Bash Advanced Scripting: Cron Jobs and Task Automation - A Complete Beginner's Guide (Part 4)

Automation is the key to efficient system administration. Cron is Linux's time-based job scheduler that allows you to run scripts and commands automatically at specified times. This comprehensive guide explains cron from the ground up, with every concept explained in detail for absolute beginners.

⏰ What is Cron?

Cron is a time-based job scheduler in Unix-like operating systems. It runs in the background (as a daemon called crond) and executes commands or scripts at scheduled intervals.

Why Use Cron?

Without cron, you'd have to:

• Manually run backups every night
• Remember to clean up temporary files
• Stay awake to run reports at midnight
• Manually check system health every hour

With cron, you can:

• Schedule automatic backups
• Clean up files while you sleep
• Generate reports at any time
• Monitor systems 24/7

Cron handles repetitive tasks so you don't have to!

🎯 Understanding Cron Syntax

Every cron job follows a specific time format. Let's break it down completely.

The Five Time Fields

* * * * * command-to-execute
│ │ │ │ │
│ │ │ │ └─── Day of week (0-7, where 0 and 7 are Sunday)
│ │ │ └──────── Month (1-12)
│ │ └────────────── Day of month (1-31)
│ └──────────────────── Hour (0-23)
└────────────────────────── Minute (0-59)

Field Value Ranges

Special Characters in Cron

📚 Cron Schedule Examples Explained

Let's look at real examples to understand the syntax:

Example 1: Every Minute

* * * * * /path/to/script.sh

Breaking it down:

• * (minute) = Every minute (0-59)
• * (hour) = Every hour (0-23)
• * (day) = Every day (1-31)
• * (month) = Every month (1-12)
• * (weekday) = Every day of the week (0-7)

Result: Script runs every single minute of every hour, every day.

Example 2: Every Hour at 30 Minutes Past

30 * * * * /path/to/script.sh

Breaking it down:

• 30 (minute) = At exactly 30 minutes past
• * (hour) = Every hour
• * (day) = Every day
• * (month) = Every month
• * (weekday) = Every weekday

Result: Runs at 00:30, 01:30, 02:30, ... 23:30 every day.

Example 3: Daily at 2:30 AM

30 2 * * * /path/to/backup.sh

Breaking it down:

• 30 (minute) = At 30 minutes
• 2 (hour) = At 2 AM
• * (day) = Every day of the month
• * (month) = Every month
• * (weekday) = Any day of the week

Result: Runs at 2:30 AM every single day.

Example 4: Every Monday at 9:00 AM

0 9 * * 1 /path/to/weekly-report.sh

Breaking it down:

• 0 (minute) = At 0 minutes (on the hour)
• 9 (hour) = At 9 AM
• * (day) = Any day of month
• * (month) = Any month
• 1 (weekday) = Monday (0=Sunday, 1=Monday, etc.)

Result: Runs at 9:00 AM every Monday.

Example 5: First Day of Every Month

0 0 1 * * /path/to/monthly-cleanup.sh

Breaking it down:

• 0 (minute) = At minute 0
• 0 (hour) = At midnight (00:00)
• 1 (day) = On the 1st day of the month
• * (month) = Every month
• * (weekday) = Any weekday

Result: Runs at midnight on the 1st of every month.

Example 6: Every 15 Minutes

*/15 * * * * /path/to/monitor.sh

Breaking it down:

• */15 (minute) = Every 15 minutes (0, 15, 30, 45)
• * (hour) = Every hour
• * (day) = Every day
• * (month) = Every month
• * (weekday) = Every weekday

Result: Runs at :00, :15, :30, :45 of every hour, every day.

The / operator: */15 means "start at 0, then step by 15". So: 0, 15, 30, 45, then repeat.

Example 7: Weekdays Only at 6 PM

0 18 * * 1-5 /path/to/workday-report.sh

Breaking it down:

• 0 (minute) = At minute 0
• 18 (hour) = At 6 PM (18:00 in 24-hour format)
• * (day) = Any day of month
• * (month) = Any month
• 1-5 (weekday) = Monday through Friday

Result: Runs at 6:00 PM only on weekdays (Monday-Friday).

The - operator: 1-5 specifies a range from 1 (Monday) to 5 (Friday).

