Scheduled data collection with cron in Raspberry Pi - Step-by-Step Execution

Concept Flow - Scheduled data collection with cron

Write data collection script

↓

Create cron job entry

↓

Cron daemon checks schedule

↓

Run script

↓

Store data in DB

↓

Repeat on schedule

The Raspberry Pi runs a script at scheduled times using cron, which collects data and stores it in a database repeatedly.

Execution Sample

Raspberry Pi

#!/bin/bash
# data_collect.sh
python3 /home/pi/collect_data.py

# Cron entry
# * * * * * /home/pi/data_collect.sh

A script runs every minute via cron to collect data and save it to the database.

Execution Table

Step	Cron Time Check	Condition (Is time to run?)	Action	Result
1	00:00	Yes	Run data_collect.sh	Data collected and stored
2	00:01	Yes	Run data_collect.sh	Data collected and stored
3	00:02	Yes	Run data_collect.sh	Data collected and stored
4	00:03	Yes	Run data_collect.sh	Data collected and stored
5	00:04	Yes	Run data_collect.sh	Data collected and stored
6	00:05	Yes	Run data_collect.sh	Data collected and stored
7	00:06	Yes	Run data_collect.sh	Data collected and stored
8	00:07	Yes	Run data_collect.sh	Data collected and stored
9	00:08	Yes	Run data_collect.sh	Data collected and stored
10	00:09	Yes	Run data_collect.sh	Data collected and stored
11	00:10	Yes	Run data_collect.sh	Data collected and stored
12	00:11	Yes	Run data_collect.sh	Data collected and stored
13	00:12	Yes	Run data_collect.sh	Data collected and stored
14	00:13	Yes	Run data_collect.sh	Data collected and stored
15	00:14	Yes	Run data_collect.sh	Data collected and stored
16	00:15	Yes	Run data_collect.sh	Data collected and stored
17	00:16	Yes	Run data_collect.sh	Data collected and stored
18	00:17	Yes	Run data_collect.sh	Data collected and stored
19	00:18	Yes	Run data_collect.sh	Data collected and stored
20	00:19	Yes	Run data_collect.sh	Data collected and stored
21	00:20	Yes	Run data_collect.sh	Data collected and stored
22	00:21	Yes	Run data_collect.sh	Data collected and stored
23	00:22	Yes	Run data_collect.sh	Data collected and stored
24	00:23	Yes	Run data_collect.sh	Data collected and stored
25	00:24	Yes	Run data_collect.sh	Data collected and stored
26	00:25	Yes	Run data_collect.sh	Data collected and stored
27	00:26	Yes	Run data_collect.sh	Data collected and stored
28	00:27	Yes	Run data_collect.sh	Data collected and stored
29	00:28	Yes	Run data_collect.sh	Data collected and stored
30	00:29	Yes	Run data_collect.sh	Data collected and stored
31	00:30	Yes	Run data_collect.sh	Data collected and stored
32	00:31	Yes	Run data_collect.sh	Data collected and stored
33	00:32	Yes	Run data_collect.sh	Data collected and stored
34	00:33	Yes	Run data_collect.sh	Data collected and stored
35	00:34	Yes	Run data_collect.sh	Data collected and stored
36	00:35	Yes	Run data_collect.sh	Data collected and stored
37	00:36	Yes	Run data_collect.sh	Data collected and stored
38	00:37	Yes	Run data_collect.sh	Data collected and stored
39	00:38	Yes	Run data_collect.sh	Data collected and stored
40	00:39	Yes	Run data_collect.sh	Data collected and stored
41	00:40	Yes	Run data_collect.sh	Data collected and stored
42	00:41	Yes	Run data_collect.sh	Data collected and stored
43	00:42	Yes	Run data_collect.sh	Data collected and stored
44	00:43	Yes	Run data_collect.sh	Data collected and stored
45	00:44	Yes	Run data_collect.sh	Data collected and stored
46	00:45	Yes	Run data_collect.sh	Data collected and stored
47	00:46	Yes	Run data_collect.sh	Data collected and stored
48	00:47	Yes	Run data_collect.sh	Data collected and stored
49	00:48	Yes	Run data_collect.sh	Data collected and stored
50	00:49	Yes	Run data_collect.sh	Data collected and stored
51	00:50	Yes	Run data_collect.sh	Data collected and stored
52	00:51	Yes	Run data_collect.sh	Data collected and stored
53	00:52	Yes	Run data_collect.sh	Data collected and stored
54	00:53	Yes	Run data_collect.sh	Data collected and stored
55	00:54	Yes	Run data_collect.sh	Data collected and stored
56	00:55	Yes	Run data_collect.sh	Data collected and stored
57	00:56	Yes	Run data_collect.sh	Data collected and stored
58	00:57	Yes	Run data_collect.sh	Data collected and stored
59	00:58	Yes	Run data_collect.sh	Data collected and stored
60	00:59	Yes	Run data_collect.sh	Data collected and stored
61	01:00	Yes	Run data_collect.sh	Data collected and stored

💡 Cron runs the script every minute as scheduled, so it continues indefinitely.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
Cron Time	00:00	00:01	00:02	00:03	00:04	00:05	continues
Script Run	Yes	Yes	Yes	Yes	Yes	Yes	Yes
Data Stored	Yes	Yes	Yes	Yes	Yes	Yes	Yes

Key Moments - 3 Insights

Why does the script run every minute even though the cron entry looks simple?

Because the cron schedule '* * * * *' means run every minute, as shown in execution_table rows where 'Condition' is always Yes.

What happens if the script fails to run at the scheduled time?

Cron will try again at the next scheduled minute; the execution_table shows each minute as a separate check and run.

How does the data get stored in the database?

The script 'data_collect.sh' runs the Python script which collects and stores data; the 'Result' column in execution_table confirms data is stored after each run.

Visual Quiz - 3 Questions

Look at the execution_table, what is the action taken at step 10?

A Wait and do nothing

B Run data_collect.sh script

C Stop cron daemon

D Delete stored data

💡 Hint

Check the 'Action' column at row 10 in execution_table.

According to variable_tracker, what is the value of 'Script Run' after the third step?

A No

B Unknown

C Yes

D Error

💡 Hint

Look at 'Script Run' row and 'After 3' column in variable_tracker.

If the cron schedule changed to '*/5 * * * *', how would the execution_table change?

A Script runs every 5 minutes only

B Script runs every minute as before

C Script never runs

D Script runs twice every minute

💡 Hint

Understand cron syntax and check the 'Condition' column logic in execution_table.

Concept Snapshot

Cron runs scripts on a schedule.
Use '* * * * *' to run every minute.
Write a script to collect data.
Add script to cron with crontab.
Cron triggers script, data saved in DB.
Runs repeatedly as scheduled.

Full Transcript

This visual execution shows how cron schedules a data collection script on a Raspberry Pi. First, you write a script that collects data and stores it in a database. Then you create a cron job entry with '* * * * *' to run the script every minute. The cron daemon checks the time every minute. If the time matches the schedule, it runs the script. The script collects and stores data. This repeats every minute indefinitely. The execution table shows each minute's check and script run. The variable tracker shows how 'Cron Time', 'Script Run', and 'Data Stored' change over time. Key moments clarify why the script runs every minute and what happens if it fails. The quiz tests understanding of cron timing and script execution. This helps beginners see step-by-step how scheduled data collection works with cron on Raspberry Pi.

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