scheduler

Introduction to CRON

What is CRON?

CRON is a time-based job scheduler in Unix-like operating systems. It allows users to schedule scripts or commands to run automatically at specified intervals.

CRON Syntax

A CRON job is defined using the following syntax:

* * * * * /path/to/command
- - - - -
| | | | |
| | | | +---- Day of the week (0 - 7, Sunday is both 0 and 7)
| | | +------ Month (1 - 12)
| | +-------- Day of the month (1 - 31)
| +---------- Hour (0 - 23)
+------------ Minute (0 - 59)

Examples:

• 0 5 * * * /path/to/script.sh: Runs the script every day at 5:00 AM.
• */15 * * * * /path/to/script.sh: Runs the script every 15 minutes.

Scheduling job.sh in CRON

To schedule job.sh using CRON:

1. Open the CRON editor:

   crontab -e

2. Add the following line to schedule job.sh (e.g., every day at 2:00 AM):

   0 2 * * * /path/to/job.sh >> /path/to/job.log 2>&1

Explanation:

• 0 2 * * *: Runs the script at 2:00 AM every day.
• /path/to/job.sh: Full path to the job.sh script.
• >> /path/to/job.log 2>&1: Appends output and errors to job.log.

Checking CRON Jobs

To view scheduled CRON jobs:

crontab -l

Notes:

• Ensure job.sh is executable:

  chmod +x /path/to/job.sh

• Use absolute paths in job.sh to avoid issues with relative paths.

• Ensure that crontab has access to the file. On macOS (e.g., macOS Sequoia), crontab does not have access to folders or files by default. You need to grant access explicitly in the System Settings:

1. Go to System Settings > Privacy & Security > Full Disk Access.
2. Add your terminal application (e.g., Terminal, iTerm) to the list.
3. Restart the terminal and reconfigure your crontab if necessary.