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

Technology

SystemD Usage Guide