Cron Expression Generator

How it works

The page stores five strings and joins them in the standard order: minute, hour, day of month, month, and day of week. The field validator accepts stars, comma-separated pieces, numeric ranges, and numeric steps, then checks the numbers against the common Unix bounds. It is intentionally a shape check rather than a full scheduler emulator because cron dialects disagree about names, seconds, timezone declarations, and special macros.

The preset values are ordinary expressions, not hidden server templates. Selecting one fills the same five fields a person can edit. That makes the result inspectable: the labels remain next to the expression and the output can be copied into a review or pull request without relying on a generated English sentence that might omit a boundary.

It reads as well as writes. Paste an existing line from a crontab into the expression box and the five fields split out of it, because the common job is not composing a schedule from nothing — it is changing one part of a schedule that already exists. The expression is the single value the page holds; the fields are a second way into it, so editing either keeps the two in agreement. A six-field expression with seconds in front keeps its first five, which is the closest honest reading of a dialect this page does not claim to build.

Test the schedule before relying on it

A cron string does not include the command, its environment, or the destination's timezone. A job that works in an interactive shell may fail under cron because PATH, HOME, credentials, or the working directory differ. Log the command, use absolute paths where appropriate, and run the exact expression through a scheduler-specific test tool when the job matters.

For daylight-saving transitions, think in instants rather than wall-clock promises. A local hour can occur twice or not at all. For complex business schedules, a calendar-aware scheduler with explicit timezone support is safer than piling more punctuation into a five-field expression.

Day-of-month and day-of-week are ORed, not ANDed

This is the single most common way a cron expression does something other than what it looks like, and it is worth checking every time both fields are set.

Every other pair of fields narrows the schedule. Set the hour to 9 and the minute to 30 and you get one moment a day, because the fields combine with AND. Day-of-month and day-of-week are the exception: when both are restricted, cron runs the job when either matches.

So 0 9 1 * 1 does not mean "the first of the month, if it is a Monday". It means "the first of every month, and every Monday" — roughly five times as often as intended. The behavior is specified rather than a bug, and it dates back to the original Unix implementation, but nothing in the syntax hints at it.

The safe habit is to leave one of the two as * whenever you constrain the other. If you genuinely need "the first Monday of the month", cron cannot express it; put the day check in the script — run every Monday and exit early unless the date is in the first seven — or use a scheduler that models calendars.

Silence is the normal failure

A cron job that fails does not usually announce itself. It writes to stdout or stderr, cron mails that output to the local user, and on a modern server nothing is reading that mailbox. The job stops working and everything looks fine.

The pattern that avoids this is to make the job responsible for its own reporting. Redirect both streams to a file you will actually look at — >> /var/log/thing.log 2>&1 — and, for anything that matters, have the script signal success somewhere you monitor rather than relying on the absence of an error.

The second half is the environment. Cron runs with a minimal PATH and none of the variables your shell profile sets, so a command that works when you type it can fail with "not found" under cron and be logged nowhere. Use absolute paths, set PATH explicitly at the top of the crontab, and test the way cron will run it: env -i /bin/sh -c 'your command here' reproduces the stripped environment far better than running it in your own terminal.