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Systemd, systemctl & journalctl – Deep Practical Guide

Systemd, systemctl & journalctl – Deep Practical Guide

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