man/systemd.generator.xml

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   8
   9 <refentry id="systemd.generator">
  10   <refentryinfo>
  11     <title>systemd.generator</title>
  12     <productname>systemd</productname>
  13   </refentryinfo>
  14
  15   <refmeta>
  16     <refentrytitle>systemd.generator</refentrytitle>
  17     <manvolnum>7</manvolnum>
  18   </refmeta>
  19
  20   <refnamediv>
  21     <refname>systemd.generator</refname>
  22     <refpurpose>systemd unit generators</refpurpose>
  23   </refnamediv>
  24
  25   <refsynopsisdiv>
  26     <cmdsynopsis>
  27       <command index='false'>/path/to/generator</command>
  28       <arg choice="plain"><replaceable>normal-dir</replaceable></arg>
  29       <arg choice="option"><replaceable>early-dir</replaceable></arg>
  30       <arg choice="option"><replaceable>late-dir</replaceable></arg>
  31     </cmdsynopsis>
  32
  33     <para>
  34       <literallayout><filename>/run/systemd/system-generators/*</filename>
  35 <filename>/etc/systemd/system-generators/*</filename>
  36 <filename>/usr/local/lib/systemd/system-generators/*</filename>
  37 <filename>&SYSTEM_GENERATOR_DIR;/*</filename></literallayout>
  38     </para>
  39
  40     <para>
  41       <literallayout><filename>/run/systemd/user-generators/*</filename>
  42 <filename>/etc/systemd/user-generators/*</filename>
  43 <filename>/usr/local/lib/systemd/user-generators/*</filename>
  44 <filename>&USER_GENERATOR_DIR;/*</filename></literallayout>
  45     </para>
  46   </refsynopsisdiv>
  47
  48   <refsect1>
  49     <title>Description</title>
  50     <para>Generators are small executables placed in <filename>&SYSTEM_GENERATOR_DIR;/</filename> and other
  51     directories listed above.
  52     <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will execute
  53     these binaries very early at bootup and at configuration reload time — before unit files are
  54     loaded. Their main purpose is to convert configuration and execution context parameters that are not
  55     native to the service manager into dynamically generated unit files, symlinks or unit file drop-ins, so
  56     that they can extend the unit file hierarchy the service manager subsequently loads and operates
  57     on.</para>
  58
  59     <para><command>systemd</command> will call each generator with three directory paths that are to be used
  60     for generator output. In these three directories, generators may dynamically generate unit files (regular
  61     ones, instances, as well as templates), unit file <filename>.d/</filename> drop-ins, and create symbolic
  62     links to unit files to add additional dependencies, create aliases, or instantiate existing templates.
  63     Those directories are included in the unit load path, allowing generated configuration to extend or
  64     override existing definitions. For tests, generators may be called with just one argument; the generator
  65     should assume that all three paths are the same in that case.</para>
  66
  67     <para>Directory paths for generator output differ by priority: <filename>…/generator.early</filename> has
  68     priority higher than the admin configuration in <filename>/etc/</filename>, while
  69     <filename>…/generator</filename> has lower priority than <filename>/etc/</filename> but higher than
  70     vendor configuration in <filename>/usr/</filename>, and <filename>…/generator.late</filename> has
  71     priority lower than all other configuration. See the next section and the discussion of unit load paths
  72     and unit overriding in
  73     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
  74     </para>
  75
  76     <para>Generators are loaded from a set of paths determined during compilation, as listed above. System
  77     and user generators are loaded from directories with names ending in
  78     <filename>system-generators/</filename> and <filename>user-generators/</filename>,
  79     respectively. Generators found in directories listed earlier override the ones with the same name in
  80     directories lower in the list. A symlink to <filename>/dev/null</filename> or an empty file can be used
  81     to mask a generator, thereby preventing it from running. Please note that the order of the two
  82     directories with the highest priority is reversed with respect to the unit load path, and generators in
  83     <filename>/run/</filename> overwrite those in <filename>/etc/</filename>.</para>
  84
  85     <para>After installing new generators or updating the configuration, <command>systemctl
  86     daemon-reload</command> may be executed. This will delete the previous configuration created by
  87     generators, re-run all generators, and cause <command>systemd</command> to reload units from disk. See
  88     <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for more
  89     information.
