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LaTerral Williams
LaTerral Williams

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🕰️Become a Timecop with Linux Task Scheduling: at, cron, and crontab

#cloudwhistler #linux #cloud #opensource

Imagine if you had the power to schedule events in time like Max Walker from Timecop. Only instead of fighting crime in 1994, you're automating tasks in Linux.

In this article, we'll review how to:

  • Set up a Linux environment for practice

  • Use at for one-time tasks

  • Use cron and crontab for recurring tasks

  • Work with system-level scheduling using /etc/cron.*

  • Write and schedule your own scripts

  • Investigate and manage scheduled tasks

Let’s hop into our time machine (terminal) and begin.

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📚 Table of Contents

🖥️ Setting Up Your Timecop Training Ground (optional)

🧪 Step 1: Install RHEL 9

  • Download RHEL 9
  • Use VirtualBox, VMware, or install on a spare machine
  • Register the system (for updates): 
  sudo subscription-manager register
  sudo subscription-manager attach --auto
  sudo dnf update -y
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⏰ Scene 1: Scheduling a One-Time Mission with at

🧠 The Mission:

Max needs to send a system alert exactly 2 minutes from now.

✅ Step 1: Install and enable at service 

sudo dnf install at -y
sudo systemctl enable --now atd
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✅ Step 2: Schedule a task 

echo "Timecop alert at $(date)" >> /tmp/timecop_alert.log | at now + 2 minutes
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✅ Step 3: View and manage at jobs 

atq         # View queue
atrm <job>  # Remove job
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♻️ Scene 2: Scheduling Recurring Missions with cron

🧠 The Mission:

Max must run a time anomaly scan every minute.

✅ Step 1: Create a script 

mkdir -p ~/timecop/scripts
vim ~/timecop/scripts/scan.sh
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#!/bin/bash
echo "Anomaly scan run at $(date)" >> /tmp/timecop_scan.log
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✅ Step 2: Make the script executable 

chmod +x ~/timecop/scripts/scan.sh
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✅ Step 3: Schedule with crontab 

crontab -e
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Add this line:

* * * * * /home/yourusername/timecop/scripts/scan.sh
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Feel free to set the schedule as you prefer. I set to run every minute in order to confirm the cronjob executes.

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If you find your cronjob is not running. The cron service may not be installed and/or running. The steps to remedy this will vary based on the Linux distro you are using.

Here's a few examples for AlmaLinux:

sudo dnf install cronie

sudo systemctl start crond.service

sudo systemctl enable crond.service

🧭 Diagram: Understanding Crontab Format 

# ┌───────────── Minute (0 - 59)
# │ ┌───────────── Hour (0 - 23)
# │ │ ┌───────────── Day of month (1 - 31)
# │ │ │ ┌───────────── Month (1 - 12)
# │ │ │ │ ┌───────────── Day of week (0 - 7) (Sunday = 0 or 7)
# │ │ │ │ │
# │ │ │ │ │
# * * * * * command_to_execute
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🔍 Example:

15 14 1 * * /home/max/run_report.sh
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Runs the script at 2:15 PM on the 1st day of every month.

🧮 Diagram: Cron Schedule Examples

Schedule Crontab Expression Description
Every minute * * * * * Runs every minute
Every 5 minutes */5 * * * * Every 5 minutes
Every day at 1 AM 0 1 * * * Daily at 1:00 AM
Every Monday at noon 0 12 * * 1 Every Monday at 12:00 PM
First of every month 0 0 1 * * Midnight on the 1st of each month

🏛️ Scene 3: Using System-Level Missions with /etc/cron.*

RHEL has system directories that run scripts at fixed intervals.

/etc/cron.* folders:

  • /etc/cron.hourly/
  • /etc/cron.daily/
  • /etc/cron.weekly/
  • /etc/cron.monthly/

🧠 The Mission:

Create a system cleanup that runs weekly.

You will need to use sudo to direct changes related to /etc/cron.*

✅ Step 1: Create a cleanup script 

sudo vim /etc/cron.weekly/timecop_cleanup.sh
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#!/bin/bash
echo "System cleanup at $(date)" >> /var/log/timecop_cleanup.log
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✅ Step 2: Set executable permission 

sudo chmod +x /etc/cron.weekly/timecop_cleanup.sh
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🔎 Scene 4: Investigating Scheduled Missions

✅ View user's cron jobs 

crontab -l
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✅ View system crontab 

cat /etc/crontab
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✅ View scripts in system cron folders 

ls /etc/cron.daily/
ls /etc/cron.weekly/
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✅ View logs (RHEL-specific) 

sudo grep CRON /var/log/cron
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✅ View at jobs 

atq
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👤 Scene 5: User vs Root: Who Runs What?

🧠 Mission Clarification:

  • crontab -e runs as your user.
  • /etc/crontab allows you to specify a user explicitly.

Example /etc/crontab line: 

30 2 * * * root /usr/local/bin/timecop_backup.sh
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💡 Tip: Use root only when needed... don’t disrupt the timeline with elevated permissions!

🎯 Scene 6: Testing Your Scripts

Let’s simulate a full time jump.

✅ Create a test script 

vim ~/timecop/scripts/test_jump.sh
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#!/bin/bash
echo "Time jump occurred at $(date)" >> /tmp/timecop_jump.log
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✅ Make it executable 

chmod +x ~/timecop/scripts/test_jump.sh
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✅ Run once with at 

echo "/home/yourusername/timecop/scripts/test_jump.sh" | at now + 1 minute
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✅ Schedule via crontab 

crontab -e
# Add:
*/1 * * * * /home/yourusername/timecop/scripts/test_jump.sh
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🧾 Timecop's Summary Table

Tool Purpose Use Case
at One-time task Run a job once in the future
cron + crontab Recurring tasks Daily/weekly backups, reports
/etc/cron.* System-wide scripts Maintenance jobs
atq, crontab -l, logs Investigation Check the timeline for jobs

🕹️ Final Thoughts: Time Is on Your Side

With at, cron, and a few bash scripts, you're now equipped to become a Linux Timecop; defending your system from disorganization and delay.

You can’t rewrite history, but with scheduling, you can plan for the future.

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