8.2 System Management

1. 🔧 Run Level (System Management Concept – Traditional SysVinit Systems)

Run levels are predefined states of a Unix/Linux system that define what services and processes are running. They’re part of the traditional SysVinit initialization system (before systemd became the standard).

🔹 Common Run Levels (SysVinit-based Systems)

Runlevel	Description
0	Halt (shutdown)
1	Single-user mode (maintenance)
2	Multi-user mode (no networking)
3	Multi-user mode (with networking)
4	Unused (custom)
5	Multi-user + GUI (graphical login)
6	Reboot

Note: Different distributions may define levels 2–5 differently.

🔹 How It Worked

• At boot, the system enters a default run level (defined in /etc/inittab).
• Each run level starts/stops specific services using scripts in /etc/rcX.d/ (where X is the run level).

🔹 Example Commands (SysVinit)

# Check current run level
runlevel

# Change to run level 3
sudo init 3

# Reboot the system (runlevel 6)
sudo init 6

⚠️ Modern Systems (systemd)

Most modern Linux distributions now use systemd, which replaces run levels with “targets” (like graphical.target, multi-user.target, etc.).

Example systemd equivalents:

# Show current target
systemctl get-default

# Set to graphical target (like runlevel 5)
sudo systemctl set-default graphical.target

# Change current target (no reboot)
sudo systemctl isolate multi-user.target

✅ Summary

Term	SysVinit	systemd
Run level 3	CLI, multi-user	multi-user.target
Run level 5	GUI login	graphical.target
Run level 0	Shutdown	poweroff.target

Tip

Once the run level has been decided, the scripts to manage all the relevant services are called rc (run control) scripts. The typical location for these scripts are /etc/init.d/ OR /etc/rc.d/. (Should be /etc/init.d/ for Ubuntu OS). Please do not modify them without any good reason.

2. Shutdown

2.1 Basics of shutdown command

🔹 1. Halt the System Immediately

sudo shutdown -h now

• -h = halt (shut down and power off)
• now = do it immediately

📌 Alternative:

sudo poweroff

🔹 2. Reboot the System in 1 Minute

sudo shutdown -r +1

• -r = reboot
• +1 = wait 1 minute before rebooting

💡 You can also add a message:

sudo shutdown -r +1 "Rebooting in 1 minute for maintenance"

🧯 3. Cancel a Scheduled Shutdown (if you change your mind):

sudo shutdown -c

2.2 systemctl Equivalents

Here are the systemctl equivalents to the shutdown commands:

🔹 1. Halt the System Immediately

sudo systemctl halt

• Halts the system — stops all CPUs but does not necessarily power off.

🔹 To Power Off (Shutdown) Immediately

sudo systemctl poweroff

• This is the modern equivalent to shutdown -h now.

🔹 2. Reboot the System in 1 Minute

systemctl does not have a built-in delay, but you can use sleep to schedule it:

sleep 60 && sudo systemctl reboot

• This waits 60 seconds, then reboots.

📌 If you want to run it in the background so your terminal isn’t stuck:

(sleep 60 && sudo systemctl reboot) &

🔹 Cancel a Scheduled Reboot (if using shutdown -r +X)

sudo shutdown -c

Note

If you scheduled the reboot using sleep, you’ll need to manually find and kill the sleep process.

2.3 Why systemctl and shutdown?

Both shutdown and systemctl can be used to shut down or reboot a Linux system — but they exist for different historical and technical reasons.

🧱 1. Why shutdown exists

• shutdown is a classic command from SysVinit days.
• It schedules or triggers system power actions (halt, poweroff, reboot).
• It’s user-friendly and still widely used.

Example:

sudo shutdown -h now

⚙️ 2. Why systemctl is used

• systemctl is part of systemd, the modern init system used by most Linux distros today (like Ubuntu, Fedora, Arch).

• It replaces older tools (shutdown, reboot, init, etc.) with a unified interface to manage:

  • System boot and shutdown
  • Services (daemons)
  • System targets (replacing runlevels)

Example:

sudo systemctl poweroff

🔄 3. They both still work — but systemctl is preferred in modern systems

• On modern distros, the shutdown command is often just a wrapper for systemctl.

• You can check this:

  which shutdown
  ls -l $(which shutdown)

  It often links to systemctl or is configured to call it under the hood.

✅ So, which should you use?

