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๐Ÿ”ง Chapter 19: Systemd & Service Management

Advanced Chapter 19

๐Ÿ“‘ Table of Contents

What is Systemd?

systemd is the init system (PID 1) on most modern Linux distros. It manages services, mounts, timers, devices, and more.

systemd Components

Component Description
systemd Init system and service manager
systemctl Control systemd and services
journald Logging daemon
journalctl Query logs
networkd Network management
resolved DNS resolution
logind Login management
timedatectl Time and timezone
hostnamectl Hostname management

Managing Services

# Start/Stop/Restart
sudo systemctl start nginx
sudo systemctl stop nginx
sudo systemctl restart nginx         # Stop + Start
sudo systemctl reload nginx          # Reload config (no downtime)
sudo systemctl reload-or-restart nginx  # Prefer reload, fallback to restart

# Enable/Disable (auto-start at boot)
sudo systemctl enable nginx          # Start on boot
sudo systemctl disable nginx         # Don't start on boot
sudo systemctl enable --now nginx    # Enable AND start
sudo systemctl disable --now nginx   # Disable AND stop

# Status
systemctl status nginx               # Detailed status
systemctl is-active nginx            # Just "active" or "inactive"
systemctl is-enabled nginx           # "enabled" or "disabled"
systemctl is-failed nginx            # "failed" or "active"

# List services
systemctl list-units --type=service              # Running services
systemctl list-units --type=service --all        # All services
systemctl list-unit-files --type=service          # All service files
systemctl list-units --failed                     # Failed units

# Mask/Unmask (prevent starting entirely)
sudo systemctl mask nginx            # Cannot be started (even manually)
sudo systemctl unmask nginx          # Allow starting again

Unit Files

Systemd uses unit files to define services, timers, mounts, and more.

# Find unit files
systemctl show -p FragmentPath nginx.service     # Location of unit file
systemctl cat nginx.service                       # View the unit file

# Unit file locations (priority order):
# /etc/systemd/system/        โ† Admin customizations (highest priority)
# /run/systemd/system/        โ† Runtime units
# /usr/lib/systemd/system/    โ† Package-installed units

Unit File Structure

[Unit]
Description=My Application Server
Documentation=https://docs.example.com
After=network.target postgresql.service
Wants=postgresql.service
Requires=network-online.target

[Service]
Type=simple
User=appuser
Group=appgroup
WorkingDirectory=/opt/myapp
Environment=NODE_ENV=production
EnvironmentFile=/opt/myapp/.env
ExecStartPre=/opt/myapp/pre-start.sh
ExecStart=/opt/myapp/server
ExecStartPost=/opt/myapp/post-start.sh
ExecReload=/bin/kill -HUP $MAINPID
ExecStop=/bin/kill -SIGTERM $MAINPID
Restart=always
RestartSec=5
StandardOutput=journal
StandardError=journal

[Install]
WantedBy=multi-user.target

Key Directives

Section Directive Purpose
[Unit] After Start after these units
[Unit] Requires Hard dependency (fail if dep fails)
[Unit] Wants Soft dependency (ok if dep fails)
[Service] Type simple, forking, oneshot, notify
[Service] ExecStart Command to run
[Service] Restart always, on-failure, on-abnormal
[Service] User/Group Run as this user/group
[Install] WantedBy Target to enable under

Service Types

Type Description
simple Process stays in foreground (default)
forking Process forks and parent exits (traditional daemons)
oneshot Runs once and exits (scripts)
notify Like simple but sends readiness notification
idle Starts after all other jobs finish

Creating Custom Services

Example: Node.js Application

# /etc/systemd/system/myapp.service
[Unit]
Description=My Node.js Application
After=network.target

[Service]
Type=simple
User=www-data
WorkingDirectory=/opt/myapp
ExecStart=/usr/bin/node /opt/myapp/server.js
Restart=on-failure
RestartSec=10
Environment=PORT=3000
Environment=NODE_ENV=production

[Install]
WantedBy=multi-user.target
# Deploy
sudo systemctl daemon-reload         # Reload unit files
sudo systemctl enable --now myapp    # Enable and start
systemctl status myapp               # Check status
journalctl -u myapp -f               # Follow logs

Example: Python Script Watchdog

# /etc/systemd/system/monitor.service
[Unit]
Description=System Monitor Script
After=network.target

[Service]
Type=simple
ExecStart=/usr/bin/python3 /opt/monitor/monitor.py
Restart=always
RestartSec=30
StandardOutput=append:/var/log/monitor.log
StandardError=append:/var/log/monitor-error.log

[Install]
WantedBy=multi-user.target

Overriding Existing Services

# Create a drop-in override (don't edit original files!)
sudo systemctl edit nginx

# This creates: /etc/systemd/system/nginx.service.d/override.conf
# Add your overrides:
[Service]
LimitNOFILE=65536

Targets (Runlevels)

Targets group units together โ€” they define system states.

Target Old Runlevel Description
poweroff.target 0 Halt system
rescue.target 1 Single user mode
multi-user.target 3 CLI multi-user
graphical.target 5 GUI desktop
reboot.target 6 Reboot
emergency.target โ€” Minimal shell 
systemctl get-default                         # Current default target
sudo systemctl set-default multi-user.target  # Boot to CLI
sudo systemctl set-default graphical.target   # Boot to GUI
sudo systemctl isolate rescue.target          # Switch to rescue NOW

Journald (Logging)

# Configuration
sudo vim /etc/systemd/journald.conf

# Common settings:
[Journal]
Storage=persistent        # Store logs on disk (survives reboot)
SystemMaxUse=500M         # Max disk usage
MaxRetentionSec=1month    # Max retention
Compress=yes              # Compress logs

sudo systemctl restart systemd-journald

Socket Activation

Start services on-demand when a connection arrives.

# /etc/systemd/system/myapp.socket
[Unit]
Description=My App Socket

[Socket]
ListenStream=8080

[Install]
WantedBy=sockets.target
# /etc/systemd/system/myapp.service
[Unit]
Description=My App
Requires=myapp.socket

[Service]
ExecStart=/opt/myapp/server
sudo systemctl enable --now myapp.socket
# Service starts only when port 8080 receives a connection!

Resource Control (cgroups)

Limit CPU, memory, and I/O per service.

# In the [Service] section:
[Service]
CPUQuota=50%               # Max 50% of one CPU
MemoryMax=512M             # Max 512 MB RAM
MemoryHigh=256M            # Throttle above 256 MB
IOWeight=100               # I/O priority (1-10000)
TasksMax=100               # Max 100 processes
# View resource usage per service
systemd-cgtop                # Top for cgroups

# View resource config
systemctl show nginx -p CPUQuota,MemoryMax,TasksMax

# Runtime override
sudo systemctl set-property nginx CPUQuota=25%

๐Ÿ‹๏ธ Practice Exercises

  1. Create: Write a custom service that runs a simple script on boot
  2. Status: Check the status of sshd with systemctl
  3. Logs: View the last 50 log entries for a specific service
  4. Target: Check your current boot target and understand what it means
  5. Override: Create a drop-in override for an existing service
  6. Resources: Set memory limit on a service using cgroups
  7. Failed: Find all failed services and investigate one
  8. Timer: Create a timer that runs a cleanup script every 6 hours

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