What Is The Difference Between Cron And Crontab?

Defining Cron and Its Function

Cron is a time-based job scheduler in Unix-like operating systems. It lets users run commands or scripts automatically at set times, dates, or intervals. Cron runs as a background service, checking the current time against scheduled tasks.

In Unix-like systems, cron reads configuration files called crontabs. These files contain instructions for when and what commands to run. The cron service checks these files regularly to determine which tasks to execute.

The cron daemon, or crond, is the program that runs in the background and manages the execution of scheduled tasks. It starts when the system boots up and runs silently in the background. The cron daemon's main job is to:

• Read the crontab files
• Compare the current time with the scheduled times in the crontabs
• Execute the specified commands when their scheduled time arrives

The cron daemon typically checks the crontab files every minute. This allows for precise scheduling of tasks, down to the minute level. By using cron, system administrators and users can automate routine tasks, such as backups, log rotations, or other repetitive processes that need to occur at regular intervals.

* * * * * command_to_execute

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

Explaining Crontab and Its Purpose

A crontab file is a text file with a list of commands to run at set times. The name "crontab" comes from "cron table," as it uses a table format to organize scheduled tasks.

Crontab files have each line representing a single task or job. Each line in a crontab file typically follows this format:

* * * * * command_to_be_executed

The five asterisks at the start of each line represent different time units:

1. Minute (0-59)
2. Hour (0-23)
3. Day of the month (1-31)
4. Month (1-12)
5. Day of the week (0-7, where both 0 and 7 represent Sunday)

After these time fields, the command to be executed is specified.

Crontab files are usually stored in specific locations on Unix-like systems. The exact location can vary depending on the system, but common locations include:

• /var/spool/cron/crontabs/ (for individual user crontabs)
• /etc/crontab (for system-wide crontabs)
• /etc/cron.d/ (for additional system crontabs)

You can edit your personal crontab files using the crontab -e command, which opens the file in the default text editor. This method is better than direct editing, as it helps prevent syntax errors and automatically notifies the cron daemon of changes.

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

How Cron and Crontab Work Together

Cron and crontab work together to schedule and run tasks automatically. Here's how they interact:

Cron uses crontab files for its setup. The cron daemon reads these files to know which tasks to run and when. When the system starts, the cron daemon loads all the crontab files. It then checks the current time against the schedules in these files every minute.

When scheduling a job using cron and crontab, the process typically involves these steps:

1. Create or edit a crontab file using the crontab -e command.
2. Add a new line to the crontab file, specifying the schedule and the command to run.
3. Save the crontab file.
4. The cron daemon detects the changes in the crontab file.
5. At the set time, the cron daemon runs the command.

For example, to schedule a daily backup at 3 AM, you would add this line to your crontab:

0 3 * * * /path/to/backup_script.sh

After saving the file, the cron daemon will run the backup script every day at 3 AM automatically.

Cron keeps checking the schedules in all crontab files. When the current time matches a scheduled task, cron runs the specified command. This process allows for automated, time-based task execution in Unix-like systems.