Example 8: Multiple Specific Times

0,30 8,17 * * * /path/to/twice-daily.sh

Breaking it down:

• 0,30 (minute) = At minute 0 AND minute 30
• 8,17 (hour) = At 8 AM AND 5 PM
• * (day) = Every day
• * (month) = Every month
• * (weekday) = Every weekday

Result: Runs at 8:00, 8:30, 17:00, and 17:30 every day.

The , operator: Lists multiple specific values. 0,30 means "at 0 and at 30".

🚀 Creating Your First Cron Job

Let's create a practical cron job step by step.

Step 1: Create a Test Script

First, create a simple script that writes to a file:

mkdir cron
cd cron
nano cron_test.sh

Step 2: Write the Script

#!/bin/bash
echo "Cron test" >> ~/cron_output.txt

Understanding the Script

#!/bin/bash

Purpose: Tells the system to use Bash to execute this script.

echo "Cron test" >> ~/cron_output.txt

Purpose: Appends "Cron test" to a file in the home directory.

Breaking it down:

• echo "Cron test" - Outputs the text "Cron test"
• >> - Append operator (doesn't overwrite)
• ~/cron_output.txt - File in your home directory (~ expands to home path)

Step 3: Make the Script Executable

chmod +x cron_test.sh

What this does: Adds execute permission so cron can run the script.

Step 4: Get the Full Script Path

Cron requires absolute paths (full paths from root), not relative paths.

pwd

Output:

/home/centos9/Razzaq-Labs-II/random/random/error_handling/cron

Your full script path:

/home/centos9/Razzaq-Labs-II/random/random/error_handling/cron/cron_test.sh

Step 5: Open Crontab Editor

crontab -e

What this does: Opens your personal crontab (cron table) in the default text editor.

On first run: The system might ask you to choose an editor. Select nano or your preferred editor.

The crontab command:

• crontab -e - Edit your crontab
• crontab -l - List your crontab entries
• crontab -r - Remove your crontab (be careful!)

Step 6: Add the Cron Job

In the editor, add this line:

* * * * * /home/centos9/Razzaq-Labs-II/random/random/error_handling/cron/cron_test.sh

This schedule:

• * * * * * = Every minute
• Followed by the absolute path to your script

Save and exit the editor.

Output after saving:

crontab: installing new crontab

What happened: Your cron job is now active and will run every minute!

Step 7: Verify the Cron Job

crontab -l

Output:

* * * * * /home/centos9/Razzaq-Labs-II/random/random/error_handling/cron/cron_test.sh

What this shows: Your currently scheduled cron jobs.

Step 8: Wait and Check Results

Wait at least 60 seconds, then check the output file:

sleep 60
cat ~/cron_output.txt

Output:

Cron test
Cron test

What happened: The script ran twice (once per minute for 2 minutes)!

Check again after a few more minutes:

cat ~/cron_output.txt

Output:

Cron test
Cron test
Cron test
Cron test
Cron test
Cron test

Perfect! Each line represents one execution.

Step 9: Understanding What Happened

Let's trace the entire flow:

1. Cron daemon (crond) is always running in the background
2. Every minute, it checks all crontabs
3. At second 0 of each minute, it sees your job needs to run
4. Cron executes your script in a new shell process
5. The script appends "Cron test" to the file
6. Process completes, cron waits for the next minute
7. Cycle repeats indefinitely

🛑 Stopping Your Test Cron Job

Since this job runs every minute, you'll want to stop it:

crontab -e

Delete the line or comment it out by adding # at the start:

# * * * * * /home/centos9/Razzaq-Labs-II/random/random/error_handling/cron/cron_test.sh

Save and exit.

Verify it's removed:

crontab -l

Output: Should show no active jobs (or just commented lines).

