| 1 | <?xml version="1.0" encoding="UTF-8"?>
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| 2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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| 3 | "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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| 4 | <!ENTITY % general-entities SYSTEM "../general.ent">
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| 5 | %general-entities;
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| 6 | ]>
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| 7 |
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| 8 | <sect1 id="ch-config-systemd-custom" revision="systemd">
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| 9 | <?dbhtml filename="systemd-custom.html"?>
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| 10 |
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| 11 | <title>Systemd Usage and Configuration</title>
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| 12 |
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| 13 | <indexterm zone="ch-config-systemd-custom">
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| 14 | <primary sortas="e-Systemd">Systemd Customization</primary>
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| 15 | </indexterm>
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| 16 |
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| 17 | <sect2>
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| 18 | <title>Basic Configuration</title>
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| 19 |
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| 20 | <para>The <filename>/etc/systemd/system.conf</filename> file contains a set
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| 21 | of options to control basic systemd operations. The default file has all
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| 22 | entries commented out with the default settings indicated. This file is
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| 23 | where the log level may be changed as well as some basic logging settings.
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| 24 | See the <ulink role='man'
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| 25 | url='&man;systemd-system.conf.5'>systemd-system.conf(5)</ulink> manual
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| 26 | page for details on each configuration option.</para>
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| 27 |
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| 28 | </sect2>
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| 29 |
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| 30 | <sect2>
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| 31 | <title>Disabling Screen Clearing at Boot Time</title>
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| 32 |
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| 33 | <para>The normal behavior for systemd is to clear the screen at
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| 34 | the end of the boot sequence. If desired, this behavior may be
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| 35 | changed by running the following command:</para>
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| 36 |
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| 37 | <screen role="nodump"><userinput>mkdir -pv /etc/systemd/system/getty@tty1.service.d
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| 38 |
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| 39 | cat > /etc/systemd/system/getty@tty1.service.d/noclear.conf << EOF
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| 40 | <literal>[Service]
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| 41 | TTYVTDisallocate=no</literal>
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| 42 | EOF</userinput></screen>
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| 43 |
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| 44 | <para>The boot messages can always be reviewed by using the
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| 45 | <userinput>journalctl -b</userinput> command as the
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| 46 | <systemitem class="username">root</systemitem> user.</para>
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| 47 |
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| 48 | </sect2>
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| 49 |
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| 50 | <sect2 id='systemd-no-tmpfs'>
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| 51 | <title>Disabling tmpfs for /tmp</title>
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| 52 |
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| 53 | <para>By default, <filename class="directory">/tmp</filename> is created as
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| 54 | a tmpfs. If this is not desired, it can be overridden by executing the
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| 55 | following command:</para>
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| 56 |
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| 57 | <screen role="nodump"><userinput>ln -sfv /dev/null /etc/systemd/system/tmp.mount</userinput></screen>
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| 58 |
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| 59 | <para>Alternatively, if a separate partition for
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| 60 | <filename class="directory">/tmp</filename> is desired, specify that
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| 61 | partition in a <filename>/etc/fstab</filename> entry.</para>
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| 62 |
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| 63 | <warning>
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| 64 | <para>
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| 65 | Do not create the symbolic link above if a separate partition is used
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| 66 | for <filename class="directory">/tmp</filename>. This will prevent the
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| 67 | root file system (/) from being remounted r/w and make the system
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| 68 | unusable when booted.
