Scheduled data collection with cron in Raspberry Pi - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is cron used for on a Raspberry Pi?

Cron is a tool that runs commands or scripts automatically at scheduled times or intervals on a Raspberry Pi.

beginner

How do you edit the cron jobs for the current user?

You use the command crontab -e to open and edit the list of scheduled tasks for the current user.

intermediate

What does the cron time format */5 * * * * mean?

It means the task runs every 5 minutes, every hour, every day, every month, and every weekday.

intermediate

Why is it important to use full paths in cron jobs?

Because cron runs with a limited environment, using full paths ensures the commands and scripts run correctly without missing files or programs.

intermediate

How can you check if your cron job ran successfully?

You can check the cron logs using grep CRON /var/log/syslog or redirect output to a file in your cron job.

Which command opens the cron editor for the current user?

A crontab -e

B cronedit

C cron -open

D editcron

The command crontab -e opens the cron editor for the current user.

What does the asterisk (*) symbol mean in a cron schedule?

A Skip this field

B No value

C Only once

D Every possible value for that time field

An asterisk (*) means the task runs for every possible value of that time field.

If you want a script to run every day at 3 AM, which cron time would you use?

A 0 3 * * *

B 3 0 * * *

C * 3 * * *

D 0 0 3 * *

The format is minute hour day month weekday. So 0 3 * * * runs at 3:00 AM every day.

Why should you use full paths in cron commands?

A To make the command shorter

B Because cron has a limited environment and may not find commands without full paths

C To speed up the cron job

D It is not necessary to use full paths

Cron runs with a limited environment, so full paths ensure commands run correctly.

Where can you find logs to check if cron jobs ran successfully?

A /home/user/cron.log

B /etc/cron.log

C /var/log/syslog

D /var/log/cronjobs

Cron logs are usually found in /var/log/syslog on Raspberry Pi systems.

Explain how to schedule a Python script to run every hour on a Raspberry Pi using cron.

Think about the cron time format and environment paths.

Describe how you would troubleshoot if your scheduled data collection script does not run as expected with cron.

Look for logs and environment issues.

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