man/systemd.generator.xml

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  11   Copyright 2015 Zbigniew Jędrzejewski-Szmek
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  26
  27 <refentry id="systemd.generator">
  28   <refentryinfo>
  29     <title>systemd.generator</title>
  30     <productname>systemd</productname>
  31
  32     <authorgroup>
  33       <author>
  34         <contrib>Developer</contrib>
  35         <firstname>Lennart</firstname>
  36         <surname>Poettering</surname>
  37         <email>lennart@poettering.net</email>
  38       </author>
  39     </authorgroup>
  40   </refentryinfo>
  41
  42   <refmeta>
  43     <refentrytitle>systemd.generator</refentrytitle>
  44     <manvolnum>7</manvolnum>
  45   </refmeta>
  46
  47   <refnamediv>
  48     <refname>systemd.generator</refname>
  49     <refpurpose>Systemd unit generators</refpurpose>
  50   </refnamediv>
  51
  52   <refsynopsisdiv>
  53     <cmdsynopsis>
  54       <command>/path/to/generator</command>
  55       <arg choice="plain"><replaceable>normal-dir</replaceable></arg>
  56       <arg choice="plain"><replaceable>early-dir</replaceable></arg>
  57       <arg choice="plain"><replaceable>late-dir</replaceable></arg>
  58     </cmdsynopsis>
  59
  60     <para>
  61       <literallayout><filename>/run/systemd/system-generators/*</filename>
  62 <filename>/etc/systemd/system-generators/*</filename>
  63 <filename>/usr/local/lib/systemd/system-generators/*</filename>
  64 <filename>&systemgeneratordir;/*</filename></literallayout>
  65     </para>
  66
  67     <para>
  68       <literallayout><filename>/run/systemd/user-generators/*</filename>
  69 <filename>/etc/systemd/user-generators/*</filename>
  70 <filename>/usr/local/lib/systemd/user-generators/*</filename>
  71 <filename>&usergeneratordir;/*</filename></literallayout>
  72     </para>
  73   </refsynopsisdiv>
  74
  75   <refsect1>
  76     <title>Description</title>
  77     <para>Generators are small binaries that live in
  78     <filename>&usergeneratordir;/</filename> and other directories
  79     listed above.
  80     <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  81     will execute those binaries very early at bootup and at
  82     configuration reload time — before unit files are loaded.
  83     Generators can dynamically generate unit files or create symbolic
  84     links to unit files to add additional dependencies, thus extending
  85     or overriding existing definitions. Their main purpose is to
  86     convert configuration files that are not native unit files
  87     dynamically into native unit files.</para>
  88
  89     <para>Generators are loaded from a set of paths determined during
  90     compilation, as listed above. System and user generators are loaded
  91     from directories with names ending in
  92     <filename>system-generators/</filename> and
  93     <filename>user-generators/</filename>, respectively. Generators
  94     found in directories listed earlier override the ones with the
  95     same name in directories lower in the list. A symlink to
  96     <filename>/dev/null</filename> or an empty file can be used to
  97     mask a generator, thereby preventing it from running. Please note
  98     that the order of the two directories with the highest priority is
  99     reversed with respect to the unit load path, and generators in
 100     <filename>/run</filename> overwrite those in
 101     <filename>/etc</filename>.</para>
 102
 103     <para>After installing new generators or updating the
 104     configuration, <command>systemctl daemon-reload</command> may be
 105     executed. This will delete the previous configuration created by
 106     generators, re-run all generators, and cause
 107     <command>systemd</command> to reload units from disk. See
 108     <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 109     for more information.
 110     </para>
 111   </refsect1>
 112
 113   <refsect1>
 114     <title>Writing generators</title>
 115
 116     <para>Generators are invoked with three arguments: paths to
 117     runtime directories where generators can place their generated
 118     unit files or symlinks.</para>
 119
 120     <orderedlist>
 121       <listitem>
 122         <para><parameter>normal-dir</parameter></para>
 123         <para>argv[1] may be used to override unit files in
 124         <filename>/usr</filename>, but not those in
 125         <filename>/run</filename> or in <filename>/etc</filename>.
