crontab syntax in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this crontab entry?

Given the crontab line below, what time(s) will the command run?

30 2 * * 1 /usr/bin/backup.sh

Linux CLI

30 2 * * 1 /usr/bin/backup.sh

A Runs every Monday at 2:30 AM

B Runs every day at 2:30 AM

C Runs every Monday at 12:30 PM

D Runs every day at 12:30 PM

💡 Hint

Remember the order: minute, hour, day of month, month, day of week.

The first field is minutes (30), second is hour (2), and the last field '1' means Monday. So it runs at 2:30 AM every Monday.

💻 Command Output

intermediate

What does this crontab entry do?

What is the effect of this crontab line?

0 */4 * * * /usr/bin/cleanup.sh

Linux CLI

0 */4 * * * /usr/bin/cleanup.sh

A Runs every 4 hours at the start of the hour

B Runs every 4 minutes

C Runs every 4 days at midnight

D Runs every day at 4 AM

💡 Hint

The hour field uses '*/4' which means every 4 hours.

The minute is 0, hour is every 4 hours (0,4,8,12,16,20), so it runs at the start of those hours daily.

📝 Syntax

advanced

Which crontab entry runs a command every weekday at 6:15 PM?

Select the correct crontab line to run a script at 6:15 PM Monday through Friday.

A 15 6 * * 1-5 /usr/bin/script.sh

B 18 15 * * 1-5 /usr/bin/script.sh

C 15 18 * * 1-5 /usr/bin/script.sh

D 15 18 * * 0-6 /usr/bin/script.sh

💡 Hint

Remember the order: minute, hour, day of month, month, day of week.

Minute is 15, hour is 18 (6 PM), and days 1-5 are Monday to Friday.

🔧 Debug

advanced

Why does this crontab entry not run at midnight as expected?

A user wants to run a script at midnight every day but wrote:

0 24 * * * /usr/bin/midnight.sh

What is the problem?

A Minute field cannot be 0

B Hour field '24' is invalid; valid range is 0-23

C Day of month field is missing

D Command path is incorrect

💡 Hint

Check the valid ranges for hour in crontab.

Hours must be between 0 and 23. '24' is invalid and causes the job not to run.

🚀 Application

expert

How many times will this crontab entry run in a week?

Given this crontab line:

*/15 9-17 * * 1-5 /usr/bin/task.sh

How many times does the command run in one week?

A 240 times

B 200 times

C 160 times

D 180 times

💡 Hint

Calculate runs per hour, hours per day, and days per week.

Runs every 15 minutes (4 times per hour) from 9 to 17 hours (9 hours) Monday to Friday (5 days). 4 * 9 * 5 = 180. But 9-17 means hours 9 through 17 inclusive, which is 9 hours (9,10,11,12,13,14,15,16,17). So 4 runs/hour * 9 hours/day * 5 days = 180 runs. The correct answer is 180.