📋 Special Time Shortcuts

Cron provides convenient shortcuts for common schedules:

Example usage:

@daily /path/to/daily-backup.sh
@reboot /path/to/startup-script.sh
@hourly /usr/local/bin/check-system-health.sh

🔧 Best Practices for Cron Scripts

1. Always Use Absolute Paths

# Wrong - relative path
cron_job.log

# Right - absolute path
/home/username/logs/cron_job.log

2. Set Environment Variables

Cron runs with a minimal environment. Set variables your script needs:

#!/bin/bash
PATH=/usr/local/bin:/usr/bin:/bin
HOME=/home/username
export PATH HOME

# Rest of your script

3. Redirect Output for Debugging

Capture both stdout and stderr:

* * * * * /path/to/script.sh >> /path/to/logfile.log 2>&1

Breaking it down:

• >> /path/to/logfile.log - Append stdout to log
• 2>&1 - Redirect stderr (2) to wherever stdout (1) is going

4. Email Notifications

By default, cron emails output to the user. To disable:

MAILTO=""
* * * * * /path/to/script.sh

To send to specific email:

MAILTO="admin@example.com"
* * * * * /path/to/script.sh

5. Lock Files to Prevent Overlap

Prevent multiple instances of the same job:

#!/bin/bash
LOCKFILE="/tmp/myscript.lock"

if [ -e "$LOCKFILE" ]; then
    echo "Script already running"
    exit 1
fi

touch "$LOCKFILE"
# Your script logic here
rm "$LOCKFILE"

🐛 Debugging Cron Jobs

Check if Cron is Running

systemctl status crond

Expected output: Should show "active (running)".

Check Cron Logs

# On systems with systemd
journalctl -u crond

# On older systems
tail -f /var/log/cron

Test Your Script Manually First

Before adding to cron:

# Run the script directly
bash /full/path/to/script.sh

# Check if it works
echo $?  # Should return 0 for success

Common Issues and Solutions

🎯 Best Practices

📝 Command Cheat Sheet

Crontab Management

# Edit your crontab
crontab -e

# List your crontab
crontab -l

# Remove your crontab (careful!)
crontab -r

# Edit another user's crontab (as root)
crontab -u username -e

Common Cron Schedules

# Every minute
* * * * * /path/to/script.sh

# Every 5 minutes
*/5 * * * * /path/to/script.sh

# Every hour at minute 30
30 * * * * /path/to/script.sh

# Every day at 2:30 AM
30 2 * * * /path/to/script.sh

# Every Sunday at 3:00 AM
0 3 * * 0 /path/to/script.sh

# Every weekday at 6:00 PM
0 18 * * 1-5 /path/to/script.sh

# First day of every month at midnight
0 0 1 * * /path/to/script.sh

# Every 15 minutes during work hours (9am-5pm)
*/15 9-17 * * * /path/to/script.sh

# Twice a day (8am and 8pm)
0 8,20 * * * /path/to/script.sh

Using Special Time Shortcuts

# At system startup
@reboot /path/to/startup-script.sh

# Once a day at midnight
@daily /path/to/daily-backup.sh

# Once an hour
@hourly /path/to/hourly-check.sh

# Once a week
@weekly /path/to/weekly-report.sh

# Once a month
@monthly /path/to/monthly-cleanup.sh

Output Redirection in Cron

# Redirect output to log file
* * * * * /path/to/script.sh >> /var/log/script.log 2>&1

# Discard all output
* * * * * /path/to/script.sh > /dev/null 2>&1

# Send only errors to log
* * * * * /path/to/script.sh 2>> /var/log/script-errors.log

# Append to log with timestamps
* * * * * echo "$(date): Running script" >> /var/log/cron.log && /path/to/script.sh

🎯 Real-World Automation Examples

Example 1: Daily Database Backup

# Run every day at 2:30 AM
30 2 * * * /home/admin/scripts/backup-database.sh >> /var/log/backup.log 2>&1

Example 2: Clean Temporary Files

# Run every Sunday at 3:00 AM
0 3 * * 0 /usr/local/bin/cleanup-temp-files.sh

Example 3: Check Disk Space Every Hour

# Run every hour
@hourly /home/admin/scripts/check-disk-space.sh

Example 4: Generate Report on First of Month

# Run at midnight on the 1st of every month
0 0 1 * * /home/admin/reports/monthly-report.sh

🚀 Summary

💬 Discussion

I'd love to hear about your experience:

• What tasks are you automating with cron?
• Have you encountered any tricky cron scheduling challenges?
• What automation projects are you planning?
• What other Bash topics would you like to learn?

Connect with me:

• 🐙 GitHub - Example scripts and more
• 📧 Contact - Bash scripting questions and discussions