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316 lines
13 KiB
XML
316 lines
13 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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<!ENTITY % general-entities SYSTEM "../general.ent">
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%general-entities;
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]>
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<sect1 id="ch-config-systemd-custom" revision="systemd">
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<?dbhtml filename="systemd-custom.html"?>
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<title>Systemd Usage and Configuration</title>
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<indexterm zone="ch-config-systemd-custom">
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<primary sortas="e-Systemd">Systemd Customization</primary>
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</indexterm>
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<sect2>
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<title>Basic Configuration</title>
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<para>The <filename>/etc/systemd/system.conf</filename> file contains a set
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of options to control basic systemd operations. The default file has all
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entries commented out with the default settings indicated. This file is
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where the log level may be changed as well as some basic logging settings.
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See the <filename>systemd-system.conf(5)</filename> manual page for details
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on each configuration option.</para>
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</sect2>
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<sect2>
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<title>Disabling Screen Clearing at Boot Time</title>
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<para>The normal behavior for systemd is to clear the screen at
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the end of the boot sequence. If desired, this behavior may be
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changed by running the following command:</para>
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<screen role="nodump"><userinput>mkdir -pv /etc/systemd/system/getty@tty1.service.d
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cat > /etc/systemd/system/getty@tty1.service.d/noclear.conf << EOF
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<literal>[Service]
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TTYVTDisallocate=no</literal>
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EOF</userinput></screen>
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<para>The boot messages can always be reviewed by using the
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<userinput>journalctl -b</userinput> command as the
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<systemitem class="username">root</systemitem> user.</para>
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</sect2>
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<sect2>
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<title>Disabling tmpfs for /tmp</title>
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<para>By default, <filename class="directory">/tmp</filename> is created as
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a tmpfs. If this is not desired, it can be overridden by executing the
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following command:</para>
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<screen role="nodump"><userinput>ln -sfv /dev/null /etc/systemd/system/tmp.mount</userinput></screen>
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<para>Alternatively, if a separate partition for
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<filename class="directory">/tmp</filename> is desired, specify that
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partition in a <filename>/etc/fstab</filename> entry.</para>
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<warning>
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<para>
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Do not create the symbolic link above if a separate partition is used
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for <filename class="directory">/tmp</filename>. This will prevent the
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root file system (/) from being remounted r/w and make the system
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unusable when booted.
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</para>
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</warning>
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</sect2>
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<sect2>
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<title>Configuring Automatic File Creation and Deletion</title>
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<para>There are several services that create or delete files or
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directories:</para>
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<itemizedlist>
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<listitem><para>systemd-tmpfiles-clean.service</para></listitem>
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<listitem><para>systemd-tmpfiles-setup-dev.service</para></listitem>
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<listitem><para>systemd-tmpfiles-setup.service</para></listitem>
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</itemizedlist>
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<para>The system location for the configuration files is
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<filename>/usr/lib/tmpfiles.d/*.conf</filename>. The local
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configuration files are in
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<filename class="directory">/etc/tmpfiles.d</filename>. Files in
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<filename class="directory">/etc/tmpfiles.d</filename> override
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files with the same name in
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<filename class="directory">/usr/lib/tmpfiles.d</filename>. See
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<filename>tmpfiles.d(5)</filename> manual page for file format
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details.</para>
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<para>
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Note that the syntax for the
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<filename>/usr/lib/tmpfiles.d/*.conf</filename> files can be
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confusing. For example, the default deletion of files in the /tmp directory
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is located in <filename>/usr/lib/tmpfiles.d/tmp.conf</filename> with
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the line:
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<screen role="nodump">q /tmp 1777 root root 10d</screen>
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The type field, q, discusses creating a subvolume with quotas which
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is really only applicable to btrfs filesystems. It references type v
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which in turn references type d (directory). This then creates the
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specified directory if it is not present and adjusts the permissions
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and ownership as specified. Contents of the directory will be
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subject to time based cleanup if the age argument is specified.
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</para>
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<para>
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If the default parameters are not desired, then the file should
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be copied to <filename class="directory">/etc/tmpfiles.d</filename>
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and edited as desired. For example:
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<screen role="nodump"><userinput>mkdir -p /etc/tmpfiles.d
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cp /usr/lib/tmpfiles.d/tmp.conf /etc/tmpfiles.d</userinput></screen>
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</para>
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</sect2>
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<sect2>
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<title>Overriding Default Services Behavior</title>
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<para>The parameters of a unit can be overridden by creating a directory
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and a configuration file in <filename
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class="directory">/etc/systemd/system</filename>. For example:</para>
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<screen role="nodump"><userinput>mkdir -pv /etc/systemd/system/foobar.service.d
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cat > /etc/systemd/system/foobar.service.d/foobar.conf << EOF
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<literal>[Service]
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Restart=always
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RestartSec=30</literal>
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EOF</userinput></screen>
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<para>See <filename>systemd.unit(5)</filename> manual page for more
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information. After creating the configuration file, run
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<userinput>systemctl daemon-reload</userinput> and <userinput>systemctl
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restart foobar</userinput> to activate the changes to a service.</para>
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</sect2>
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<sect2>
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<title>Debugging the Boot Sequence</title>
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<para>Rather than plain shell scripts used in SysVinit or BSD style init
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systems, systemd uses a unified format for different types of startup
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files (or units). The command <command>systemctl</command> is used to
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enable, disable, control state, and obtain status of unit files. Here
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are some examples of frequently used commands:</para>
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<itemizedlist>
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<listitem>
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<para><command>systemctl list-units -t <replaceable><service></replaceable> [--all]</command>:
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lists loaded unit files of type service.</para>
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</listitem>
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<listitem>
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<para><command>systemctl list-units -t <replaceable><target></replaceable> [--all]</command>:
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lists loaded unit files of type target.</para>
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</listitem>
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<listitem>
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<para><command>systemctl show -p Wants <replaceable><multi-user.target></replaceable></command>:
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shows all units that depend on the multi-user target. Targets are
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special unit files that are analogous to runlevels under
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SysVinit.</para>
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</listitem>
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<listitem>
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<para><command>systemctl status <replaceable><servicename.service></replaceable></command>:
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shows the status of the servicename service. The .service extension
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can be omitted if there are no other unit files with the same name,
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such as .socket files (which create a listening socket that provides
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similar functionality to inetd/xinetd).</para>
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</listitem>
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</itemizedlist>
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</sect2>
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<sect2>
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<title>Working with the Systemd Journal</title>
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<para>Logging on a system booted with systemd is handled with
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systemd-journald (by default), rather than a typical unix syslog daemon.
