Schedule Tasks with Precision with Cron Jobs

📅 Cron Jobs Cheat Sheet: Schedule Tasks with Precision

🛠️ Cron Syntax Format

* * * * * /path/to/command arg1 arg2
│ │ │ │ │
│ │ │ │ └───── Day of the Week (0 - 7) (Sunday = 0 or 7)
│ │ │ └──────── Month (1 - 12)
│ │ └──────────── Day of the Month (1 - 31)
│ └──────────────── Hour (0 - 23)
└──────────────────── Minute (0 - 59)

🔁 Special Time Strings

String	Meaning	Equivalent To
@reboot	Run once, at startup	-
@yearly	Once a year	0 0 1 1 *
@annually	Same as @yearly	0 0 1 1 *
@monthly	Once a month	0 0 1 * *
@weekly	Once a week	0 0 * * 0
@daily	Once a day	0 0 * * *
@midnight	Same as @daily	0 0 * * *
@hourly	Once an hour	0 * * * *

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📋 Common Cron Job Examples

📂 File & Directory Tasks

Task	Cron Schedule	Command
Backup /home daily at 2 AM	0 2 * * *	tar -czf /backup/home.tar.gz /home
Clean tmp folder every day at midnight	0 0 * * *	rm -rf /tmp/*
Sync files every 10 minutes	*/10 * * * *	rsync -av /source /destination

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💾 System Maintenance

Task	Cron Schedule	Command
Update package list daily at 1 AM	0 1 * * *	sudo apt update
Reboot system every Sunday at 4 AM	0 4 * * 0	sudo reboot
Monitor disk usage every 30 min	*/30 * * * *	df -h >> /var/log/disk_usage.log

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🧪 Script Execution

Task	Cron Schedule	Command
Run a script at 3 PM every day	0 15 * * *	/home/user/myscript.sh
Run script on the first day of the month	0 0 1 * *	bash /path/to/monthly_task.sh
Run every 5 minutes	*/5 * * * *	/home/user/script.sh

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📈 Logging & Monitoring

Task	Cron Schedule	Command
Append date to file every Sunday at 6:15 PM	15 18 * * 0	date >> /home/user/sundays.txt
Log user sessions daily at midnight	0 0 * * *	who >> /var/log/user_sessions.log

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📧 Email Notification

To send output to your email, add this to the top of your crontab:

MAILTO="[email protected]"

Example:

0 9 * * * /path/to/backup.sh

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⚙️ Crontab Commands

Command	Description
crontab -e	Edit current user’s crontab
crontab -l	List current user’s crontab
crontab -r	Remove current user’s crontab
crontab -u [user] -l	List another user’s crontab (as root)
crontab -u [user] -e	Edit another user’s crontab (as root)

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📬 Step-by-Step Guide: Email Notification from Cron Jobs

✅ 1. Install a Mail Transfer Agent (MTA)

You need an MTA like mailutils or sendmail:

• Ubuntu/Debian:

  sudo apt update
  sudo apt install mailutils
  

• CentOS/RHEL/Fedora:

  sudo yum install mailx
  

• Arch Linux:

  sudo pacman -S mailutils
  

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✍️ 2. Set the MAILTO Variable in Your Crontab

Open your crontab:

crontab -e

At the top, add:

MAILTO="[email protected]"

Then write a cron job below it. For example:

MAILTO="[email protected]"
0 5 * * * /home/user/backup.sh

This will send standard output (stdout) and standard error (stderr) of backup.sh to your email.

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📁 3. Ensure Your Script Produces Output

Only scripts that generate output will trigger emails.

Example script:

#!/bin/bash
echo "Backup started at $(date)"
tar -czf /backup/home.tar.gz /home
echo "Backup completed."

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🔧 4. Test It

Create a simple script:

echo -e '#!/bin/bash\necho "Test email from cron at $(date)"' > ~/testcron.sh
chmod +x ~/testcron.sh

Add to crontab:

MAILTO="[email protected]"
*/2 * * * * /home/yourusername/testcron.sh

Check your email after 2–3 minutes.

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🛠 Troubleshooting

Problem	Solution
Not receiving mail	Check your spam folder. Also try running the command manually and verify it produces output.
Cron not sending mail	Ensure mailutils is installed. Try `echo “body” | mail -s “Subject” [email protected]` to test manually.
External SMTP needed	Use tools like msmtp, sSMTP, or configure postfix with Gmail SMTP.

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📬 Step-by-Step: Send Cron Job Emails via Gmail SMTP

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✅ 1. Create an App Password in Gmail

Gmail no longer allows direct login from less secure apps. You must use an App Password.

Steps:

1. Go to https://myaccount.google.com/
2. Enable 2-Step Verification if not already enabled.
3. Go to Security > App passwords
4. Generate a new password (select “Other”, name it cronmail, click “Generate”)
5. Copy the 16-character password — you’ll need this soon

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✅ 2. Install msmtp and msmtp-mta

• Debian/Ubuntu:

  sudo apt install msmtp msmtp-mta
  

• Arch Linux:

  sudo pacman -S msmtp
  

• Fedora/RHEL:

  sudo dnf install msmtp
  

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✅ 3. Configure msmtp

Create a config file:

mkdir -p ~/.config/msmtp
nano ~/.config/msmtp/config

Paste this into the file (replace with your Gmail info):

defaults
auth           on
tls            on
tls_trust_file /etc/ssl/certs/ca-certificates.crt
logfile        ~/.config/msmtp/msmtp.log

account        gmail
host           smtp.gmail.com
port           587
from           [email protected]
user           [email protected]
password       your_app_password

account default : gmail

Replace [email protected] and your_app_password accordingly.

🛡 Important: Secure the file!

chmod 600 ~/.config/msmtp/config

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✅ 4. Set msmtp as the Mailer

Ensure mail command uses msmtp. Create or edit this file:

nano ~/.mailrc

Add:

set sendmail="/usr/bin/msmtp"
set use_from=yes
set realname="Your Name"
set from=[email protected]

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✅ 5. Test the Setup

Run this command:

echo "This is a test from msmtp" | mail -s "Test Email" [email protected]

Check your Gmail inbox!

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✅ 6. Add to Crontab

Edit your crontab:

crontab -e

Add this:

MAILTO="[email protected]"
* * * * * echo "Hello from cron at $(date)"

Check if you receive an email every minute.

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🛠️ Troubleshooting

Issue	Fix
No email received	Check Gmail spam folder. Check ~/.config/msmtp/msmtp.log for logs.
TLS errors	Make sure ca-certificates package is installed and path to certs is correct.
Cron output not triggering email	Ensure the command prints output (stdout or stderr).

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🧠 Tips

• Use full paths to commands and scripts.
• Redirect output to a log: command >> /path/to/logfile.log 2>&1
• Test cron expressions with: https://crontab.guru

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