SystemD Usage Guide

Systemd Usage Guide on RHEL 7
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Table of Contents
Overview.......................................................................................................................................................1
Applies To..................................................................................................................................................1
Pre-Requisites ...........................................................................................................................................1
Systemd – Generic Information....................................................................................................................1
Version......................................................................................................................................................1
Binaries......................................................................................................................................................1
Process System..........................................................................................................................................2
Systemd – Units ............................................................................................................................................2
Systemd – Boot Process................................................................................................................................3
Analyze systemd – Boot Process...............................................................................................................3
Analyze systemd – Each Boot Process......................................................................................................4
Analyze systemd – Critical Chain at Boot..................................................................................................5
Analyze systemd – Critical Chain by Service Unit .....................................................................................6
Analyze systemd – Critical Chain by Target ..............................................................................................6
Systemd Units Commands............................................................................................................................7
List Units – Available.................................................................................................................................7
List Units – Running ..................................................................................................................................8
List Units – Failed ......................................................................................................................................8
List Units – By Type Service.......................................................................................................................9
List Units – By Type Mount .......................................................................................................................9
List Units – By Type Socket .....................................................................................................................10
List Units – By Type Slice.........................................................................................................................10
List Units – By Type Swap........................................................................................................................11
List Units – By Type Snapshot .................................................................................................................11
List Units – By Type Path.........................................................................................................................11
List Units – By Type Timer.......................................................................................................................12
So What’s systemctl / systemd unit Files....................................................................................................12
List Service – All Properties.....................................................................................................................13
List Service – Specific Property - CPUShares...........................................................................................13
List Service – Specific Property - CanReload...........................................................................................14
List Service – Specific Property - Conflicts ..............................................................................................14

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Service Dependencies.................................................................................................................................14
List Dependencies...................................................................................................................................15
List Service Dependencies by Service .....................................................................................................16
List Dependencies by Target...................................................................................................................16
List Service Dependencies – For Service.................................................................................................17
List Service Dependencies – Before........................................................................................................17
List Service Dependencies – After...........................................................................................................18
List Service Dependencies – Reverse......................................................................................................18
List Service Dependencies – Target ........................................................................................................19
List Service Dependencies – Plot ............................................................................................................19

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Overview
Systemctl is a systemd utility which is responsible for controlling the systemd system and service manager.
Systemd is a collection of system management daemons, utilities and libraries which serves as a
replacement of SystemV init daemon.
Systemd functions as central management and configuration platform for UNIX like system.
Applies To
Tested on RHEL 7, CentOS 7.
Pre-Requisites
 None
Systemd – Generic Information
Version
To find the version of systemctl executable, run the command;
systemctl --version
Binaries
To find the binaries of systemctl executable, run the command;
whereis systemctl

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Process System
To know the process associated with systemd run the command;
ps -eaf | grep [s]ystemd
Systemd – Units
There are different units that are managed by systemd and it accepts following units.
Systemctl Units Purpose
systemd.service Manage System Service;
A unit configuration file whose name ends in .service encodes information about
a process controlled and supervised by systemd.
systemd.socket Manage sockets for each service;
A unit configuration file whose name ends in ".socket" encodes information
about an IPC or network socket or a file system FIFO controlled and supervised
by systemd, for socket-based activation.
systemd.device Manage devices;
A unit configuration file whose name ends in ".device" encodes information
about a device unit as exposed in the sysfs/udev device tree.
systemd.mount Manage mount points;
A unit configuration file whose name ends in ".mount" encodes information
about a file system mount point controlled and supervised by systemd.
systemd.target Manage targets;
A unit configuration file whose name ends in ".target" encodes information
about a target unit of systemd, which is used for grouping units and as well-
known synchronization points during start-up.
Target units do not offer any additional functionality on top of the generic
functionality provided by units. They exist merely to group units via
dependencies (useful as boot targets), and to establish standardized names for
synchronization points used in dependencies between units. Among other

