Cron jobs for scheduled tasks in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this cron schedule expression?

Given the cron schedule expression 30 14 * * 1-5, when will the cron job run?

A At 2:30 PM every Saturday and Sunday

B At 2:30 PM every weekday (Monday to Friday)

C At 2:30 AM every day

D At 14:00 every weekday (Monday to Friday)

💡 Hint

Remember, the fields are: minute, hour, day of month, month, day of week.

The expression means minute 30, hour 14 (2 PM), every day of month, every month, and days Monday (1) through Friday (5). So it runs at 2:30 PM on weekdays.

❓ Predict Output

intermediate

What will this cron job output?

A cron job runs the command echo $(date +"%H:%M") > /home/pi/time.txt every hour at minute 0. What will be the content of time.txt after the job runs at 3 PM?

A 15:00

B 03:00

C 3 PM

D The current date and time in full

💡 Hint

The format string %H:%M gives hour in 24-hour format and minutes.

The command outputs the current hour and minute in 24-hour format, so at 3 PM it writes '15:00' to the file.

🔧 Debug

advanced

Why does this cron job not run?

A user added this line to their crontab: 0 9 * * * /home/pi/myscript.sh. The script has execute permission and runs fine manually. But the cron job never runs. What is the most likely reason?

A The script path is incorrect or missing environment variables in cron

B The cron job syntax is invalid

C The script does not have execute permission

D The cron daemon is not installed on Raspberry Pi

💡 Hint

Cron jobs run with a limited environment and may not have the same PATH as your user.

Cron runs with a minimal environment. If the script relies on environment variables or relative paths, it may fail. The path is correct and script is executable, so environment is the likely cause.

🧠 Conceptual

advanced

How to schedule a cron job to run every 15 minutes?

Which cron schedule expression correctly runs a job every 15 minutes?

A 0,15,30,45 * * * *

B 15 * * * *

C */15 * * * *

D 0-15 * * * *

💡 Hint

The slash (/) operator in cron means 'every'.

The expression '*/15 * * * *' means every 15 minutes (0,15,30,45), every hour, every day, etc. A ('0-15 * * * *') runs every minute from 0 to 15. B runs only at minute 15 each hour. C explicitly lists the minutes.

📝 Syntax

expert

What error does this crontab line cause?

Consider this crontab line: 0 25 * * * /home/pi/script.sh. What happens when cron tries to run this?

A The cron daemon ignores the line and runs other jobs

B The job runs at 1 AM (25 mod 24)

C The job runs at midnight

D The cron job will not run because hour 25 is invalid

💡 Hint

Hours in cron must be between 0 and 23.

The hour field must be 0-23. 25 is invalid, so cron will reject the line and not run the job.