How to Schedule Script with Cron

Quick Answer: Schedule Script with Cron

To schedule a script, edit your crontab with crontab -e and add a line like: 0 9 * * * /path/to/script.sh. This runs the script daily at 9:00 AM. The five fields are: minute, hour, day of month, month, day of week. Use * for “any” and comma-separated lists for multiple values.

Quick Comparison: Scheduling Methods

Frequency	Cron Expression	Best For	Complexity
Every minute	* * * * *	Frequent tasks	Simple
Hourly	0 * * * *	Regular checks	Simple
Daily	0 9 * * *	Daily jobs	Simple
Weekly	0 9 * * 0	Weekly tasks	Simple
Complex	Multiple entries	Many schedules	Moderate

Bottom line: Use crontab -e to edit; use simple expressions for common schedules.

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Schedule Bash scripts to run periodically using cron. Learn cron syntax, crontab management, and practical scheduling examples.

Cron Syntax

Cron uses five fields to define scheduling:

┌───────────── minute (0 - 59)
│ ┌───────────── hour (0 - 23)
│ │ ┌───────────── day of month (1 - 31)
│ │ │ ┌───────────── month (1 - 12)
│ │ │ │ ┌───────────── day of week (0 - 7) (0 or 7 is Sunday)
│ │ │ │ │
│ │ │ │ │
* * * * *  command_to_run

Basic Examples

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

# Run every hour
0 * * * * /path/to/script.sh

# Run every 15 minutes
*/15 * * * * /path/to/script.sh

# Run at 9:00 AM on weekdays
0 9 * * 1-5 /path/to/script.sh

# Run at midnight
0 0 * * * /path/to/script.sh

Edit Crontab

# Edit your crontab
crontab -e

# List current cron jobs
crontab -l

# Remove all cron jobs
crontab -r

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

Cron Fields Explained

Field	Range	Example
Minute	0-59	30 = 30 minutes
Hour	0-23	14 = 2 PM
Day	1-31	15 = 15th of month
Month	1-12	3 = March
Day of Week	0-7	5 = Friday (0,7 = Sunday)

Special Characters

*     # Any value
,     # Multiple values (1,3,5)
-     # Range (1-5)
/     # Step value (*/5 = every 5)

Practical Example: Daily Backup

#!/bin/bash

# File: daily_backup.sh

backup_dir="/backups"
source_dir="/home/user/documents"
date=$(date +%Y%m%d)

mkdir -p "$backup_dir"

# Create backup
tar -czf "$backup_dir/documents_$date.tar.gz" "$source_dir"

# Keep only last 7 backups
find "$backup_dir" -name "documents_*.tar.gz" -mtime +7 -delete

echo "Backup completed: documents_$date.tar.gz"

Add to crontab:

# Run daily at 3:00 AM
0 3 * * * /path/to/daily_backup.sh

$ crontab -e
# Add the line above, save, and exit

Practical Example: Log Cleanup

#!/bin/bash

# File: cleanup_logs.sh

log_dir="/var/log/myapp"
keep_days=30

if [ ! -d "$log_dir" ]; then
  echo "ERROR: Directory not found: $log_dir"
  exit 1
fi

# Remove logs older than 30 days
find "$log_dir" -name "*.log" -mtime +$keep_days -delete

# Count remaining logs
count=$(find "$log_dir" -name "*.log" | wc -l)
echo "Cleanup completed: $count logs remaining"

Add to crontab:

# Run daily at 4:00 AM
0 4 * * * /path/to/cleanup_logs.sh

Common Cron Expressions

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

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

# Every hour
0 * * * * /path/to/script.sh

# Every day at 6 AM
0 6 * * * /path/to/script.sh

# Every Monday at 9 AM
0 9 * * 1 /path/to/script.sh

# First day of month at 8 AM
0 8 1 * * /path/to/script.sh

# Every quarter (Jan, Apr, Jul, Oct)
0 0 1 1,4,7,10 * /path/to/script.sh

# Every weekday at 5 PM
0 17 * * 1-5 /path/to/script.sh

# Twice daily (6 AM and 6 PM)
0 6,18 * * * /path/to/script.sh

Cron with Output Redirection

# Run at 2 AM, save output to log file
0 2 * * * /path/to/script.sh >> /var/log/script.log 2>&1

# Run and email output
0 3 * * * /path/to/script.sh | mail -s "Backup Report" admin@example.com

# Append with timestamp
0 4 * * * echo "Running at $(date)" >> /tmp/cron.log && /path/to/script.sh >> /tmp/cron.log 2>&1

Monitor Cron Jobs

# Check if cron is running
systemctl status cron

# View system cron log
sudo tail -f /var/log/syslog | grep CRON

# Check cron execution history
sudo grep CRON /var/log/syslog

Practical Example: Monitoring Script

#!/bin/bash

# File: monitor_service.sh

service_name="myapp"
log_file="/var/log/service_monitor.log"
email="admin@example.com"

# Check if service is running
if ! systemctl is-active --quiet "$service_name"; then
  message="ERROR: $service_name is down!"
  echo "$(date): $message" >> "$log_file"

  # Send alert
  echo "$message" | mail -s "Service Alert" "$email"

  # Attempt restart
  systemctl restart "$service_name"
fi

Add to crontab:

# Check every 5 minutes
*/5 * * * * /path/to/monitor_service.sh

Practical Example: Hourly Report

#!/bin/bash

# File: hourly_report.sh

report_file="/tmp/hourly_report_$(date +%Y%m%d_%H).txt"

{
  echo "=== Hourly System Report ==="
  echo "Time: $(date)"
  echo ""
  echo "=== Disk Usage ==="
  df -h /

  echo ""
  echo "=== Memory Usage ==="
  free -h

  echo ""
  echo "=== Top Processes ==="
  ps aux | sort -rk 3,3 | head -5
} > "$report_file"

echo "Report saved: $report_file"

Add to crontab:

# Run every hour
0 * * * * /path/to/hourly_report.sh

Environment Variables in Cron

# Set environment variables before cron command
SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin
MAILTO=admin@example.com

# Then add your cron jobs
0 2 * * * /path/to/script.sh

Debugging Cron Issues

#!/bin/bash

# Script with debugging output

LOG="/tmp/cron_debug_$(date +%s).log"

{
  echo "Starting at: $(date)"
  echo "User: $(whoami)"
  echo "Current dir: $(pwd)"
  echo "PATH: $PATH"

  # Your commands here
  /path/to/script.sh

  echo "Ended at: $(date)"
} >> "$LOG" 2>&1

Crontab for Specific Users

# View root's crontab
sudo crontab -u root -l

# Edit another user's crontab
sudo crontab -u username -e

# System-wide crontab
sudo vi /etc/crontab

Common Mistakes

1. Not using full paths - cron doesn’t have PATH set like interactive shell
2. Forgetting to make script executable - chmod +x script.sh
3. Wrong time format - use 0-59 for minutes, 0-23 for hours
4. MAILTO not set - cron won’t send emails silently
5. Scripts creating files - need proper permissions in cron context

Performance Tips

• Avoid running too many jobs at once
• Schedule heavy jobs during off-hours
• Use flock to prevent concurrent runs
• Redirect output to prevent email floods
• Test script manually before scheduling

Key Points

• Five fields: minute, hour, day, month, day_of_week
• Use full paths in cron jobs
• Redirect output to log files
• Test scripts before adding to crontab
• Check syslog for cron execution errors

Summary

Cron is powerful for scheduling automated tasks. Always use full paths, test scripts first, and monitor execution. Proper logging helps debug issues. Remember that cron environment is minimal compared to interactive shell.