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You can also add a normal syslog daemon and have both operate side by
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side if desired. The systemd-journald program stores journal entries in a
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binary format rather than a plain text log file. To assist with
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parsing the file, the command <command>journalctl</command> is provided.
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Here are some examples of frequently used commands:</para>
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<itemizedlist>
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<listitem>
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<para><command>journalctl -r</command>: shows all contents of the
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journal in reverse chronological order.</para>
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</listitem>
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<listitem>
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<para><command>journalctl -u <replaceable>UNIT</replaceable></command>:
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shows the journal entries associated with the specified UNIT
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file.</para>
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</listitem>
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<listitem>
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<para><command>journalctl -b[=ID] -r</command>: shows the journal
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entries since last successful boot (or for boot ID) in reverse
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chronological order.</para>
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</listitem>
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<listitem>
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<para><command>journalctl -f</command>: provides functionality similar
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to tail -f (follow).</para>
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</listitem>
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</itemizedlist>
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</sect2>
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<sect2>
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<title>Working with Core Dumps</title>
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<para>Core dumps are useful to debug crashed programs, especially
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when a daemon process crashes. On systemd booted systems the core
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dumping is handled by <command>systemd-coredump</command>. It will
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log the core dump in the journal and store the core dump itself in
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<filename class="directory">/var/lib/systemd/coredump</filename>.
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To retrieve and process core dumps, the <command>coredumpctl</command>
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tool is provided. Here are some examples of frequently used commands:
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</para>
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<itemizedlist>
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<listitem>
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<para><command>coredumpctl -r</command>: lists all core dumps in
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reverse chronological order.</para>
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</listitem>
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<listitem>
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<para><command>coredumpctl -1 info</command>: shows the information
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from the last core dump.</para>
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</listitem>
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<listitem>
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<para><command>coredumpctl -1 debug</command>: loads the last core
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dump into <ulink url="&blfs-book;general/gdb.html">GDB</ulink>.
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</para>
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</listitem>
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</itemizedlist>
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<para>Core dumps may use a lot of disk space. The maximum disk space
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used by core dumps can be limited by creating a configuration file in
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<filename class="directory">/etc/systemd/coredump.conf.d</filename>.
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For example:</para>
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<screen role="nodump"><userinput>mkdir -pv /etc/systemd/coredump.conf.d
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cat > /etc/systemd/coredump.conf.d/maxuse.conf << EOF
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<literal>[Coredump]
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MaxUse=5G</literal>
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EOF</userinput></screen>
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<para>See the <filename>systemd-coredump(8)</filename>,
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<filename>coredumpctl(1)</filename>, and
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<filename>coredump.conf.d(5)</filename> manual pages for more
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information.</para>
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</sect2>
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<sect2>
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<title>Long Running Processes</title>
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<para>Beginning with systemd-230, all user processes are killed when a user
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session is ended, even if nohup is used, or the process uses the
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<function>daemon()</function> or <function>setsid()</function> functions.
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This is a deliberate change from a historically permissive environment to a
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more restrictive one. The new behavior may cause issues if you depend on
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long running programs (e.g., <command>screen</command> or
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<command>tmux</command>) to remain active after ending your user session.
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There are three ways to enable lingering processes to remain after a user
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session is ended.</para>
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<itemizedlist>
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<listitem>
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<para>
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<emphasis>Enable process lingering for only selected users</emphasis>:
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Normal users have permission to enable process lingering
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with the command <command>loginctl enable-linger</command> for their
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own user. System administrators can use the same command with a
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<parameter>user</parameter> argument to enable for a user. That user
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can then use the <command>systemd-run</command> command to start
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long running processes. For example: <command>systemd-run --scope
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--user /usr/bin/screen</command>. If you enable lingering for your
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user, the user@.service will remain even after all login sessions are
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closed, and will automatically start at system boot. This has the
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advantage of explicitly allowing and disallowing processes to run
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after the user session has ended, but breaks backwards compatibility
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with tools like <command>nohup</command> and utilities that use
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<function>daemon()</function>.
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</para>
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</listitem>
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<listitem>
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<para>
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<emphasis>Enable system-wide process lingering</emphasis>:
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You can set <parameter>KillUserProcesses=no</parameter> in
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<filename>/etc/systemd/logind.conf</filename> to enable process lingering
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globally for all users. This has the benefit of leaving the old
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method available to all users at the expense of explicit control.
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</para>
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</listitem>
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<listitem>
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<para>
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<emphasis>Disable at build-time</emphasis>: You can disable
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lingering by default while building systemd by adding the switch
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<parameter>-Ddefault-kill-user-processes=false</parameter> to the
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<command>meson</command> command for systemd. This completely
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disables the ability of systemd to kill user processes at session
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end.
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</para>
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</listitem>
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</itemizedlist>
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</sect2>
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</sect1>
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