crontab syntax in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the crontab command in Linux?

The crontab command is used to schedule tasks (called cron jobs) to run automatically at specified times or intervals.

beginner

In crontab syntax, what does the asterisk (*) symbol mean?

The asterisk (*) means "every" possible value for that time field. For example, * in the minute field means every minute.

beginner

Explain the five time fields in a crontab entry.

The five time fields are: minute (0-59), hour (0-23), day of month (1-31), month (1-12), and day of week (0-7, where 0 and 7 both mean Sunday). They define when the command runs.

intermediate

How do you schedule a cron job to run every 5 minutes?

Use */5 in the minute field, like: */5 * * * *. This means every 5 minutes, every hour, every day, every month, every weekday.

intermediate

What does this crontab line do? <br>0 0 1 * * /home/user/backup.sh

It runs the script /home/user/backup.sh at midnight (00:00) on the first day of every month.

What does the following crontab time field mean? <br>30 14 * * *

A Run at 14:30 on the first day of every month

B Run every 30 minutes starting at 2 PM

C Run every minute between 2 PM and 3 PM

D Run at 2:30 PM every day

The fields mean minute=30, hour=14 (2 PM), and the rest are wildcards meaning every day, month, and weekday.

Which symbol in crontab means 'every' value for a time field?

A #

B *

C /

D -

The asterisk (*) means every possible value for that time field.

How do you specify a cron job to run only on Sundays?

A Both A and B

B * * * * 7

C * * * * 1

D * * * * 0

In crontab, both 0 and 7 represent Sunday in the day of week field.

What does this crontab entry do? <br>15 10 * * 1-5 /usr/bin/script.sh

A Runs script.sh at 10:15 AM Monday to Friday

B Runs script.sh at 10:15 AM every day

C Runs script.sh every minute from 10:00 to 10:15 on weekdays

D Runs script.sh at 10:15 AM on weekends

The day of week field 1-5 means Monday to Friday.

Which crontab field controls the month when a job runs?

A First field

B Second field

C Fourth field

D Fifth field

The fourth field in crontab syntax is the month (1-12).

Describe the structure of a crontab entry and what each time field means.

Think about the order and range of each time field.

Explain how to schedule a cron job to run every day at 6:30 AM.

Use exact numbers for hour and minute, wildcards for others.