 126         This means that unit files placed
 127         in this directory take precedence over vendor unit
 128         configuration but not over native user/administrator unit
 129         configuration.</para>
 130       </listitem>
 131
 132       <listitem>
 133         <para><parameter>early-dir</parameter></para>
 134         <para>argv[2] may be used to override unit files in
 135         <filename>/usr</filename>, in <filename>/run</filename> and in
 136         <filename>/etc</filename>. This means that unit files placed
 137         in this directory take precedence over all configuration,
 138         both vendor and user/administrator.</para>
 139       </listitem>
 140
 141       <listitem>
 142         <para><parameter>late-dir</parameter></para>
 143         <para>argv[3] may be used to extend the unit file tree without
 144         overriding any other unit files. Any native configuration
 145         files supplied by the vendor or user/administrator take
 146         precedence over the generated ones placed in this directory.
 147         </para>
 148       </listitem>
 149     </orderedlist>
 150
 151     <refsect2>
 152       <title>Notes</title>
 153
 154       <itemizedlist>
 155         <listitem>
 156           <para>
 157             All generators are executed in parallel. That means all
 158             executables are started at the very same time and need to
 159             be able to cope with this parallelism.
 160           </para>
 161         </listitem>
 162
 163         <listitem>
 164           <para>
 165             Generators are run very early at boot and cannot rely on
 166             any external services. They may not talk to any other
 167             process. That includes simple things such as logging to
 168             <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 169             or <command>systemd</command> itself (this means: no
 170             <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>)!
 171             Non-essential file systems like
 172             <filename>/var</filename> and <filename>/home</filename>
 173             are mounted after generators have run. Generators
 174             can however rely on the most basic kernel functionality to be
 175             available, including a mounted <filename>/sys</filename>,
 176             <filename>/proc</filename>, <filename>/dev</filename>,
 177             <filename>/usr</filename>.
 178           </para>
 179         </listitem>
 180
 181         <listitem>
 182           <para>
 183             Units written by generators are removed when the configuration
 184             is reloaded. That means the lifetime of the generated
 185             units is closely bound to the reload cycles of
 186             <command>systemd</command> itself.
 187           </para>
 188         </listitem>
 189
 190         <listitem>
 191           <para>
 192             Generators should only be used to generate unit files, not
 193             any other kind of configuration. Due to the lifecycle
 194             logic mentioned above, generators are not a good fit to
 195             generate dynamic configuration for other services. If you
 196             need to generate dynamic configuration for other services,
 197             do so in normal services you order before the service in
 198             question.
 199           </para>
 200         </listitem>
 201
 202         <listitem>
 203           <para>
 204             Since
 205             <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 206             is not available (see above), log messages have to be
 207             written to <filename>/dev/kmsg</filename> instead.
 208           </para>
 209         </listitem>
 210
 211         <listitem>
 212           <para>
 213             It is a good idea to use the
 214             <varname>SourcePath=</varname> directive in generated unit
 215             files to specify the source configuration file you are
 216             generating the unit from. This makes things more easily
 217             understood by the user and also has the benefit that
 218             systemd can warn the user about configuration files that
 219             changed on disk but have not been read yet by systemd.
 220           </para>
 221         </listitem>
 222
 223         <listitem>
 224           <para>
 225             Generators may write out dynamic unit files or just hook
 226             unit files into other units with the usual
 227             <filename>.wants/</filename> or
 228             <filename>.requires/</filename> symlinks. Often, it is
 229             nicer to simply instantiate a template unit file from
 230             <filename>/usr</filename> with a generator instead of
 231             writing out entirely dynamic unit files. Of course, this
 232             works only if a single parameter is to be used.
