Scheduled data collection with cron in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this cron schedule?

Given the cron expression 30 2 * * 1-5, when will the scheduled task run?

A At 2:30 AM every weekday (Monday to Friday)

B At 2:30 PM every day

C At 2:00 AM every Monday to Friday

D At 30 minutes past every hour on weekdays

💡 Hint

Remember the order of fields in a cron expression: minute, hour, day of month, month, day of week.

The cron expression means: minute 30, hour 2, every day of month, every month, days Monday to Friday (1-5). So it runs at 2:30 AM on weekdays.

📝 Syntax

intermediate

Identify the syntax error in this cron job entry

Which option shows a syntax error in a cron job entry?

A 0 25 * * * /home/pi/script.sh

B 0 0 * * 0 /home/pi/script.sh

C 15 14 1 * * /home/pi/script.sh

D */10 * * * * /home/pi/script.sh

💡 Hint

Check the valid range for the hour field in cron.

The hour field must be between 0 and 23. '25' is invalid and causes a syntax error.

❓ optimization

advanced

Optimize this cron schedule for hourly data collection

You want to collect data every hour at 15 minutes past the hour. Which cron expression is the most efficient and correct?

A */15 * * * * /home/pi/collect_data.sh

B 0 15 * * * /home/pi/collect_data.sh

C 15 * * * * /home/pi/collect_data.sh

D 15 0-23 * * * /home/pi/collect_data.sh

💡 Hint

Remember the order: minute, hour, day, month, weekday.

Option C runs the script at minute 15 of every hour, every day, which matches the requirement.

🔧 Debug

advanced

Why does this cron job not run as expected?

A cron job entry is: 0 6 * * * /home/pi/data_collect.sh. The script runs fine manually but does not run at 6 AM. What is the most likely cause?

A The script path is incorrect because cron uses a different working directory

B Cron does not support running scripts at 6 AM

C The cron daemon is not installed on Raspberry Pi

D The script lacks execute permissions for the user running cron

💡 Hint

Check permissions and environment differences between manual and cron runs.

If the script is not executable by the cron user, it won't run even if scheduled correctly.

🧠 Conceptual

expert

How to ensure data collection runs even if the previous run is still active?

You schedule a data collection script every 10 minutes with cron. Sometimes the script takes longer than 10 minutes. Which approach ensures no overlapping runs?

A Run multiple instances in parallel to speed up data collection

B Use a lock file in the script to check if a previous instance is running and exit if so

C Restart the Raspberry Pi before each scheduled run

D Schedule the cron job every 5 minutes instead of 10

💡 Hint

Think about how to prevent multiple script instances from running simultaneously.

Using a lock file prevents overlapping runs by letting the script detect if another instance is active.

Practice

1. What is the main purpose of using cron on a Raspberry Pi for database tasks?

easy

A. To create database backups only when the Pi is restarted

B. To manually start database queries

C. To store data permanently

D. To automatically run scripts at scheduled times

Solution

1. Step 1: Understand cron's role

   Cron is a tool that runs commands or scripts automatically at set times without user intervention.

2. Step 2: Connect cron to database tasks

   Using cron, you can schedule database scripts to run regularly, like collecting data or backups.

3. Final Answer:

   To automatically run scripts at scheduled times -> Option D

4. Quick Check:

   Cron automates tasks = A [OK]

Hint: Cron schedules tasks automatically, no manual start needed [OK]

Common Mistakes:

• Thinking cron runs scripts only manually
• Confusing cron with data storage
• Assuming cron runs only at reboot

2. Which of the following is the correct command to edit the cron jobs on a Raspberry Pi?

easy

A. crontab -e

B. cronedit

C. cron -edit

D. editcron

Solution

1. Step 1: Recall the standard cron command

   The command to edit cron jobs is crontab -e, which opens the cron table for editing.

2. Step 2: Verify other options

   Other options like cronedit, cron -edit, and editcron are not valid commands.

3. Final Answer:

   crontab -e -> Option A

4. Quick Check:

   Edit cron jobs = crontab -e [OK]

Hint: Use 'crontab -e' to edit cron jobs every time [OK]

Common Mistakes:

• Typing 'cronedit' instead of 'crontab -e'
• Using wrong command syntax
• Confusing cron commands with other tools

3. Given this cron job entry:
0 * * * * /home/pi/scripts/collect_data.sh
What does this cron job do?

medium

A. Runs the script every minute

B. Runs the script every hour at minute 0

C. Runs the script once a day at midnight

D. Runs the script every day at noon

Solution

1. Step 1: Understand cron time format

   The five fields represent minute, hour, day of month, month, and day of week. '0 * * * *' means minute 0 of every hour.

2. Step 2: Interpret the schedule

   This means the script runs at the start of every hour (e.g., 1:00, 2:00, 3:00, etc.).

3. Final Answer:

   Runs the script every hour at minute 0 -> Option B

4. Quick Check:

   Minute 0 every hour = hourly run [OK]

Hint: Minute 0 with * hour means hourly at start [OK]

Common Mistakes:

• Thinking it runs every minute
• Confusing hour and minute fields
• Assuming daily or noon run

4. A user added this cron job:
*/15 * * * * collect_data.sh
But the script does not run. What is the most likely problem?

medium

A. The script path is missing; cron needs full path

B. The cron syntax is invalid

C. The script runs only once a day

D. Cron does not support scripts with .sh extension

Solution

1. Step 1: Check cron job command

   The command collect_data.sh lacks a full path. Cron requires full paths to scripts.

2. Step 2: Understand cron environment

   Cron runs with a minimal environment and does not know where to find scripts without full paths.

3. Final Answer:

   The script path is missing; cron needs full path -> Option A

4. Quick Check:

   Always use full script path in cron [OK]

Hint: Always specify full script path in cron jobs [OK]

Common Mistakes:

• Assuming cron finds scripts in current directory
• Thinking .sh extension is unsupported
• Believing syntax is wrong when it is correct

5. You want to collect sensor data every day at 6:30 AM and save it to a database using a script at /home/pi/scripts/save_data.sh. Which cron entry correctly schedules this?

hard

A. 30 06 * * 1-5 /home/pi/scripts/save_data.sh

B. 6 30 * * * /home/pi/scripts/save_data.sh

C. 30 6 * * * /home/pi/scripts/save_data.sh

D. 0 6 * * * /home/pi/scripts/save_data.sh

Solution

1. Step 1: Understand cron time fields

   The format is: minute hour day month weekday. To run at 6:30 AM daily, minute=30 and hour=6.

2. Step 2: Check each option

   30 6 * * * /home/pi/scripts/save_data.sh uses '30 6 * * *' which means 6:30 AM every day. 6 30 * * * /home/pi/scripts/save_data.sh reverses minute and hour. 30 06 * * 1-5 /home/pi/scripts/save_data.sh restricts to weekdays only. 0 6 * * * /home/pi/scripts/save_data.sh runs at 6:00 AM.

3. Final Answer:

   30 6 * * * /home/pi/scripts/save_data.sh -> Option C

4. Quick Check:

   Minute 30, hour 6 daily = 30 6 * * * /home/pi/scripts/save_data.sh [OK]

Hint: Minute first, then hour in cron time fields [OK]

Common Mistakes:

• Swapping hour and minute fields
• Forgetting to specify full script path
• Limiting days unintentionally