  90     </para>
  91   </refsect1>
  92
  93   <refsect1>
  94     <title>Output directories</title>
  95
  96     <para>Generators are invoked with three arguments: paths to directories where generators can place their
  97     generated unit files or symlinks. By default those paths are runtime directories that are included in the
  98     search path of <command>systemd</command>, but a generator may be called with different paths for
  99     debugging purposes. If only one argument is provided, the generator should use the same directory as the
 100     the three output paths.</para>
 101
 102     <orderedlist>
 103       <listitem>
 104         <para><parameter>normal-dir</parameter></para>
 105         <para>In normal use this is <filename>/run/systemd/generator</filename> in case of the system
 106         generators and <filename>$XDG_RUNTIME_DIR/generator</filename> in case of the user generators. Unit
 107         files placed in this directory take precedence over vendor unit configuration but not over native
 108         user/administrator unit configuration.
 109         </para>
 110       </listitem>
 111
 112       <listitem>
 113         <para><parameter>early-dir</parameter></para>
 114         <para>In normal use this is <filename>/run/systemd/generator.early</filename> in case of the system
 115         generators and <filename>$XDG_RUNTIME_DIR/generator.early</filename> in case of the user
 116         generators. Unit files placed in this directory override unit files in <filename>/usr/</filename>,
 117         <filename>/run/</filename> and <filename>/etc/</filename>. This means that unit files placed in this
 118         directory take precedence over all normal configuration, both vendor and user/administrator.</para>
 119       </listitem>
 120
 121       <listitem>
 122         <para><parameter>late-dir</parameter></para>
 123         <para>In normal use this is <filename>/run/systemd/generator.late</filename> in case of the system
 124         generators and <filename>$XDG_RUNTIME_DIR/generator.late</filename> in case of the user
 125         generators. This directory may be used to extend the unit file tree without overriding any other unit
 126         files. Any native configuration files supplied by the vendor or user/administrator take
 127         precedence.</para>
 128       </listitem>
 129     </orderedlist>
 130   </refsect1>
 131
 132   <refsect1>
 133     <title>Environment</title>
 134
 135     <para>The service manager sets a number of environment variables when invoking generator
 136     executables. They carry information about the execution context of the generator, in order to simplify
 137     conditionalizing generators to specific environments. The following environment variables are set:</para>
 138
 139     <variablelist class='environment-variables'>
 140       <varlistentry>
 141         <term><varname>$SYSTEMD_SCOPE</varname></term>
 142
 143         <listitem><para>If the generator is invoked from the system service manager this variable is set to
 144         <literal>system</literal>; if invoked from the per-user service manager it is set to
 145         <literal>user</literal>.</para></listitem>
 146       </varlistentry>
 147
 148       <varlistentry>
 149         <term><varname>$SYSTEMD_IN_INITRD</varname></term>
 150
 151         <listitem><para>If the generator is run as part of an initial RAM file system (initrd) this is set to
 152         <literal>1</literal>. If it is run from the regular host (i.e. after the transition from initrd to
 153         host) it is set to <literal>0</literal>. This environment variable is only set for system
 154         generators.</para></listitem>
 155       </varlistentry>
 156
 157       <varlistentry>
 158         <term><varname>$SYSTEMD_FIRST_BOOT</varname></term>
 159
 160         <listitem><para>If this boot-up cycle is considered a "first boot", this is set to
 161         <literal>1</literal>; if it is a subsequent, regular boot it is set to <literal>0</literal>. For
 162         details see the documentation of <varname>ConditionFirstBoot=</varname> in
 163         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>. This
 164         environment variable is only set for system generators.</para></listitem>
 165       </varlistentry>
 166
 167       <varlistentry>
 168         <term><varname>$SYSTEMD_VIRTUALIZATION</varname></term>
 169
 170         <listitem><para>If the service manager is run in a virtualized environment,
 171         <varname>$SYSTEMD_VIRTUALIZATION</varname> is set to a pair of strings, separated by a colon. The
 172         first string is either <literal>vm</literal> or <literal>container</literal>, categorizing the type
 173         of virtualization. The second string identifies the implementation of the virtualization
 174         technology. If no virtualization is detected this variable will not be set. This data is identical to
 175         what
 176         <citerefentry><refentrytitle>systemd-detect-virt</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 177         detects and reports, and uses the same vocabulary of virtualization implementation
 178         identifiers.</para></listitem>
 179       </varlistentry>
 180
 181       <varlistentry>
 182         <term><varname>$SYSTEMD_ARCHITECTURE</varname></term>
 183
 184         <listitem><para>This variable is set to a short identifier of the reported architecture of the
 185         system. For details about defined values, see documentation of
 186         <varname>ConditionArchitecture=</varname> in
 187         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 188       </varlistentry>
 189     </variablelist>
 190   </refsect1>
 191
 192   <refsect1>
 193     <title>Notes about writing generators</title>
 194
 195     <itemizedlist>
 196       <listitem>
 197         <para>All generators are executed in parallel. That means all executables are started at the very
 198         same time and need to be able to cope with this parallelism.