 233           </para>
 234         </listitem>
 235
 236         <listitem>
 237           <para>
 238             If you are careful, you can implement generators in shell
 239             scripts. We do recommend C code however, since generators
 240             are executed synchronously and hence delay the
 241             entire boot if they are slow.
 242           </para>
 243         </listitem>
 244
 245         <listitem>
 246           <para>Regarding overriding semantics: there are two rules we
 247           try to follow when thinking about the overriding semantics:
 248           </para>
 249
 250           <orderedlist numeration="lowerroman">
 251             <listitem>
 252               <para>User configuration should override vendor
 253               configuration. This (mostly) means that stuff from
 254               <filename>/etc</filename> should override stuff from
 255               <filename>/usr</filename>.</para>
 256             </listitem>
 257
 258             <listitem>
 259               <para>Native configuration should override non-native
 260               configuration. This (mostly) means that stuff you
 261               generate should never override native unit files for the
 262               same purpose.</para>
 263             </listitem>
 264           </orderedlist>
 265
 266           <para>Of these two rules the first rule is probably the more
 267           important one and breaks the second one sometimes. Hence,
 268           when deciding whether to user argv[1], argv[2], or argv[3],
 269           your default choice should probably be argv[1].</para>
 270         </listitem>
 271
 272         <listitem>
 273           <para>
 274             Instead of heading off now and writing all kind of
 275             generators for legacy configuration file formats, please
 276             think twice! It is often a better idea to just deprecate
 277             old stuff instead of keeping it artificially alive.
 278           </para>
 279         </listitem>
 280       </itemizedlist>
 281     </refsect2>
 282   </refsect1>
 283
 284   <refsect1>
 285     <title>Examples</title>
 286     <example>
 287       <title>systemd-fstab-generator</title>
 288
 289       <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 290       converts <filename>/etc/fstab</filename> into native mount
 291       units. It uses argv[1] as location to place the generated unit
 292       files in order to allow the user to override
 293       <filename>/etc/fstab</filename> with her own native unit files,
 294       but also to ensure that <filename>/etc/fstab</filename>
 295       overrides any vendor default from <filename>/usr</filename>.
 296       </para>
 297
 298       <para>After editing <filename>/etc/fstab</filename>, the user
 299       should invoke <command>systemctl daemon-reload</command>. This
 300       will re-run all generators and cause <command>systemd</command>
 301       to reload units from disk. To actually mount new directories
 302       added to <filename>fstab</filename>, <command>systemctl start
 303       <replaceable>/path/to/mountpoint</replaceable></command> or
 304       <command>systemctl start local-fs.target</command> may be used.
 305       </para>
 306     </example>
 307
 308     <example>
 309       <title>systemd-system-update-generator</title>
 310
 311       <para><citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 312       temporarily redirects <filename>default.target</filename> to
 313       <filename>system-update.target</filename>, if a system update is
 314       scheduled. Since this needs to override the default user
 315       configuration for <filename>default.target</filename>, it uses
 316       argv[2]. For details about this logic, see
 317       <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 318       </para>
 319     </example>
 320
 321     <example>
 322       <title>Debugging a generator</title>
 323
 324       <programlisting>dir=$(mktemp -d)
 325 SYSTEMD_LOG_LEVEL=debug &systemgeneratordir;/systemd-fstab-generator \
 326         "$dir" "$dir" "$dir"
 327 find $dir</programlisting>
 328     </example>
 329   </refsect1>
 330
 331   <refsect1>
 332     <title>See also</title>
 333
 334     <para>
 335       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 336       <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 337       <citerefentry><refentrytitle>systemd-debug-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 338       <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 339       <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 340       <citerefentry><refentrytitle>systemd-getty-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 341       <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 342       <citerefentry><refentrytitle>systemd-hibernate-resume-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 343       <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 344       <citerefentry><refentrytitle>systemd-sysv-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 345       <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 346       <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 347       <citerefentry><refentrytitle>systemd.environment-generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 348     </para>
 349   </refsect1>
 350 </refentry>