Cron jobs for scheduled tasks in Raspberry Pi

Introduction

Cron jobs help you run commands or scripts automatically at set times. This saves you from doing repetitive tasks manually.

You want to turn on a light every day at 7 PM.

You need to back up files every night without forgetting.

You want to check your system status every hour.

You want to run a script to update weather info every morning.

You want to clean temporary files every week automatically.

Syntax

Raspberry Pi

minute hour day_of_month month day_of_week command_to_run

Each part is a number or symbol that tells when to run the command.

Use * to mean 'every' for that time part.

Examples

Runs the script at 7:00 AM every day.

Raspberry Pi

0 7 * * * /home/pi/turn_on_light.sh

Runs backup script at 10:30 PM every Monday.

Raspberry Pi

30 22 * * 1 /home/pi/backup.sh

Runs the status check every 15 minutes.

Raspberry Pi

*/15 * * * * /home/pi/check_status.sh

Sample Program

This example shows how to edit the cron table to schedule a script. You open the cron editor, add a line with the time and command, then save it.

Raspberry Pi

# Open terminal on Raspberry Pi
crontab -e

# Add this line to run a script every day at 8 AM
0 8 * * * /home/pi/myscript.sh

# Save and exit the editor

# The script /home/pi/myscript.sh will run automatically at 8 AM daily

Important Notes

Make sure your script has execute permission: chmod +x /home/pi/myscript.sh

Use full paths in your scripts because cron runs with limited environment.

Check cron logs if your job does not run: grep CRON /var/log/syslog

Summary

Cron jobs automate tasks by running commands at scheduled times.

Use the crontab editor to add or change cron jobs.

Always test your scripts manually before scheduling them.

Practice

1. What is the main purpose of a cron job on a Raspberry Pi?

easy

A. To update the Raspberry Pi operating system

B. To manually execute commands in the terminal

C. To schedule tasks to run automatically at specific times

D. To install new software packages

Solution

1. Step 1: Understand what cron jobs do

   Cron jobs are used to automate tasks by running commands or scripts at scheduled times without manual intervention.

2. Step 2: Compare options with cron job purpose

   Only To schedule tasks to run automatically at specific times describes scheduling tasks automatically, which matches the purpose of cron jobs.

3. Final Answer:

   To schedule tasks to run automatically at specific times -> Option C

4. Quick Check:

   Cron job purpose = schedule tasks automatically [OK]

Hint: Cron jobs run commands automatically on schedule [OK]

Common Mistakes:

• Confusing cron jobs with manual command execution
• Thinking cron updates the system automatically
• Assuming cron installs software

2. Which of the following is the correct syntax to edit the cron jobs for the current user on a Raspberry Pi?

easy

A. cronedit

B. cron -edit

C. editcron

D. crontab -e

Solution

1. Step 1: Recall the command to edit cron jobs

   The standard command to edit cron jobs for the current user is crontab -e.

2. Step 2: Verify other options

   Options A, B, and D are not valid Linux commands for editing cron jobs.

3. Final Answer:

   crontab -e -> Option D

4. Quick Check:

   Edit cron jobs command = crontab -e [OK]

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

Common Mistakes:

• Typing incorrect commands like 'cronedit'
• Using 'cron -edit' which is invalid
• Assuming 'editcron' is a command

3. Given the cron job entry 30 2 * * * /home/pi/backup.sh, what does it do?

medium

A. Runs backup.sh every day at 2:30 AM

B. Runs backup.sh every 30 minutes

C. Runs backup.sh every day at 2:00 PM

D. Runs backup.sh every 2 hours and 30 minutes

Solution

1. Step 1: Understand cron time fields

   The fields are: minute (30), hour (2), day of month (*), month (*), day of week (*). This means the command runs at 2:30 AM every day.

2. Step 2: Match the time to options

   Runs backup.sh every day at 2:30 AM correctly states it runs daily at 2:30 AM. Other options misunderstand the timing.

3. Final Answer:

   Runs backup.sh every day at 2:30 AM -> Option A

4. Quick Check:

   Cron time 30 2 * * * = 2:30 AM daily [OK]

Hint: Minute then hour in cron means 30 2 is 2:30 AM [OK]

Common Mistakes:

• Confusing AM and PM hours
• Thinking it runs every 30 minutes
• Misreading the asterisks as specific days

4. You added this cron job: 0 25 * * * /home/pi/script.sh but it never runs. What is the problem?

medium

A. The hour value 25 is invalid in cron syntax

B. The script path is incorrect

C. The minute value 0 is invalid

D. Cron jobs cannot run scripts

Solution

1. Step 1: Check the hour field in cron syntax

   The hour field must be between 0 and 23. 25 is invalid and causes the job not to run.

2. Step 2: Verify other options

   The script path might be correct, minute 0 is valid, and cron can run scripts, so only the hour value is the problem.

3. Final Answer:

   The hour value 25 is invalid in cron syntax -> Option A

4. Quick Check:

   Cron hour must be 0-23, 25 is invalid [OK]

Hint: Hour in cron is 0-23, not 25 [OK]

Common Mistakes:

• Using invalid hour values above 23
• Assuming minute 0 is wrong
• Thinking cron can't run scripts

5. You want to run a Python script every weekday at 7:15 AM on your Raspberry Pi. Which cron job entry is correct?

hard

A. 7 15 * * 1-5 /usr/bin/python3 /home/pi/script.py

B. 15 7 * * 1-5 /usr/bin/python3 /home/pi/script.py

C. 15 7 * * 6,7 /usr/bin/python3 /home/pi/script.py

D. 15 7 1-5 * * /usr/bin/python3 /home/pi/script.py

Solution

1. Step 1: Understand cron fields for time and days

   The format is minute hour day_of_month month day_of_week. To run at 7:15 AM, minute=15, hour=7. Weekdays are days 1-5 (Monday to Friday).

2. Step 2: Check each option

   15 7 * * 1-5 /usr/bin/python3 /home/pi/script.py correctly uses '15 7 * * 1-5' for 7:15 AM on weekdays. 7 15 * * 1-5 /usr/bin/python3 /home/pi/script.py swaps hour and minute. 15 7 * * 6,7 /usr/bin/python3 /home/pi/script.py uses weekend days (6,7). 15 7 1-5 * * /usr/bin/python3 /home/pi/script.py uses day_of_month 1-5, not weekdays.

3. Final Answer:

   15 7 * * 1-5 /usr/bin/python3 /home/pi/script.py -> Option B

4. Quick Check:

   Weekday cron = 1-5, time 7:15 = 15 7 [OK]

Hint: Weekdays are 1-5 in cron day_of_week field [OK]

Common Mistakes:

• Swapping hour and minute fields
• Using day_of_month instead of day_of_week for weekdays
• Confusing weekend days with weekdays