 199         </para>
 200       </listitem>
 201
 202       <listitem>
 203         <para>Generators are run very early at boot and cannot rely on any external services. They may not
 204         talk to any other process. That includes simple things such as logging to <citerefentry
 205         project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>, or
 206         <command>systemd</command> itself (this means: no
 207         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>)!
 208         Non-essential file systems like <filename>/var/</filename> and <filename>/home/</filename> are
 209         mounted after generators have run. Generators can however rely on the most basic kernel functionality
 210         to be available, as well as mounted <filename>/sys/</filename>, <filename>/proc/</filename>,
 211         <filename>/dev/</filename>, <filename>/usr/</filename> and <filename>/run/</filename> file systems.
 212         </para>
 213       </listitem>
 214
 215       <listitem>
 216         <para>Units written by generators are removed when the configuration is reloaded. That means the
 217         lifetime of the generated units is closely bound to the reload cycles of <command>systemd</command>
 218         itself.</para>
 219       </listitem>
 220
 221       <listitem>
 222         <para>Generators should only be used to generate unit files, <filename>.d/*.conf</filename> drop-ins
 223         for them and symlinks to them, not any other kind of non-unit related configuration. Due to the
 224         lifecycle logic mentioned above, generators are not a good fit to generate dynamic configuration for
 225         other services. If you need to generate dynamic configuration for other services, do so in normal
 226         services you order before the service in question.</para>
 227
 228         <para>Note that using the <varname>StandardInputData=</varname>/<varname>StandardInputText=</varname>
 229         settings of service unit files (see
 230         <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>), it
 231         is possible to make arbitrary input data (including daemon-specific configuration) part of the unit
 232         definitions, which often might be sufficient to embed data or configuration for other programs into
 233         unit files in a native fashion.</para>
 234       </listitem>
 235
 236       <listitem>
 237         <para>Since
 238         <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 239
 240         is not available (see above), log messages have to be written to <filename>/dev/kmsg</filename>
 241         instead.</para>
 242       </listitem>
 243
 244       <listitem>
 245         <para>The generator should always include its own name in a comment at the top of the generated file,
 246         so that the user can easily figure out which component created or amended a particular unit.</para>
 247
 248         <para>The <varname>SourcePath=</varname> directive should be used in generated files to specify the
 249         source configuration file they are generated from. This makes things more easily understood by the
 250         user and also has the benefit that systemd can warn the user about configuration files that changed
 251         on disk but have not been read yet by systemd. The <varname>SourcePath=</varname> value does not have
 252         to be a file in a physical filesystem. For example, in the common case of the generator looking at
 253         the kernel command line, <option>SourcePath=/proc/cmdline</option> should be used.</para>
 254       </listitem>
 255
 256       <listitem>
 257         <para>Generators may write out dynamic unit files or just hook unit files into other units with the
 258         usual <filename>.wants/</filename> or <filename>.requires/</filename> symlinks. Often, it is nicer to
 259         simply instantiate a template unit file from <filename>/usr/</filename> with a generator instead of
 260         writing out entirely dynamic unit files. Of course, this works only if a single parameter is to be
 261         used.</para>
 262       </listitem>
 263
 264       <listitem>
 265         <para>If you are careful, you can implement generators in shell scripts. We do recommend C code
 266         however, since generators are executed synchronously and hence delay the entire boot if they are
 267         slow.</para>
 268       </listitem>
 269
 270       <listitem>
 271         <para>Regarding overriding semantics: there are two rules we try to follow when thinking about the
 272         overriding semantics:</para>
 273
 274         <orderedlist numeration="lowerroman">
