Cron Job in Linux

Understanding Cron, Crontabs, and Cron Jobs

1. Cron

• Definition: Cron is a time-based job scheduler in Unix-like operating systems. It allows users to automate repetitive tasks by executing commands or scripts at specified times and intervals.

• Usage: Commonly used for system maintenance, backups, sending emails, or running scripts.

2. Crontab (Cron Table)

• Definition: Crontab is a file where the cron jobs (tasks) are defined. Each user, including the system, can have their own crontab file.

• Command:

  • crontab -e: Edit the user's crontab.

  • crontab -l: List the current user's crontab entries.

  • crontab -r: Remove the current user's crontab.

To see more details, run

cat /etc/crontab

3. Cron Jobs

• Definition: A cron job is a specific task or command scheduled to run at a particular time and interval using cron.

• Syntax:

    * * * * * /path/to/command_or_script
  

  The * symbols represent time fields in this order:

  • Minute (0-59)

  • Hour (0-23)

  • Day of the Month (1-31)

  • Month (1-12 or names like Jan, Feb)

  • Day of the Week (0-6 or names like Sun, Mon)

Example Cron Job

Task: Run a script daily at 2:30 AM.

1. Create the script:

    #!/bin/bash
    echo "Backup started at $(date)" >> /path/to/backup.log
    tar -czf /path/to/backup.tar.gz /important/directory
    echo "Backup completed at $(date)" >> /path/to/backup.log
   

2. Edit the crontab: Open crontab using:

    crontab -e
   

3. Add the cron job:

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

Advanced Example with Environment Variables

• Task: Schedule a Python script to run every Monday at 9:00 AM.

    MAILTO="admin@example.com"  # Send output/errors via email
    0 9 * * 1 python3 /path/to/script.py
  

Common Use Cases

1. System Cleanup:
   Clear temporary files every hour:

    0 * * * * rm -rf /tmp/*
   

2. Server Health Check:
   Run a script to monitor CPU usage every minute:

    * * * * * /path/to/monitor_cpu.sh
   

3. Database Backup:
   Backup a database weekly on Sunday at midnight:

    0 0 * * 0 /path/to/db_backup.sh
    # here last 0 = sunday
   

By leveraging cron, you can automate repetitive tasks, ensuring efficiency and reducing manual intervention.

Cron Job for Backup

First write a script

#!/bin/bash
alert = 90
backup_date = $(date +"%m-%d-%Y-%H-%M-%S")
df -h | awk '{print $5 " " $1}' | while read output;
do
    usage = $(echo $ouput | awk '{print $1}' | cut -d'%' -f1) 
    file_sys = $(echo $ouput | awk '{print $2}') 
    if [ $usage -ge $alert ]
    then 
        echo "CRITICAL for $file_sys on $backup_date"
    fi
done

Then create crontab

crontab -e

* * * * * bash /home/ubuntu/backup.sh >> /home/ubuntu/backup.logs

Done now check in the specified path you will find that script…