Scheduled data collection with cron in Raspberry Pi - Mini Project: Build & Apply

Scheduled Data Collection with cron on Raspberry Pi

📖 Scenario: You have a Raspberry Pi that collects temperature data from a sensor. You want to save this data into a simple text database file every minute automatically.This project will guide you to set up a cron job that runs a script to append the current temperature and timestamp to a data file.

🎯 Goal: Create a cron job on your Raspberry Pi that runs a script every minute to collect temperature data and save it to a file called temperature_data.txt.

📋 What You'll Learn

Create a bash script called collect_temp.sh that appends the current timestamp and a simulated temperature value to temperature_data.txt.

Make the script executable.

Create a cron job that runs the script every minute.

Verify the cron job is scheduled correctly.

💡 Why This Matters

🌍 Real World

Automating data collection on devices like Raspberry Pi is common in home automation, weather stations, and IoT projects.

💼 Career

Understanding cron jobs and scripting is essential for system administrators, DevOps engineers, and IoT developers to automate tasks and manage data collection.

1

Create the data collection script

Create a bash script called collect_temp.sh that appends the current date and time plus a simulated temperature value (use $((RANDOM % 30 + 10)) to generate a temperature between 10 and 39) to a file named temperature_data.txt. Use the format: YYYY-MM-DD HH:MM:SS Temperature.

Raspberry Pi

# Create the collect_temp.sh script
# Your code here

Hint

Use date '+%Y-%m-%d %H:%M:%S' to get the current timestamp. Use $((RANDOM % 30 + 10)) to simulate temperature.

2

Make the script executable

Make the collect_temp.sh script executable by adding the correct permission using the chmod command.

Raspberry Pi

#!/bin/bash

echo "$(date '+%Y-%m-%d %H:%M:%S') $((RANDOM % 30 + 10))" >> temperature_data.txt

# Make the script executable
# Your code here

Hint

Use chmod +x collect_temp.sh to make the script executable.

3

Schedule the cron job

Add a cron job that runs the collect_temp.sh script every minute. Use crontab -e to edit the cron jobs and add the line: * * * * * /path/to/collect_temp.sh. Replace /path/to/collect_temp.sh with the full path to your script.

Raspberry Pi

# Add cron job line below
# Your code here

Hint

Use * * * * * /full/path/collect_temp.sh to run the script every minute.

4

Verify the cron job setup

Verify that the cron job is scheduled by running crontab -l and ensure the line * * * * * /home/pi/collect_temp.sh appears. Also, check that the temperature_data.txt file is being updated every minute.

Raspberry Pi

# Verify cron job with crontab -l
# Your code here

Hint

Run crontab -l to list scheduled cron jobs and confirm your job is listed.

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