Scheduled data collection with cron in Raspberry Pi

Introduction

Scheduling data collection helps you automatically gather information at set times without doing it manually.

You want to record temperature every hour from a sensor.

You need to save system performance data daily.

You want to back up your database every night.

You want to collect data from a website every 10 minutes.

You want to run a script to clean old data weekly.

Syntax

Raspberry Pi

crontab -e

# Each line in crontab has this format:
# * * * * * command_to_run
# | | | | |
# | | | | +---- Day of week (0-7, Sunday=0 or 7)
# | | | +------ Month (1-12)
# | | +-------- Day of month (1-31)
# | +---------- Hour (0-23)
# +------------ Minute (0-59)

Use 'crontab -e' to edit your scheduled tasks.

The five stars (*) mean 'every' unit of time (every minute, hour, etc.).

Examples

Runs the script 'collect_data.sh' every hour at minute 0.

Raspberry Pi

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

Runs the script every 10 minutes.

Raspberry Pi

*/10 * * * * /home/pi/collect_data.sh

Runs the backup script every Sunday at midnight.

Raspberry Pi

0 0 * * 0 /home/pi/backup_db.sh

Sample Program

This example sets up a script that records the Raspberry Pi's CPU temperature every 15 minutes into a text file.

Raspberry Pi

# Step 1: Create a script to collect data
# File: /home/pi/collect_data.sh
#!/bin/bash
DATE=$(date '+%Y-%m-%d %H:%M:%S')
TEMP=$(vcgencmd measure_temp | egrep -o '[0-9]*\.[0-9]*')
echo "$DATE Temperature: $TEMP C" >> /home/pi/temperature_log.txt

# Step 2: Make the script executable
# Run in terminal:
chmod +x /home/pi/collect_data.sh

# Step 3: Schedule the script to run every 15 minutes
# Run in terminal:
crontab -e
# Add this line:
*/15 * * * * /home/pi/collect_data.sh

Important Notes

Make sure your script has execute permission (chmod +x).

Cron runs commands with limited environment; use full paths in scripts.

Check cron logs if your scheduled task does not run as expected.

Summary

Cron helps automate tasks by running scripts at set times.

Use 'crontab -e' to add or edit scheduled jobs.

Scripts must be executable and use full paths for reliability.

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