Nano Ink

What is systemd?

systemd is the modern init system and service manager used in Linux systems such as Fedora, Ubuntu, RHEL, Arch, Debian, etc.

Problems before systemd (SysVinit)

  • Slow sequential boot
  • No proper dependency management
  • No unified logging
  • Hard to supervise daemons
  • Poor service recovery

Why systemd?

  • Faster parallel boot
  • Dependency control
  • Integrated logging
  • Easy service supervision
  • Resource control with cgroups
  • Socket & timer activation

What systemd does

  • Acts as PID 1
  • Starts services
  • Manages dependencies
  • Controls startup targets
  • Handles system logging
  • Manages resources
  • Starts services on-demand
  • Replaces cron for many tasks

Boot flow

  1. Kernel boots
  2. Kernel executes /sbin/init → systemd
  3. systemd loads default target
  4. Resolves dependencies
  5. Starts services in parallel
  6. System becomes usable

Systemd Units

Everything in systemd is a “unit”.
Types of units:

  • .service → services
  • .socket → socket activation
  • .target → runlevel groups
  • .timer → scheduled jobs
  • .mount → mount points
  • .swap → swap devices
  • .device → hardware devices
  • .path → filesystem event watches

Targets (Runlevel equivalents)

Runlevel mapping:
0 → poweroff.target
1 → rescue.target
3 → multi-user.target
5 → graphical.target
6 → reboot.target
Check default target:

systemctl get-default

Set default target:

sudo systemctl set-default graphical.target

systemctl (service control command)

Start / stop / restart services

sudo systemctl start SERVICE
sudo systemctl stop SERVICE
sudo systemctl restart SERVICE
sudo systemctl reload SERVICE

Start and enable at boot:

sudo systemctl enable --now SERVICE

Disable autostart:

sudo systemctl disable SERVICE

Completely block service:

sudo systemctl mask SERVICE

Unmask:

sudo systemctl unmask SERVICE

Service status and listing

Show status:

systemctl status SERVICE

List running services:

systemctl list-units --type=service

List installed service files:

systemctl list-unit-files --type=service

List failed units:

systemctl --failed

Kill service processes:

sudo systemctl kill SERVICE

Force kill:

sudo systemctl kill -s SIGKILL SERVICE

Service files locations

/usr/lib/systemd/system/ → package installed
/etc/systemd/system/ → admin installed
~/.config/systemd/user/ → per-user services

Example custom service

File:
/etc/systemd/system/myapp.service
Content:

[Unit]
Description=My App
After=network.target
[Service]
ExecStart=/usr/bin/python3 /opt/app/app.py
Restart=always
User=myuser
[Install]
WantedBy=multi-user.target

Reload and enable:

sudo systemctl daemon-reload
sudo systemctl enable --now myapp

Cgroups with systemd

Used for:

  • resource limits
  • isolation
  • CPU control
  • memory control
    Commands:
systemd-cgls
systemd-cgtop

Systemd timers (cron alternative)

List timers:

systemctl list-timers

Create timer unit:
/etc/systemd/system/backup.timer
Content:

[Unit]
Description=Run backup daily
[Timer]
OnCalendar=daily
Persistent=true
[Install]
WantedBy=timers.target

Create related service:
/etc/systemd/system/backup.service
Enable:

sudo systemctl enable --now backup.timer

journald and journalctl

journald = system logging system used by systemd
Log storage:
/var/log/journal/

journalctl usage

Show all logs:

journalctl

Follow logs live:

journalctl -f

Logs for current boot:

journalctl -b

Previous boot:

journalctl -b -1

Logs for specific service:

journalctl -u nginx

Follow service logs:

journalctl -u nginx -f

Logs since specific time:

journalctl --since "2 hours ago"
journalctl --since yesterday
journalctl --since "2026-01-03" --until "2026-01-04"

Logs by PID:

journalctl _PID=1234

Logs by priority:

journalctl -p err

Priority level meanings:
0 emerg
1 alert
2 crit
3 err
4 warning
5 notice
6 info
7 debug

Enable persistent logging

sudo mkdir -p /var/log/journal
sudo systemctl restart systemd-journald

Debugging with systemd

systemctl status SERVICE
journalctl -xe
journalctl -u SERVICE

Summary table

systemd → init and service manager
systemctl → command to control systemd
journald → logging service
journalctl → tool to read logs

Practical real-world uses

  • debugging boot failures
  • monitoring services
  • log investigation
  • running background applications
  • replacing cron jobs
  • server administration