📅 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 * * * *`

📋 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`

💾 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`

🧪 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`

📈 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`

📧 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

⚙️ 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)

📬 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
```

✍️ 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.

📁 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."

🔧 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.

🛠 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.

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

✅ 1. Create an App Password in Gmail

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

Steps:

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

✅ 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
```

✅ 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

✅ 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 [email protected]

✅ 5. Test the Setup

Run this command:

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

Check your Gmail inbox!

✅ 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.

🛠️ 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).

🧠 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

Schedule Tasks with Precision with Cron Jobs