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| 69 | </para>
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| 70 | </warning>
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| 71 |
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| 72 | </sect2>
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| 73 |
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| 74 | <sect2>
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| 75 | <title>Configuring Automatic File Creation and Deletion</title>
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| 76 |
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| 77 | <para>There are several services that create or delete files or
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| 78 | directories:</para>
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| 79 |
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| 80 | <itemizedlist>
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| 81 | <listitem><para>systemd-tmpfiles-clean.service</para></listitem>
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| 82 | <listitem><para>systemd-tmpfiles-setup-dev.service</para></listitem>
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| 83 | <listitem><para>systemd-tmpfiles-setup.service</para></listitem>
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| 84 | </itemizedlist>
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| 85 |
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| 86 | <para>The system location for the configuration files is
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| 87 | <filename>/usr/lib/tmpfiles.d/*.conf</filename>. The local
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| 88 | configuration files are in
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| 89 | <filename class="directory">/etc/tmpfiles.d</filename>. Files in
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| 90 | <filename class="directory">/etc/tmpfiles.d</filename> override
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| 91 | files with the same name in
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| 92 | <filename class="directory">/usr/lib/tmpfiles.d</filename>. See
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| 93 | <ulink role='man' url='&man;tmpfiles.d.5'>tmpfiles.d(5)</ulink> manual
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| 94 | page for file format details.</para>
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| 95 |
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| 96 | <para>
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| 97 | Note that the syntax for the
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| 98 | <filename>/usr/lib/tmpfiles.d/*.conf</filename> files can be
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| 99 | confusing. For example, the default deletion of files in the /tmp directory
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| 100 | is located in <filename>/usr/lib/tmpfiles.d/tmp.conf</filename> with
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| 101 | the line:
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| 102 |
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| 103 | <screen role="nodump">q /tmp 1777 root root 10d</screen>
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| 104 |
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| 105 | The type field, q, discusses creating a subvolume with quotas which
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| 106 | is really only applicable to btrfs filesystems. It references type v
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| 107 | which in turn references type d (directory). This then creates the
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| 108 | specified directory if it is not present and adjusts the permissions
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| 109 | and ownership as specified. Contents of the directory will be
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| 110 | subject to time based cleanup if the age argument is specified.
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| 111 | </para>
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| 112 |
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| 113 | <para>
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| 114 | If the default parameters are not desired, then the file should
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| 115 | be copied to <filename class="directory">/etc/tmpfiles.d</filename>
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| 116 | and edited as desired. For example:
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| 117 |
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| 118 | <screen role="nodump"><userinput>mkdir -p /etc/tmpfiles.d
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| 119 | cp /usr/lib/tmpfiles.d/tmp.conf /etc/tmpfiles.d</userinput></screen>
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| 120 | </para>
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| 121 |
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| 122 | </sect2>
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| 123 |
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| 124 | <sect2>
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| 125 | <title>Overriding Default Services Behavior</title>
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| 126 |
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| 127 | <para>The parameters of a unit can be overridden by creating a directory
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| 128 | and a configuration file in <filename
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| 129 | class="directory">/etc/systemd/system</filename>. For example:</para>
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| 130 |
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| 131 | <screen role="nodump"><userinput>mkdir -pv /etc/systemd/system/foobar.service.d
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| 132 |
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| 133 | cat > /etc/systemd/system/foobar.service.d/foobar.conf << EOF
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| 134 | <literal>[Service]
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| 135 | Restart=always
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| 136 | RestartSec=30</literal>
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| 137 | EOF</userinput></screen>
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| 138 |
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| 139 | <para>See <ulink role='man'
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| 140 | url='&man;systemd.unit.5'>systemd.unit(5)</ulink> manual page for more
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| 141 | information. After creating the configuration file, run
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| 142 | <userinput>systemctl daemon-reload</userinput> and <userinput>systemctl
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| 143 | restart foobar</userinput> to activate the changes to a service.</para>
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| 144 |
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| 145 | </sect2>
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| 146 |
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| 147 | <sect2>
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| 148 | <title>Debugging the Boot Sequence</title>
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| 149 |
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| 150 | <para>Rather than plain shell scripts used in SysVinit or BSD style init
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| 151 | systems, systemd uses a unified format for different types of startup
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| 152 | files (or units). The command <command>systemctl</command> is used to
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| 153 | enable, disable, control state, and obtain status of unit files. Here
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| 154 | are some examples of frequently used commands:</para>