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things, target units are a more flexible replacement for SysV runlevels in the
classic SysV init system.
systemd.paths Check the existence of files or directories or create them as needed
systemd.slices Manage resources
systemd.snapshots Takes snapshots of the current state of the system
systemd.sockets Set up sockets to allow communication paths to processes that can remain in
place, even if the underlying process needs to restart
systemd.swaps Create and use swap files or swap partitions
Systemd.timer Trigger actions based on a timer
Systemd – Boot Process
systemd is the first process started by the kernel. It replaces the venerable SysVinit program (also
called init) and the newer Upstart init system. systemd coordinates the rest of the boot process and
configures the environment for the user.
systemd improves on other init systems with increased parallelization. It starts the process of loading all
programs it launches immediately, and manages information between interdependent programs as they
load. By dissociating programs and their means of communication, each program is able to load without
waiting for unrelated or even dependent programs to load first.
Analyze systemd – Boot Process
To find out overall boot-up performance time consumption for the boot processes under different
categories, run the command;
systemd-analyze
This information are categorized under different heads such as;
 kernel
 initrd
 userspace

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Analyze systemd – Each Boot Process
You can also analyze boot-up performance, the time consumed by each task / service during the boot
process. Run the command;
systemd-analyze blame

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Analyze systemd – Critical Chain at Boot
To analyze the boot-up processes that’s critical for the booting the system in the current target, run the
command;
systemd-analyze critical-chain
Note:
 If the list is very long the overall boot process is extended / delay in system boot.
 Time after the unit is active or started is printed after the "@" character.
 Time the unit takes to start is printed after the "+" character.

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Analyze systemd – Critical Chain by Service Unit
To analyze the boot-up process for a specific service unit and its critical chain, run the command;
systemd-analyze critical-chain sshd.service
Note:
 If the list is very long the overall boot process is extended / delay. In case of the specific service takes
longer time to start, it would impact the overall booting of the server
Analyze systemd – Critical Chain by Target
To list the “Critical Chain” for different targets, to know the analysis run the command;
systemd-analyze critical-chain basic.target | grep target
Note:

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Systemd Units Commands
Units are the objects that systemd knows how to manage it. It’s standardized representation of system
resources that can be managed by the suite of daemons and it can be manipulated by the provided
utilities.
Units in some ways can be said to similar to services or jobs in other init systems.
List Units – Available
Unit files allow systemd to address multiple units from a single configuration file. You can call a systemd
template unit file. To list from the template, unit files configured on the system along with their current
state of each unit run the command;
systemctl list-unit-files

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List Units – Running
template unit file. To list from the template, active state unit files configured on the system run the
command;
systemctl list-units --state=active
List Units – Failed
template unit file. To list from the template, failed state unit files configured on the system run the
command;
systemctl list-units --state=failed

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List Units – By Type Service
Unit files that allow systemd to manage units. In case of wherein unit name that ends with “.service”
encodes information about a process controlled and managed by systemd , to list service units run the
command;
systemctl list-units --type=service
List Units – By Type Mount
Unit files that allow systemd to manage units. In case of wherein unit name that ends with “.mount”
encodes information about a process controlled and managed by systemd , to list mount points run the
command;
systemctl list-unit-files --type=mount

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List Units – By Type Socket
Unit files that allow systemd to manage units. In case of wherein unit name that ends with “.socket”
encodes information an IPC or network socket or a file system FIFO controlled and supervised by systemd,
for socket-based activation, to list sockets that are configured and enabled, run the command;
systemctl list-unit-files --type=socket
List Units – By Type Slice
Unit files that allow systemd to manage units. In case of wherein unit name that ends with “.slice” encodes
information about a slice which is a concept for hierarchically managing resources of a group of
processes. This management is performed by creating a node in the Linux Control Group (cgroup) tree, ,
run the command;
systemctl list-unit-files --type=slice

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List Units – By Type Swap
Unit files that allow systemd to manage units. In case of wherein unit name that ends with “.swap”
encodes swap device or file for memory paging controlled and supervised by systemd, run the command;
systemctl list-unit-files --type=swap
List Units – By Type Snapshot
Snapshot units are not configured via unit configuration files. Nonetheless they are named similar to
filenames. A unit whose name ends in ".snapshot" refers to a dynamic snapshot of the systemd runtime
state.
Snapshots are not configured on disk but created dynamically via systemctl, run the command;
systemctl list-unit-files --type=snapshot
List Units – By Type Path
systemctl list-unit-files --type=path
Unit files that allow systemd to manage units. In case of wherein unit name that ends with “.path”
encodes information about path monitored by systemd, for path-based activation, run the command;

