How to Schedule Tasks in Unix

Automating repetitive tasks is a powerful feature of Unix, enabling you to schedule scripts or commands to run at specific times. Tools like cron and at make this process seamless.

In this blog, we’ll explore how to schedule tasks effectively with clear examples and practical tips.

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1. Scheduling Tasks with cron

The cron service runs tasks at specified times and intervals. You define these tasks in a crontab file.

View Your Crontab

To view or edit your scheduled tasks, use:

$ crontab -e

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Crontab Syntax

A typical crontab entry looks like this:

* * * * * command_to_run

The five * symbols represent the following time fields:

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 mean Sunday)

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Examples of Crontab Entries

1. Run a Script Every Hour:

   0 * * * * /path/to/script.sh

2. Run a Command Every Day at 7:30 AM:

   30 7 * * * echo "Good morning!" >> ~/log.txt

3. Run a Script Every Monday at 10 PM:

   0 22 * * 1 /path/to/backup.sh

4. Run a Task Every 5 Minutes:

   */5 * * * * /path/to/task.sh

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List All Crontab Jobs

To view all scheduled tasks for your user:

$ crontab -l

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2. Scheduling One-Time Tasks with at

The at command schedules a task to run once at a specified time.

Enable the at Service

First, ensure the atd service is running:

$ sudo systemctl start atd
$ sudo systemctl enable atd

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Basic Syntax

at [time]

Examples of Using at

1. Schedule a Task for 2 PM Today:

   $ at 2pm
   at> echo "Task complete!" >> ~/log.txt
   at> Ctrl+D

2. Schedule a Task for Tomorrow:

   $ at 5:30pm tomorrow
   at> /path/to/task.sh
   at> Ctrl+D

3. Schedule a Task in an Hour:

   $ at now + 1 hour
   at> echo "This is a one-time task." >> ~/task_log.txt
   at> Ctrl+D

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View Scheduled at Tasks

To see all tasks in the queue:

$ atq

Remove a Scheduled at Task

Use the atrm command followed by the task’s job number:

$ atrm 3

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3. Combining cron and at with Logs

It’s good practice to log the output of your tasks to ensure they’ve run successfully.

Log cron Outputs

Add >> to redirect output to a log file:

0 9 * * * /path/to/command.sh >> ~/cron.log 2>&1

• >> ~/cron.log: Appends the output to cron.log.
• 2>&1: Captures both standard output and errors.

Log at Outputs

You can also redirect at command outputs:

$ at 3pm
at> /path/to/script.sh >> ~/at.log 2>&1
at> Ctrl+D

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Practice Time!

1. Schedule a cron job to clean up temporary files every day at midnight:

   0 0 * * * rm -rf ~/temp/*

2. Schedule an at task to remind you in 30 minutes:

   $ at now + 30 minutes
   at> echo "Time for a break!" | wall
   at> Ctrl+D

3. View and remove all your scheduled at tasks:

   $ atq
   $ atrm [job_number]

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Summary

• Use cron for recurring tasks and at for one-time tasks.
• Crontab files allow flexible scheduling with precise time intervals.
• Log task outputs to ensure proper execution and troubleshoot errors.