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| 155 |
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| 156 | <itemizedlist>
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| 157 | <listitem>
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| 158 | <para><command>systemctl list-units -t <replaceable><service></replaceable> [--all]</command>:
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| 159 | lists loaded unit files of type service.</para>
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| 160 | </listitem>
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| 161 | <listitem>
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| 162 | <para><command>systemctl list-units -t <replaceable><target></replaceable> [--all]</command>:
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| 163 | lists loaded unit files of type target.</para>
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| 164 | </listitem>
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| 165 | <listitem>
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| 166 | <para><command>systemctl show -p Wants <replaceable><multi-user.target></replaceable></command>:
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| 167 | shows all units that depend on the multi-user target. Targets are
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| 168 | special unit files that are analogous to runlevels under
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| 169 | SysVinit.</para>
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| 170 | </listitem>
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| 171 | <listitem>
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| 172 | <para><command>systemctl status <replaceable><servicename.service></replaceable></command>:
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| 173 | shows the status of the servicename service. The .service extension
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| 174 | can be omitted if there are no other unit files with the same name,
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| 175 | such as .socket files (which create a listening socket that provides
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| 176 | similar functionality to inetd/xinetd).</para>
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| 177 | </listitem>
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| 178 | </itemizedlist>
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| 179 |
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| 180 | </sect2>
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| 181 |
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| 182 | <sect2>
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| 183 | <title>Working with the Systemd Journal</title>
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| 184 |
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| 185 | <para>Logging on a system booted with systemd is handled with
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| 186 | systemd-journald (by default), rather than a typical unix syslog daemon.
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| 187 | You can also add a normal syslog daemon and have both operate side by
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| 188 | side if desired. The systemd-journald program stores journal entries in a
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| 189 | binary format rather than a plain text log file. To assist with
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| 190 | parsing the file, the command <command>journalctl</command> is provided.
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| 191 | Here are some examples of frequently used commands:</para>
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| 192 |
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| 193 | <itemizedlist>
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| 194 | <listitem>
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| 195 | <para><command>journalctl -r</command>: shows all contents of the
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| 196 | journal in reverse chronological order.</para>
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| 197 | </listitem>
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| 198 | <listitem>
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| 199 | <para><command>journalctl -u <replaceable>UNIT</replaceable></command>:
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| 200 | shows the journal entries associated with the specified UNIT
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| 201 | file.</para>
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| 202 | </listitem>
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| 203 | <listitem>
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| 204 | <para><command>journalctl -b[=ID] -r</command>: shows the journal
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| 205 | entries since last successful boot (or for boot ID) in reverse
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| 206 | chronological order.</para>
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| 207 | </listitem>
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| 208 | <listitem>
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| 209 | <para><command>journalctl -f</command>: provides functionality similar
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| 210 | to tail -f (follow).</para>
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| 211 | </listitem>
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| 212 | </itemizedlist>
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| 213 |
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| 214 | </sect2>
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| 215 |
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| 216 | <sect2>
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| 217 | <title>Working with Core Dumps</title>
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| 218 |
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| 219 | <para>Core dumps are useful to debug crashed programs, especially
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| 220 | when a daemon process crashes. On systemd booted systems the core
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| 221 | dumping is handled by <command>systemd-coredump</command>. It will
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| 222 | log the core dump in the journal and store the core dump itself in
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| 223 | <filename class="directory">/var/lib/systemd/coredump</filename>.
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| 224 | To retrieve and process core dumps, the <command>coredumpctl</command>
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| 225 | tool is provided. Here are some examples of frequently used commands:
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| 226 | </para>
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| 227 |
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| 228 | <itemizedlist>
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| 229 | <listitem>
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| 230 | <para><command>coredumpctl -r</command>: lists all core dumps in
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| 231 | reverse chronological order.</para>
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| 232 | </listitem>
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| 233 | <listitem>
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| 234 | <para><command>coredumpctl -1 info</command>: shows the information
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| 235 | from the last core dump.</para>
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| 236 | </listitem>
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| 237 | <listitem>
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| 238 | <para><command>coredumpctl -1 debug</command>: loads the last core
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| 239 | dump into <ulink url="&blfs-book;general/gdb.html">GDB</ulink>.
