THE CRON EXPRESSION

the five schedule fields: what each one means, what values it takes, and how to read stars, lists and steps

Built as a memo rather than a table: the thing worth hanging it up for reads across the room, with the full reference in smaller type below it.

Three sizes in one archive — from a sheet on the fridge to a poster on the wall.

Formats: A4 · A3 · A2
What you get: Print-ready PDFs, one per size, with every letterform converted to outlines — nothing to install, nothing to shift.
Subject: code · beginners · reference · study

Fields, bounds, the asterisk, lists, ranges and the OR rule for the two calendar fields follow POSIX (The Open Group Base Specifications, crontab); steps, names, “7 is Sunday” and the @-shortcuts follow crontab(5), Linux man-pages (Vixie cron). Examples are verified by execution.

IT IS NOT FRIDAY THE 13TH

The two calendar fields together mean OR, not AND: when both day of month and day of week are restricted, the job runs on days matching either one (POSIX). So “0 0 13 * 5” is not Friday the 13th — it is every Friday plus every 13th.

In January 2026 that line fires six times: five Fridays and Tuesday the 13th — although the month has no Friday the 13th at all.

On the sheet

Fields, bounds, the asterisk, lists, ranges and the OR rule for the two calendar fields follow POSIX (The Open Group Base Specifications, crontab); steps, names, “7 is Sunday” and the @-shortcuts follow crontab(5), Linux man-pages (Vixie cron). Examples are verified by execution.
Colour carries meaning, but meaning never rests on colour alone — the sheet reads in black and white.
Nothing about how it was made: the sheet carries only what the reader needs.

Printing

Three sizes in the archive: A4 — 210×297 mm, A3 — 297×420 mm, A2 — 420×594 mm.
All text is converted to outlines. No fonts to install, and nothing shifts on another machine.
White background and sparing fills — it prints on a home printer without draining the cartridge.
Margins of at least 10 mm: a home printer cannot print to the edge.
No stroke thinner than 0.3 mm, so nothing disappears in print.

Why the numbers can be trusted. Every countable number here is computed from data rather than typed in, and the sheet must pass 8 independent checks before it is allowed to build — how many items there are, the order they come in, and the claim in the highlighted box itself. If one fails, there is no file.

THE CRON EXPRESSION

IT IS NOT FRIDAY THE 13TH

On the sheet

Printing

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