Systemd

systemd is the init system and service manager used by most modern Linux distributions.

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#systemd Core Utilities

#systemd utility List

Utility Description
systemctl Manages services (start, stop, restart, enable, disable, etc.)
journalctl Views and filters logs collected by the systemd journal
loginctl Manages user logins and sessions
hostnamectl Sets or queries the hostname and related machine metadata
timedatectl Sets or queries the system date, time, and NTP settings
systemd-analyze Analyzes system boot-up performance
systemd-cgtop Monitors control group resource usage in real time
systemd-run Runs a command as a transient service or scope
resolvectl Resolves domain names using the systemd-resolved service
busctl Interacts with the D-Bus message bus system

#Utility Commands

#systemctl

#systemctl - System Commands

  • Manage systemd units and control system services (start, stop, enable, etc.).
Command Description
systemctl halt Stop all running software, leave the hardware powered on.
systemctl hibernate Save contents of system memory to disk and power off the hardware.
systemctl poweroff Halt and power off the system.
systemctl reboot Reboot the system.

#systemctl - Service Commands

Command Description
systemctl start <service> Start a service immediately
systemctl stop <service> Stop a running service
systemctl restart <service> Restart a service
systemctl reload <service> Reload service configuration without restart
systemctl enable <service> Enable service to start at boot
systemctl disable <service> Disable boot-time start
systemctl status <service> Show current status and recent logs
systemctl mask <service> Prevent service from being started
systemctl unmask <service> Remove mask so it can start again
systemctl list-units List active units
systemctl list-unit-files List all unit files and their enabled state

#journalctl

  • Query and filter logs from the systemd journal.
Command Description
journalctl Show entire journal
journalctl -b Show logs from current boot
journalctl --list-boots List all boot sessions
journalctl -b -1 Logs from previous boot
journalctl -u <service> Show logs for a specific unit/service
journalctl -n 100 Show the last 100 log entries
journalctl -f Follow new log entries (tail -f style)
journalctl -p err..emerg Show only error (and above) priority logs
journalctl --disk-usage Show disk usage of journal
journalctl --since today Show logs since today

#busctl

  • Inspect and monitor D-Bus (Desktop Bus) for debugging or introspection.
Command Description
busctl list List all services/peers on the bus
busctl status [SERVICE\|PID] Show process info & credentials for service or PID
busctl monitor [SERVICE...] Dump live message traffic, Ctrl+C to stop
busctl capture [SERVICE...] Capture bus traffic to a pcap file
busctl tree [SERVICE...] Show object tree exported by service(s)

#loginctl

  • Manages user sessions and login-related settings.
Command Description
loginctl list-sessions List all active sessions
loginctl list-users List logged-in users
loginctl user-status <UID> Show all sessions & processes for user <UID>
loginctl session-status <ID> Show detailed info and logs for session <ID>
loginctl show-session <ID> Show machine-parsable properties of a session
loginctl terminate-session <ID> End all processes in session <ID>
loginctl lock-session <ID> Lock the specified session
loginctl unlock-session <ID> Unlock the specified session

#systemd-analyze

  • Analyze system boot performance and unit dependencies.
Command Description
systemd-analyze Show total boot time breakdown
systemd-analyze blame List services ordered by startup time
systemd-analyze plot > boot.svg Generate SVG visual of boot sequence

#systemd-cgtop

  • Real-time monitoring of resource usage by cgroups (CPU, memory, I/O).
Command Description
systemd-cgtop Monitor top cgroups resource usage
systemd-cgtop -c Sort cgroups by CPU usage
systemd-cgtop -d <delay> Change refresh delay interval

#timedatectl

  • Manage system date, time, timezone, and NTP synchronization.
Command Description
timedatectl status Show current date/time settings
timedatectl list-timezones List all available timezones
timedatectl set-timezone <Zone> Set system timezone
timedatectl set-time "YYYY-MM-DD" Manually set date
timedatectl set-time "HH:MM:SS" Manually set time
timedatectl set-ntp true Enable NTP sync
timedatectl set-ntp false Disable NTP sync

#resolvectl

  • Interact with systemd-resolved DNS resolver service.
Command Description
resolvectl status Show DNS configuration and link info
resolvectl query <hostname> Resolve hostname via systemd-resolved
resolvectl dns <interface> <IP> Set DNS server(s) for a specific interface
resolvectl domain <interface> <name> Set search domain(s) for an interface
resolvectl flush-caches Clear the DNS cache managed by systemd-resolved
resolvectl reset-statistics Reset all DNS resolution statistics

#hostnamectl

  • Query and set the system hostname (static, transient, pretty) and related metadata.

#↓ Commands

Command Description
hostnamectl status Show current hostname & system info
hostnamectl hostname <NAME> Set the static/transient/pretty hostname
hostnamectl set-hostname <NAME> --static Set only the static hostname
hostnamectl set-hostname <NAME> --transient Set only the transient hostname
hostnamectl set-hostname <NAME> --pretty Set the pretty (user-friendly) hostname
hostnamectl set-chassis <TYPE> Set chassis type (e.g., desktop, laptop, server)

#↓ Description

- Static - The permanent hostname stored in `/etc/hostname`. Used at boot time.
- Transient - Temporary hostname set at runtime (often by DHCP or manually). Lost on reboot.
- Pretty - A user-friendly, display name (can include spaces and special characters).

#systemd-run

  • Run a command as a transient systemd unit (service, scope, timer, etc.).
Command Description
systemd-run <command> Run command as a transient .service unit
systemd-run -p BlockIOWeight=10 updatedb Run with a specific resource limit property
systemd-run -p MemoryMax=100M script.sh Run a memory-hungry task with a memory usage limit
systemd-run --on-calendar='YYYY-MM-DD HH:MM' script.sh Schedule script to run at a specific date and time
systemd-run -E VAR=value script.sh Run a script with environment variables set

#Types of systemd limits

There are different kinds of limit that can be imposed:

- CPU Limits
- Memory Limits
- I/O Limits
- Task/Thread Limits
- Security/Isolation
- Execution/Environment

Example

systemd-run -p MemoryMax=100M -p CPUQuota=50% scriptFile.sh

#Also See