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List Units – By Type Timer
Unit files that allow systemd to manage units. In case of wherein unit name that ends with “.timer”
encodes timer controlled and supervised by systemd, for timer-based activation.
systemctl list-unit-files --type=timer
So What’s systemctl / systemd unit Files
The unit files on your system determine how systemd will start and run. Each corresponds to a single
activity or component or unit in systemd terms.
 The global systemd configuration is stored in the /etc/systemd directory.
 The service configuration files are located in the /usr/lib/systemd/system directory.
 Custom service configuration files are stored in the /etc/systemd/system directory.
Each unit file is a simple text file describing a unit, what it does, what needs to run before or afterward,
and other details. Unit files can be stored in a few different places on your system. systemd looks for
system unit files in the following order;
Path Purpose Mode
/etc/systemd/system Local Configuration System
/run/systemd/system Run-time Units System
/usr/lib/systemd/system Units provided by installed packages System
Depending on whether “--system” “--user” “--runtime” or “--global” is specified, this enables the unit for
the system, for the calling user only, for only this boot of the system, or for all future logins of all users, or
only this boot.
Note: In the last case, systemd daemon configuration is reloaded.
Similarly, systemd can also run in a user context and manage resources per user in addition to system-
side. Unit files for user units are stored similarly in folders
Path Purpose Mode
/etc/systemd/user Local Configuration User
/run/systemd/user Run-time Units User
/usr/lib/systemd/user Units provided by installed packages User
Note: The order of precedence works similarly way.

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List Service – All Properties
List all the properties of a unit. It will display a list of properties that are set for the specified unit.
systemctl show rsyslogd --all
List Service – Specific Property - CPUShares
List a specific property of a service / unit, enter the attribute name. Run the command;
systemctl show rsyslogd -p CPUShares
Note: The default each service has a CPUShare = 1024. It can be configured to increase/decrease CPU
share of a process.

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List Service – Specific Property - CanReload
List dependencies for a service / unit, if the service can automatically reload. Run the command;
systemctl show rsyslogd -p CanReload
List Service – Specific Property - Conflicts
List dependencies for a service / unit, if the service can automatically reload. Run the command;
In case sshd.service conflicts, it will invoke shutdown.target which will shutdown the unit / service.
systemctl show sshd.service -p Conflicts
Service Dependencies
One other information that can be collate or reviewed is dependencies for each target(s) or each unit(s).
Service dependencies allow you to conditionally include submodules and functionality, based on what the
service / units library require.
There are two types systemd dependencies that exists between units:
 Dependencies that affect the activation of units, that are
 Requires
 Wants
 Conflicts
 Dependencies that affect the order of units, that are
 After
 Before

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List Dependencies
To list the services, current target dependencies run the command;
systemctl list-dependencies

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List Service Dependencies by Service
To list the services dependencies run the command;
systemctl list-dependencies sshd.service
List Dependencies by Target
To list the target dependencies run the command;
systemctl list-dependencies graphical.target | grep target

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List Service Dependencies – For Service
Show the service / unit dependencies. To know the dependencies of rsyslog.service, run the command;
systemctl list-dependencies rsyslog.service
List Service Dependencies – Before
List the units that are ordered after the specified unit.
systemctl list-dependencies rsyslog.service --before

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List Service Dependencies – After
show the units that are ordered before the specified unit
systemctl list-dependencies rsyslog.service --after
List Service Dependencies – Reverse
Show reverse dependencies between units, List units with dependencies of type Wants= or Requires= on
the given unit.
systemctl list-dependencies rsyslog.service --reverse

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List Service Dependencies – Target
List critical chain for network and local-fs targets.
systemd-analyze critical-chain network.target local-fs.target
List Service Dependencies – Plot
Analyze systemd and plot into a file.
systemd-analyze plot > systemd_analyze_plot.svg

SystemD Usage Guide

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SystemD Usage Guide