• Unit
  • Unit file
  • Unit management
• System state management
  • System overview
  • Targets
• Journal
  • Time filter
• System time

unit is a basic object that systemd manages and acts upon. Unit is defined using unit file. Similar to services or jobs in other init systems. Can be used to abstract services, network resources, devices, filesystem mounts and so on.

Most common type of unit is service (indicated by a unit file name with .service suffix).

Static unit doesn't contain install section, thus, can not be enabled - usually such units perform one-off action or act as a dependency for other unit.

File locations(listed in low to high priority order):

List of unit files on the system:

$ systemctl list-unit-files

See full content of a unit file;

$ systemctl cat foo.service

edit command, by default, opens a unit file snippet, which would be a blank file that can be used to override or add directives to the unit definition. A directory at /etc/systemd/system with a service name will be created, e.g. foo.service.d with a snippet override.conf file. --full flag load the whole unit file definition; changes will be stored at /etc/systemd/system.

# Add unit file snippet
$ systemctl edit foo.service
# Modify entire content
$ systemctl edit --full foo.service
# Reload the process to pick up changes
$ systemctl daemon-reload

Show low level details of the unit's settings (outputs key-value pairs):

$ systemctl show nginx.service
# Show individual key value
$ systemctl show nginx.service -p Before

View dependency tree - which units were activated when stating the unit (the unit that are specified as Wanted or Required). --reverse would show units that depend on specified unit. Additional --before and --after options narrow the list to those that depend before or after.

$ systemctl list-dependencies foo.service
# Expand all dependencies recursively
$ systemctl list-dependencies --all foo.service
# --reverse - show units that depend on unit
# --before, --after - depend before or after

Unit can be marked as unstartable (masked). Corresponding command creates a symlink to /dev/null:

$ systemctl [mask|unmask] foo.service

When operating on services, *.service suffix can be omitted - systemd identifies service management commands and targets *.service files automatically. Commands affecting OS, require sudo privileges.

# Basic system commands
# Use reload if application can reload configuration without restarting
$ systemctl [start|stop|restart|reload] foo.service

# Use the following if unsure if app has reload functionality
$ systemctl reload-or-restart foo.service

Start services automatically at boot. Enabling creates a symbolic link at the location where systemd looks for autostart files.

$ systemctl [enable|disable] foo.service

View current state of a unit. Shortcut commands return also return exit code 0 or 1 that can be used with scripts. is-failed return active (properly running), failed (error occurred) or unknown/inactive (intentionally stopped); 0 exit code indicates failure, 1 - other states.

# Current state of a unit
$ systemctl status foo.service
# Shortcuts
$ systemctl is-active foo.service
$ systemctl is-enabled foo.service
$ systemctl is-failed foo.service

Some major states have shortcuts (rescue is single user mode, would be same as systemctl isolate rescue.target):

$ sudo systemctl [poweroff|reboot|rescue|halt]

View all unit files that are listed as "active"; list-units can be dropped, as it is the default behavior of systemctl. --all lists all units that systemd attempted to load regardless of current state. Use --state=, --type= to narrow down the list.

$ systemctl list-units
$ systemctl list-units --all
$ systemctl list-units --all --state=inactive
$ systemctl list-units --type=service

Targets, previously known as runlevels, describe a system state and act as synchronization points that the server can use to bring itself to a specific state. Unlike runlevels multiple targets can be active at the same time.

# View all available targets on the system
$ systemctl list-unit-files --type=target
# View all active targets
$ systemctl list-units --type=target

# View default target that systemd tries to reach
$ systemctl get-default
# Set new default
$ systemctl set-default multi-user.target

Stop all units that are not tied to the specified target. Make sure no essential services would be stopped before running.

$ systemctl list-dependencies multi-user.target
$ systemctl isolate multi-user.target

The system that collects and manages all kernel and userland processes' logs. Implemented as journald daemon. Can be used together with existing syslog implementation (centralized server with local journal for multiple services), or can completely replace it.

By default entries are shown in local time (make sure correct time zone is set via timedatectl utility); can also be viewed in UTC.

# Display entries with UTC timestamps
$ journalctl --utc

# List only kernel messages (typically represented by dmesg
$ journalctl -k

# List journal entries for the unit
$ journalctl -u foo.service

By default shows all log entries with a pager (usually less) starting from oldest including current and previous boots, if journald is configured to save previous boot records.

To enable storing previous boot log entries set Storage= option under [Journal] section to persistent in /etc/systemd/journald.conf file, or create persistent directory:

$ sudo mkdir -p /var/log/journal

Boot listing: first column is an offset, second is boot ID. Both can be referenced in other journalctl commands.

# List entries from current boot
$ journalctl -b

# List boots that journald is aware of
$ journalctl --list-boots
# List entries on previous boot
$ journalctl -b -1

Arbitrary time limits can be specified via --since and --until options. YYYY-MM-DD HH:MM:SS format is used to specify absolute time values. Some defaults are applied, if parts of the format are left off: current date is assumed for date, midnight time for time, 00 seconds for seconds field. Relative values such as yesterday, today, tomorrow, and now. Also prepending - or + to a numbered value or using ago in sentence construction is possible.

$ journalctl --since "2015-01-10" --until "2015-01-11 03:00"
$ journalctl --since yesterday
$ journalctl --since 09:00 --until "1 hour ago"

# Display system info, status may be ommited
$ timedatectl status

# List time zones
$ timedatectl list-timezones
# Set time zone
$ sudo timedatectl set-timezone <zone>

• Systemd Essentials: Working with Services, Units, and the Journal
• Understanding Systemd Units and Unit Files
• How To Use Systemctl to Manage Systemd Services and Units
• How To Use Journalctl to View and Manipulate Systemd Logs