Command	Use when you want…	Notes
shutdown	A familiar, older-style syntax	Still works fine, good for scheduling
systemctl	Modern, explicit control with systemd features	Preferred for scripting and consistency

3. What is systemctl?

systemctl is the command-line tool used to interact with systemd, the modern init system used by most Linux distributions (like Ubuntu, Fedora, Arch, Debian, etc.).

3.1 🔧 What is systemd?

systemd is the software that boots your system, manages system services (like networking, cron, SSH), and handles shutdowns, logins, mounts, and more. It’s the replacement for older init systems like SysVinit or Upstart.

3.2 🚀 What You Can Do With systemctl

Task Type	Command Example	Description
Start service	sudo systemctl start nginx	Starts the nginx service right now
Stop service	sudo systemctl stop sshd	Stops the SSH service
Restart service	sudo systemctl restart apache2	Restarts a service
Enable service	sudo systemctl enable bluetooth	Start at boot
Disable service	sudo systemctl disable bluetooth	Don’t start at boot
Check status	systemctl status cups	Shows whether a service is active/running
Show all units	systemctl list-units	Lists active services, mounts, etc.
Shutdown system	sudo systemctl poweroff	Shuts down the computer
Reboot system	sudo systemctl reboot	Reboots the computer
Change target	sudo systemctl isolate multi-user.target	Change run level (e.g., GUI ↔ CLI)

3.3 📌 Targets vs Runlevels

In systemd, targets replace old runlevels:

Runlevel (SysV)	systemd Target
0	poweroff.target
1	rescue.target
3	multi-user.target
5	graphical.target
6	reboot.target

3.4 ✅ Summary

• systemctl = the control tool for systemd.
• It lets you start/stop services, reboot/shutdown, and manage boot behavior.
• It’s modern, consistent, and the preferred tool on most current Linux distros.

4. Managing Services with systemctl

Here’s how you can handle both parts of your task using systemctl and cron on a modern Fedora Linux system:

To manage the cron service (called crond on Fedora):

4.1 🕒 What is the cron service?

The cron service (called crond on most Linux systems) is a background daemon that runs scheduled tasks automatically at specified times and intervals.

Think of it as Linux’s built-in task scheduler — like Windows Task Scheduler.

🔧 What It Does

• Runs commands or scripts at specific times (e.g., backups, cleanups, updates).
• Uses cron tables (crontab files) where users define what and when to run.

🧠 Terminology

Term	Meaning
cron	The overall scheduling system
crond	The background daemon that runs jobs
crontab	The configuration file that lists jobs

🗂️ Types of Crontabs

• System-wide: /etc/crontab
• User-specific: managed with crontab -e (stored in /var/spool/cron/username)
• Package-specific: jobs placed in /etc/cron.daily/, /etc/cron.hourly/, etc.

🧾 Example of a Crontab Entry

0 22 * * * /home/user/mybackup.sh

➡️ This runs mybackup.sh every day at 10:00 PM.

✅ Basic Commands

Command	What it does
sudo systemctl start crond	Start the cron service
sudo systemctl stop crond	Stop it
crontab -e	Edit your user’s crontab file
crontab -l	List scheduled jobs for your user

🛠️ Use Cases

• Automating backups
• Running cleanup scripts
• Scheduling periodic system tasks
• Sending reminder emails or reports

---

✅ Stop the cron service:

sudo systemctl stop crond

✅ Restart the cron service:

sudo systemctl restart crond

You can also use status to verify:

systemctl status crond

4.2 📅 Scheduling mybackup.sh in /etc/crontab

Let’s say your backup script is located at:

/home/username/mybackup.sh

You want it to run every day at 10:00 PM (22:00).

📂 Step 1: Make sure the script is executable

chmod +x /home/username/mybackup.sh

📄 Step 2: Edit /etc/crontab with root

Open the file with:

sudo nano /etc/crontab

You’ll see lines like this:

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin

# m h dom mon dow user command

This shows the field order:

minute hour day_of_month month day_of_week user command

🧾 Step 3: Add your cron job

To run the backup every day at 10PM as user username, add:

0 22 * * * username /home/username/mybackup.sh

🔹 Breakdown:

• 0 = minute
• 22 = hour (10PM)
• * * * = every day/month/week
• username = the user to run the command as
• /home/username/mybackup.sh = full path to the script

✅ Final Notes

• After editing /etc/crontab, no restart is required — crond automatically picks up the change.
• You can view logs in /var/log/cron or with journalctl -u crond.