Cron Service Fundamentals

Cron is a time-based job scheduler in Unix-like operating systems. The crond daemon runs in the background and executes scheduled commands at specified intervals.

# Check if cron service is running
ps aux | grep cron | grep -v 'grep'

# Verify cron service status across runlevels
chkconfig crond --list

# Monitor cron execution logs
tail -f /var/log/cron

The cron daemon checks for scheduled tasks every minute. Logs are maintained at /var/log/cron.

System-Level Cron Jobs

System cron jobs are defined in /etc/crontab and manage essential system maintenance tasks:

# /etc/crontab structure
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly

Common system maintenance scripts:

• /etc/cron.daily/: logrotate, tmpwatch, package management
• /etc/cron.weekly/: database updates, system checks
• /etc/cron.monthly/: comprehensive system maitnenance

User-Level Cron Cofniguration

Individual users can create personal cron jobs using crontab:

# Edit current user's cron jobs
crontab -e

# List scheduled tasks
crontab -l

# Manage another user's cron jobs (root only)
crontab -u username -e

User cron files are stored in /var/spool/cron/username.

Cron Time Syntax

Cron uses a five-field time specification:

* * * * * command_to_execute
│ │ │ │ └── Day of week (0-7, 0=Sunday)
│ │ │ └──── Month (1-12)
│ │ └────── Day of month (1-31)
│ └──────── Hour (0-23)
└────────── Minute (0-59)

Time pattern examples:

# Daily at 2:00 AM
00 02 * * * /path/to/command

# Monthly on 1st at 2:00 AM
00 02 1 * * /path/to/command

# Every Sunday at 2:00 AM
00 02 * * 0 /path/to/command

# Every 5 minutes
*/5 * * * * /path/to/command

# Specific days (1st, 5th, 8th) at 2:00 AM
00 02 1,5,8 * * /path/to/command

# Range of days (1st through 8th) at 2:00 AM
00 02 1-8 * * /path/to/command

Practical Example: Automated Backup System

Create a daily backup script for the /etc directory:

Backup Script Implementation

#!/bin/bash
# backup_etc.sh - Automated /etc directory backup

BACKUP_DIR="/var/backup"
TIMESTAMP=$(date +%Y-%m-%d)
ARCHIVE_NAME="${TIMESTAMP}_etc_backup.tar.gz"

# Ensure backup directory exists
if [ ! -d "$BACKUP_DIR" ]; then
    mkdir -p "$BACKUP_DIR"
fi

# Create compressed backup
tar -czf "${BACKUP_DIR}/${ARCHIVE_NAME}" /etc > /dev/null 2>&1

# Remove backups older than 5 days
find "$BACKUP_DIR" -name "*_etc_backup.tar.gz" -mtime +5 -delete

Make the script executable:

chmod +x /root/backup_etc.sh

Schedule the Backup Task

Add to crontab:

# Edit root's crontab
crontab -e

# Add daily backup at 4:00 AM
0 4 * * * /root/backup_etc.sh

Testing the Implementation

# Test script manually
/root/backup_etc.sh

# Verify backup creation
ls -la /var/backup/

# Check cron execution logs
grep "backup_etc.sh" /var/log/cron

Advanced Cron Techniques

Second-Level Execution

Cron doesn't natively support second granularity, but you can implement it:

# Execute command every 10 seconds
* * * * * /path/to/command
* * * * * sleep 10; /path/to/command
* * * * * sleep 20; /path/to/command
* * * * * sleep 30; /path/to/command
* * * * * sleep 40; /path/to/command
* * * * * sleep 50; /path/to/command

Continuous Execution with Sleep

For frequent periodic tasks, use a while loop:

#!/bin/bash
# continuous_monitor.sh
while true
do
    # Your command here
    sleep 10

done

Anacron: Missed Job Recovery

Anacron handles jobs that may have been missed during system downtime:

# /etc/anacrontab configuration
1       65      daily_jobs    run-parts /etc/cron.daily
7       70      weekly_jobs   run-parts /etc/cron.weekly
30      75      monthly_jobs  run-parts /etc/cron.monthly

The format is: frequency delay job_identifier command where frequency is in days, delay is in minutes.