 275           <listitem>
 276             <para>User configuration should override vendor configuration. This (mostly) means that stuff
 277             from <filename>/etc/</filename> should override stuff from <filename>/usr/</filename>.</para>
 278           </listitem>
 279
 280           <listitem>
 281             <para>Native configuration should override non-native configuration. This (mostly) means that
 282             stuff you generate should never override native unit files for the same purpose.</para>
 283           </listitem>
 284         </orderedlist>
 285
 286         <para>Of these two rules the first rule is probably the more important one and breaks the second one
 287         sometimes. Hence, when deciding whether to use argv[1], argv[2], or argv[3], your default choice
 288         should probably be argv[1].</para>
 289       </listitem>
 290
 291       <listitem>
 292         <para>Instead of heading off now and writing all kind of generators for legacy configuration file
 293         formats, please think twice! It is often a better idea to just deprecate old stuff instead of keeping
 294         it artificially alive.
 295         </para>
 296       </listitem>
 297     </itemizedlist>
 298   </refsect1>
 299
 300   <refsect1>
 301     <title>Examples</title>
 302     <example>
 303       <title>systemd-fstab-generator</title>
 304
 305       <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 306       converts <filename>/etc/fstab</filename> into native mount units. It uses argv[1] as location to place
 307       the generated unit files in order to allow the user to override <filename>/etc/fstab</filename> with
 308       their own native unit files, but also to ensure that <filename>/etc/fstab</filename> overrides any
 309       vendor default from <filename>/usr/</filename>.</para>
 310
 311       <para>After editing <filename>/etc/fstab</filename>, the user should invoke <command>systemctl
 312       daemon-reload</command>. This will re-run all generators and cause <command>systemd</command> to reload
 313       units from disk. To actually mount new directories added to <filename>fstab</filename>,
 314       <command>systemctl start <replaceable>/path/to/mountpoint</replaceable></command> or <command>systemctl
 315       start local-fs.target</command> may be used.</para>
 316     </example>
 317
 318     <example>
 319       <title>systemd-system-update-generator</title>
 320
 321       <para><citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 322       temporarily redirects <filename>default.target</filename> to <filename>system-update.target</filename>,
 323       if a system update is scheduled. Since this needs to override the default user configuration for
 324       <filename>default.target</filename>, it uses argv[2]. For details about this logic, see
 325       <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 326       </para>
 327     </example>
 328
 329     <example>
 330       <title>Debugging a generator</title>
 331
 332       <programlisting>dir=$(mktemp -d)
 333 SYSTEMD_LOG_LEVEL=debug &SYSTEM_GENERATOR_DIR;/systemd-fstab-generator \
 334         "$dir" "$dir" "$dir"
 335 find $dir</programlisting>
 336     </example>
 337   </refsect1>
 338
 339   <refsect1>
 340     <title>See also</title>
 341
 342     <para>
 343       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 344       <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 345       <citerefentry><refentrytitle>systemd-debug-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 346       <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 347       <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 348       <citerefentry><refentrytitle>systemd-getty-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 349       <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 350       <citerefentry><refentrytitle>systemd-hibernate-resume-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 351       <citerefentry><refentrytitle>systemd-rc-local-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 352       <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 353       <citerefentry><refentrytitle>systemd-sysv-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 354       <citerefentry><refentrytitle>systemd-xdg-autostart-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 355       <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 356       <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 357       <citerefentry><refentrytitle>systemd.environment-generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 358     </para>
 359   </refsect1>
 360 </refentry>