Question 1

What does the asterisk (*) mean in a crontab expression?

Accepted Answer

The asterisk is a wildcard operator that represents "every possible value" for that specific field. If you place an asterisk in the minute field, it means the job will run every single minute. If placed in the month field, it means the job will run in every month from January through December. It is essentially a bypass for that specific time constraint, telling the daemon to ignore that field when calculating whether it is time to execute the command.

Question 2

How do I schedule a job to run every other day?

Accepted Answer

In standard POSIX cron, scheduling a job for "every other day" is surprisingly difficult because the month lengths vary (28, 30, or 31 days). If you use a step value like 0 0 */2 * *, the job will run on odd days (1st, 3rd, 5th... 31st). However, when the month rolls over from the 31st to the 1st, the job will run on two consecutive days. To achieve true "every other day" execution, professionals usually schedule the cron job to run every day (0 0 * * *) but add a mathematical check inside the actual bash script, such as evaluating if the number of days since the Unix epoch modulo 2 equals zero.

Question 3

Why is my cron job running at the wrong time?

Accepted Answer

This is almost universally caused by a timezone mismatch. The cron daemon evaluates expressions based on the system's local timezone configuration, not the timezone of your personal computer. If your local machine is in London but your server is located in an AWS data center in Virginia (EST), a job scheduled for 9:00 AM will execute at 2:00 PM London time. You can verify the server's timezone by running the date command in the terminal. The best practice is to always configure servers to UTC and translate your expressions accordingly.

Question 4

Can I schedule a cron job to run every second?

Accepted Answer

No, not with standard Linux cron. The lowest granularity supported by the POSIX cron standard is one minute. The crond daemon literally sleeps and only wakes up at the top of the minute to check the crontab. If you require sub-minute precision, you must either use a different scheduler (like Quartz or Systemd Timers), or write a shell script that uses an infinite loop and the sleep command, though this is highly discouraged as it consumes unnecessary CPU cycles and is difficult to monitor.

Question 5

What is the difference between cron and crontab?

Accepted Answer

"Cron" is the general name for the time-based job scheduling system, and specifically refers to the background daemon process (crond) that executes the tasks. "Crontab" (cron table) refers to the actual text files where the scheduled commands and their time expressions are written. You use the crontab -e command to edit your personal user's scheduling file, which the cron daemon will then read and execute.

Question 6

How do I comment out a line in a crontab file?

Accepted Answer

You can disable a cron job or add human-readable notes by placing a hash symbol (#) at the very beginning of the line. The cron parser ignores any line that starts with a hash. This is incredibly useful for temporarily disabling a job during server maintenance without deleting the complex scheduling expression you spent time writing. Inline comments (placing a hash after the command on the same line) are technically supported by some modern cron implementations, but are generally avoided as they can cause parsing errors in older POSIX-compliant systems.

Question 7

What happens if a cron job takes longer to run than its scheduled interval?

Accepted Answer

Standard cron does not monitor the state of running jobs. If you schedule a script to run every 5 minutes, and that script takes 10 minutes to complete, cron will blindly spawn a second, concurrent instance of the script at the 5-minute mark. This can lead to database deadlocks, file corruption, and server crashes due to memory exhaustion. To prevent this, system administrators use locking utilities like flock in their cron commands (e.g., * * * * * /usr/bin/flock -n /tmp/myjob.lock /path/to/script.sh), which ensures only one instance of the script can run at a time.

Crontab Expression Validator

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