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| 240 | </para>
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| 241 | </listitem>
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| 242 | </itemizedlist>
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| 243 |
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| 244 | <para>Core dumps may use a lot of disk space. The maximum disk space
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| 245 | used by core dumps can be limited by creating a configuration file in
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| 246 | <filename class="directory">/etc/systemd/coredump.conf.d</filename>.
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| 247 | For example:</para>
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| 248 |
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| 249 | <screen role="nodump"><userinput>mkdir -pv /etc/systemd/coredump.conf.d
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| 250 |
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| 251 | cat > /etc/systemd/coredump.conf.d/maxuse.conf << EOF
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| 252 | <literal>[Coredump]
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| 253 | MaxUse=5G</literal>
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| 254 | EOF</userinput></screen>
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| 255 |
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| 256 | <para>See the <ulink role='man' url='&man;systemd-coredump.8'>systemd-coredump(8)</ulink>,
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| 257 | <ulink role='man' url='&man;coredumpctl.1'>coredumpctl(1)</ulink>, and
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| 258 | <ulink role='man'
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| 259 | url='&man;coredump.conf.d.5'>coredump.conf.d(5)</ulink> manual pages for
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| 260 | more information.</para>
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| 261 | </sect2>
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| 262 |
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| 263 | <sect2>
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| 264 | <title>Long Running Processes</title>
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| 265 |
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| 266 | <para>Beginning with systemd-230, all user processes are killed when a user
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| 267 | session is ended, even if nohup is used, or the process uses the
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| 268 | <function>daemon()</function> or <function>setsid()</function> functions.
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| 269 | This is a deliberate change from a historically permissive environment to a
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| 270 | more restrictive one. The new behavior may cause issues if you depend on
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| 271 | long running programs (e.g., <command>screen</command> or
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| 272 | <command>tmux</command>) to remain active after ending your user session.
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| 273 | There are three ways to enable lingering processes to remain after a user
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| 274 | session is ended.</para>
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| 275 |
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| 276 | <itemizedlist>
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| 277 | <listitem>
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| 278 | <para>
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| 279 | <emphasis>Enable process lingering for only selected users</emphasis>:
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| 280 | Normal users have permission to enable process lingering
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| 281 | with the command <command>loginctl enable-linger</command> for their
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| 282 | own user. System administrators can use the same command with a
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| 283 | <parameter>user</parameter> argument to enable for a user. That user
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| 284 | can then use the <command>systemd-run</command> command to start
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| 285 | long running processes. For example: <command>systemd-run --scope
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| 286 | --user /usr/bin/screen</command>. If you enable lingering for your
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| 287 | user, the user@.service will remain even after all login sessions are
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| 288 | closed, and will automatically start at system boot. This has the
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| 289 | advantage of explicitly allowing and disallowing processes to run
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| 290 | after the user session has ended, but breaks backwards compatibility
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| 291 | with tools like <command>nohup</command> and utilities that use
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| 292 | <function>daemon()</function>.
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| 293 | </para>
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| 294 | </listitem>
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| 295 | <listitem>
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| 296 | <para>
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| 297 | <emphasis>Enable system-wide process lingering</emphasis>:
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| 298 | You can set <parameter>KillUserProcesses=no</parameter> in
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| 299 | <filename>/etc/systemd/logind.conf</filename> to enable process lingering
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| 300 | globally for all users. This has the benefit of leaving the old
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| 301 | method available to all users at the expense of explicit control.
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| 302 | </para>
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| 303 | </listitem>
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| 304 | <listitem>
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| 305 | <para>
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| 306 | <emphasis>Disable at build-time</emphasis>: You can disable
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| 307 | lingering by default while building systemd by adding the switch
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| 308 | <parameter>-Ddefault-kill-user-processes=false</parameter> to the
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| 309 | <command>meson</command> command for systemd. This completely
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| 310 | disables the ability of systemd to kill user processes at session
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| 311 | end.
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| 312 | </para>
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| 313 | </listitem>
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| 314 | </itemizedlist>
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| 315 |
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| 316 | </sect2>
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| 317 |
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| 318 | </sect1>
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