Crontab Generator

Introduction:

Crontab lines are compact schedules that state when commands should run. A cron schedule generator that mirrors the field logic helps you build expressions you can trust and understand quickly.

Enter minutes, hours, and calendar limits or pick a preset that matches a common interval, then read a plain English summary before copying the line. You can paste an existing schedule to interpret it and bring its parts into view for quick edits.

Results show the five timing fields and the complete line with the command and an optional user. Names for months and weekdays are accepted, so familiar words can stand in for numbers when that improves readability.

When both the day of month and the day of week are restricted, the run time may match either condition in many environments, so confirm behavior with a harmless command first. Keep inputs consistent across edits and prefer exact times for predictable runs.

Use the summary to verify intent, then copy the expression or the full line when it says exactly what you expect.

Technical Details:

Schedules are defined by five quantities observed on a repeating calendar: minute, hour, day of month, month, and day of week. A compact expression is formed from these parts, and a full crontab line appends an optional user and the command to execute.

Computation produces three artifacts from the same inputs: the five‑field expression, a human‑readable description of the timing, and a copy‑ready line. Macros beginning with the at sign represent common schedules without expanding fields in place.

Results should be interpreted as inclusive matches on each field. Names are allowed for months and weekdays, and day of week accepts zero or seven for Sunday. Human text summarizes obvious patterns such as every five minutes or a specific time each day.

Comparisons across edits remain valid when the same command and the same field meanings are used. Advanced calendar nuances outside the accepted tokens are not interpreted by the validator.

Processing pipeline:

1. Collect five fields or a macro and the command; user is optional.
2. If a macro is present, keep the macro token as the expression.
3. Otherwise, render each field from simple or advanced mode into a token.
4. Join tokens with spaces to form the five‑field expression.
5. Build the full line by concatenating expression, optional user, and command.
6. Compute a human summary for common patterns and simple times.
7. Validate each field and surface any warnings or errors.

Validation and bounds:

I/O formats and encoding:

Networking & storage behavior:

Processing is client‑only. Copy actions write to the clipboard, and downloads create local CSV or JSON files. No data is transmitted or stored server‑side.

Diagnostics & determinism:

• Identical inputs produce identical expression, full line, and JSON.
• Inline warnings appear when both day‑of‑month and day‑of‑week are restricted.
• Per‑field validation messages explain the first failing token.
• Human summaries favor common patterns and simple daily times.

Security considerations:

• Command text is handled locally; avoid pasting secrets you would not want copied verbatim.
• User detection for six‑field lines relies on a conservative username pattern.

Assumptions & limitations:

• Five timing fields are supported; seconds are not modeled.
• Supported macros: @reboot, @hourly, @daily, @weekly, @monthly, @yearly.
• Accepted tokens are *, lists, ranges, numeric steps, and month or weekday names.
• Heads‑up 0 or 7 both represent Sunday in the day‑of‑week field.
• Names are case‑insensitive but validated against three‑letter abbreviations.
• Unknown special syntax is rejected as an unsupported token.
• User, MAILTO, SHELL, PATH, TZ, and Comment are recorded in JSON but not inserted into the generated line.
• Only the first non‑blank pasted line is parsed in single‑item mode.

Edge cases & error sources:

• Empty field values fail validation.
• Lists with missing items are malformed.
• Ranges outside allowed bounds or with start greater than end are rejected.
• Steps must be positive; zero or negative steps are invalid.
• Out‑of‑range integers and unknown names fail with “value out of bounds.”
• Day‑of‑week 7 is accepted and treated the same as 0 for Sunday.
• Six‑field user detection uses ^[a-z_][a-z0-9_-]*[$]?$; unusual accounts may not match.
• User detection can be ambiguous when a command’s first token looks like a username.
• Mixed case names are normalized for checking; display uses the original text.
• Multiple pasted lines are ignored after the first non‑blank line.

Step‑by‑Step Guide:

The schedule is defined first, then the expression and full line follow from it.

1. Choose a preset or macro, or fill the five fields by hand.
2. Use Advanced mode to enter a list, range, or step value.
3. Optionally paste an existing line to populate fields automatically.
4. Enter the Command; add a User for system crontab style.
5. Check the human summary and any warnings for intent and scope.
6. Copy the expression or the full line when satisfied.

Example: minute 0, hour */2, others * with command /usr/bin/rotate produces 0 */2 * * * /usr/bin/rotate.

Pro tip: parse a known good line, adjust one field, and copy the updated result.

FAQ:

Troubleshooting:

• Copy includes only the expression: the command field is blank.
• User not present in output: the user field is empty or you pasted a five‑field line.
• Weekday name fails: use three‑letter forms such as MON, TUE, WED.
• Range rejected: ensure start and end are within bounds and ordered correctly.
• Step rejected: enter a positive integer after */ or after a range slash.
• Parse ignored extra lines: only the first non‑blank line is considered.

Glossary:

Cron expression

The five‑field string that defines timing.

Macro

A shorthand schedule token beginning with the at sign.

Step

Repeating interval written as */n or x–y/n.

Range

Inclusive start and end written as x–y.

List

Comma‑separated values that all match the field.

User column

The account to run under in system crontab style lines.