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Crontab File and Syntax

The crontab is a command in Unix-like operating systems used to schedule tasks (cron jobs) to be executed at specified times or intervals.

Crontab Syntax

Each line in the crontab file follows this basic format:

*  *  *  *  *  command-to-execute
│  │  │  │  │
│  │  │  │  │
│  │  │  │  └───── Day of the week (0 - 7) (Sunday is 0 or 7)
│  │  │  └───────── Month (1 - 12)
│  │  └──────────── Day of the month (1 - 31)
│  └─────────────── Hour (0 - 23)
└────────────────── Minute (0 - 59)
  • *: Represents all possible values for that field (every minute, hour, day, etc.).
  • Each field must be separated by a space.

Example: Scheduling a Task to Run at 11:59 PM Every Day

We want to execute a script that retrieves system information at 11:59 PM every night and sends it to an email address.

Step 1: Create the Script

First, create a shell script (e.g., system_info.sh) to collect system information.

#!/bin/bash

# Collect system information
df -h > /tmp/system_info.txt
free -g >> /tmp/system_info.txt
nproc >> /tmp/system_info.txt
ps -ef >> /tmp/system_info.txt

# Send the information to an email
mail -s "Daily System Info" your-email@example.com < /tmp/system_info.txt

Make the script executable:

chmod +x system_info.sh

Step 2: Edit the Crontab

To run the script at 11:59 PM every night, use the following crontab entry:

59 23 * * * /path/to/system_info.sh
  • 59: The minute (59th minute).
  • 23: The hour (23:00 or 11:00 PM).
  • *: Every day of the month.
  • *: Every month.
  • *: Every day of the week.

Step 3: Add the Crontab Entry

To add this entry to your crontab, edit the crontab file by running:

crontab -e

Add the following line to schedule the script:

59 23 * * * /path/to/system_info.sh

This ensures that the script runs at 11:59 PM every night, gathering system information and emailing it to the specified email address.

Options:

 -u <user>  define user
 -e         edit user's crontab
 -l         list user's crontab
 -r         delete user's crontab
 -i         prompt before deleting
 -n <host>  set host in cluster to run users' crontabs
 -c         get host in cluster to run users' crontabs
 -T <file>  test a crontab file syntax
 -s         selinux context
 -V         print version and exit
 -x <mask>  enable debugging

Key Points

  • Use crontab -e to edit the cron table.
  • The time format is based on the 24-hour clock.
  • Ensure the script you want to run is executable.
  • Configure mail on your system to send emails from the command line. Ensure mail is installed and properly configured.

This cron setup is useful for automating system monitoring and getting daily reports via email.