What is the difference between cron and crontab?

cron is the background service (daemon) responsible for "running things on a schedule"; crontab is the configuration file it reads, and also the command used to edit that file (crontab -e). The scheduling rules you write are stored in the crontab, and the cron service runs them on time.

How many fields does a cron expression have?

A standard cron has 5 fields: minute, hour, day of month, month, and day of week. Some systems (such as Quartz and Spring) use 6 fields, adding a "seconds" field at the front.

In the day-of-week field, do both 0 and 7 mean Sunday?

In most implementations 0 means Sunday; some also accept 7 for Sunday. For compatibility, it is recommended to consistently use 0 for Sunday.

What Is Cron? crontab Expression Format Explained

What Is Cron

Cron is the background service on Linux, macOS, and other Unix-like systems responsible for running tasks on a schedule. You simply tell it “when to run and what command to run,” and it runs automatically at the specified time—no manual supervision needed. Common uses: automatically backing up a database in the early morning, cleaning up temporary files every hour, checking service status every minute, and so on.

crontab: cron’s Configuration

The configuration cron reads is called the crontab (short for cron table). At the same time, crontab is also the command used to manage this configuration:

crontab -e: edit the current user’s scheduled tasks
crontab -l: list existing scheduled tasks
crontab -r: delete all tasks (use with care)

Each line in a crontab = one cron expression + the command to run, for example:

0 3 * * * /usr/bin/backup.sh

This means: run /usr/bin/backup.sh every day at 3 AM. The leading 0 3 * * * is the cron expression, which describes “when.”

The Five Fields of a Cron Expression

A standard cron expression consists of 5 fields, separated by spaces. From left to right, they are:

Position	Field	Range	Description
1	Minute	0-59	Which minute
2	Hour	0-23	Which hour (24-hour clock)
3	Day of month	1-31	Which day of the month
4	Month	1-12	Which month
5	Day of week	0-6	Which weekday (0 = Sunday)

For example, 30 8 * * 1 means: every Monday at 8:30 AM.

Meaning of Special Characters

Within each field you can combine these symbols to build flexible time rules:

Symbol	Meaning	Example
`*`	Every value (any value)	`*` in the minute field = every minute
`,`	Enumerate multiple values	`9,18` in the hour field = 9 AM and 6 PM
`-`	Continuous range	`1-5` in the day-of-week field = Monday through Friday
`/`	Step (every N)	`*/5` in the minute field = every 5 minutes

Combined example: */10 9-18 * * 1-5 means every 10 minutes between 9 AM and 6 PM on weekdays.

Common Pitfalls

Confusing “day of month” and “day of week”: when both are set to specific values, most systems run the task “if either matches,” which can trigger more often than expected. In everyday use, just use one of them.
Forgetting that commands need absolute paths: cron’s runtime PATH is very minimal, so it’s best to write python, node, etc. as full paths.
Writing seconds into the expression: standard cron has no seconds; if you need second-level precision, use a 6-field implementation (such as Quartz).

Next Steps

Now that you understand the format, you can:

Use the Cron Expression Generator to visually build expressions and preview their run times;
Read How to Write a Cron Expression to learn to construct them from scratch;
Browse the Cron Expression Examples to copy common patterns.

What Is Cron? A Clear Guide to crontab and Expression Format