 0 and 7 are both Sunday. In the day-of-week field, 0 and 7 both mean Sunday; 1 is

Monday … 6 is Saturday.

1. Lead with a comment. Put a comment line stating the schedule in plain words directly above

the expression, so a reader does not have to decode five fields:

# every day at 03:30 30 3 * * *

Worked examples

Interval — enumerated list vs. the step form:

BEFORE   0,20,40 * * * *          # "every 20 minutes"
AFTER    */20 * * * *

Field order — a fixed daily time:

# 2:30 PM every day
30 14 * * *                        # minute=30 first, hour=14 second

Unconstrained fields stay *:

BEFORE   0 12 1-31 1-12 0-6        # "noon every day", every field spelled out
AFTER    0 12 * * *

Single day condition — leave the other day field *:

# every Tuesday at 08:00
0 8 * * 2                          # day-of-month is *, only day-of-week is set

The day trap — a union you DID want, both fields set:

# 1st of the month and every Friday, at midnight
0 0 1 * 5                          # runs on the 1st OR any Friday (cron ORs the two)

Sunday, either spelling:

# every Sunday at midnight
0 0 * * 0                          # 0 and 7 both mean Sunday

Edge cases & exceptions

• "Every N hours" fixes the minute so it fires once per mark: 0 */6 * * *, not */6 * * * *.
• Two specific times that are not a clean interval (6am and 6pm) are a list, not a step:

0 6,18 * * *.

• A business-hours window uses a range in the hour field: 0 9-17 * * 1-5.
• Wanting the 1st ONLY when it is a weekday is an INTERSECTION cron cannot express with both day

fields — restrict day-of-month and guard the weekday in the command, or accept the union.

• Month and day-of-week accept names (JAN, MON), but numbers are the portable default.
• @daily, @hourly macros exist but the explicit five fields are the reviewable form.

Do / Don't

• Do keep the five fields in minute-hour-dom-month-dow order. Don't put the hour before the minute.
• Do use * for an unconstrained field. Don't enumerate every value to mean "every".
• Do write a regular interval as */n. Don't spell it as a list or an equivalent range.
• Do restrict only one day field unless a union is intended. Don't set both day-of-month and

day-of-week by reflex.

• Do lead with a plain-words comment. Don't ship a bare five-field line with no explanation.

Common mistakes

• Hour and minute swapped — 14 30 * * * for 2:30 PM instead of 30 14 * * *.
• "Every 15 minutes" written as 0,15,30,45 * * * * or 0-45/15 * * * * instead of */15.
• Both day fields pinned (0 9 1 * 1) expecting "1st AND Monday" — cron runs it on the 1st OR any

Monday.

• Every field spelled out (0-59 0-23 1-31 1-12 0-6) instead of * * * * *.
• Forgetting that 0 and 7 both mean Sunday.
• No leading comment, so the schedule is unreadable at a glance.

Quick checklist

• Five fields, order minute · hour · day-of-month · month · day-of-week.
• * for unconstrained; */n for a regular interval.
• Only one day field set unless the union is intended.
• 0 or 7 for Sunday; a leading plain-words comment.