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>/etc</filename>. This means that unit files placed
 126         in this directory take precedence over vendor unit
 127         configuration but not over native user/administrator unit
 128         configuration.</para>
 129       </listitem>
 130
 131       <listitem>
 132         <para><parameter>early-dir</parameter></para>
 133         <para>argv[2] may be used to override unit files in
 134         <filename>/usr</filename> and in
 135         <filename>/etc</filename>. This means that unit files placed
 136         in this directory take precedence over all configuration,
 137         both vendor and user/administrator.</para>
 138       </listitem>
 139
 140       <listitem>
 141         <para><parameter>late-dir</parameter></para>
 142         <para>argv[3] may be used to extend the unit file tree without
 143         overriding any other unit files. Any native configuration
 144         files supplied by the vendor or user/administrator take
 145         precedence over the generated ones placed in this directory.
 146         </para>
 147       </listitem>
 148     </orderedlist>
 149
 150     <refsect2>
 151       <title>Notes</title>
 152
 153       <itemizedlist>
 154         <listitem>
 155           <para>
 156             All generators are executed in parallel. That means all
 157             executables are started at the very same time and need to
 158             be able to cope with this parallelism.
 159           </para>
 160         </listitem>
 161
 162         <listitem>
 163           <para>
 164             Generators are run very early at boot and cannot rely on
 165             any external services. They may not talk to any other
 166             process. That includes simple things such as logging to
 167             <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 168             or <command>systemd</command> itself (this means: no
 169             <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>)!
 170             Non-essential file systems like
 171             <filename>/var</filename> and <filename>/home</filename>
 172             are mounted after generators have run. Generators
 173             can however rely on the most basic kernel functionality to be
 174             available, including a mounted <filename>/sys</filename>,
 175             <filename>/proc</filename>, <filename>/dev</filename>,
 176             <filename>/usr</filename>.
 177           </para>
 178         </listitem>
 179
 180         <listitem>
 181           <para>
 182             Units written by generators are removed when the configuration
 183             is reloaded. That means the lifetime of the generated
 184             units is closely bound to the reload cycles of
 185             <command>systemd</command> itself.
 186           </para>
 187         </listitem>
 188
 189         <listitem>
 190           <para>
 191             Generators should only be used to generate unit files, not
 192             any other kind of configuration. Due to the lifecycle
 193             logic mentioned above, generators are not a good fit to
 194             generate dynamic configuration for other services. If you
 195             need to generate dynamic configuration for other services,
 196             do so in normal services you order before the service in
 197             question.
 198           </para>
 199         </listitem>
 200
 201         <listitem>
 202           <para>
 203             Since
 204             <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 205             is not available (see above), log messages have to be
 206             written to <filename>/dev/kmsg</filename> instead.
 207           </para>
 208         </listitem>
 209
 210         <listitem>
 211           <para>
 212             It is a good idea to use the
 213             <varname>SourcePath=</varname> directive in generated unit
 214             files to specify the source configuration file you are
 215             generating the unit from. This makes things more easily
 216             understood by the user and also has the benefit that
 217             systemd can warn the user about configuration files that
 218             changed on disk but have not been read yet by systemd.
 219           </para>
 220         </listitem>
 221
 222         <listitem>
 223           <para>
 224             Generators may write out dynamic unit files or just hook
 225             unit files into other units with the usual
 226             <filename>.wants/</filename> or
 227             <filename>.requires/</filename> symlinks. Often, it is
 228             nicer to simply instantiate a template unit file from
 229             <filename>/usr</filename> with a generator instead of
 230             writing out entirely dynamic unit files. Of course, this
 231             works only if a single parameter is to be used.
 232           </para>
 233         </listitem>
 234
 235         <listitem>
 236           <para>
 237             If you are careful, you can implement generators in shell
 238             scripts. We do recommend C code however, since generators
 239             are executed synchronously and hence delay the
 240             entire boot if they are slow.
 241           </para>
 242         </listitem>
 243
 244         <listitem>
 245           <para>Regarding overriding semantics: there are two rules we
 246           try to follow when thinking about the overriding semantics:
 247           </para>
 248
 249           <orderedlist numeration="lowerroman">
 250             <listitem>
 251               <para>User configuration should override vendor
 252               configuration. This (mostly) means that stuff from
 253               <filename>/etc</filename> should override stuff from
 254               <filename>/usr</filename>.</para>
 255             </listitem>
 256
 257             <listitem>
 258               <para>Native configuration should override non-native
 259               configuration. This (mostly) means that stuff you
 260               generate should never override native unit files for the
 261               same purpose.</para>
 262             </listitem>
 263           </orderedlist>
 264
 265           <para>Of these two rules the first rule is probably the more
 266           important one and breaks the second one sometimes. Hence,
 267           when deciding whether to user argv[1], argv[2], or argv[3],
 268           your default choice should probably be argv[1].</para>
 269         </listitem>
 270
 271         <listitem>
 272           <para>
 273             Instead of heading off now and writing all kind of
 274             generators for legacy configuration file formats, please
 275             think twice! It is often a better idea to just deprecate
 276             old stuff instead of keeping it artificially alive.
 277           </para>
 278         </listitem>
 279       </itemizedlist>
 280     </refsect2>
 281   </refsect1>
 282
 283   <refsect1>
 284     <title>Examples</title>
 285     <example>
 286       <title>systemd-fstab-generator</title>
 287
 288       <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 289       converts <filename>/etc/fstab</filename> into native mount
 290       units. It uses argv[1] as location to place the generated unit
 291       files in order to allow the user to override
 292       <filename>/etc/fstab</filename> with her own native unit files,
 293       but also to ensure that <filename>/etc/fstab</filename>
 294       overrides any vendor default from <filename>/usr</filename>.
 295       </para>
 296
 297       <para>After editing <filename>/etc/fstab</filename>, the user
 298       should invoke <command>systemctl daemon-reload</command>. This
 299       will re-run all generators and cause <command>systemd</command>
 300       to reload units from disk. To actually mount new directories
 301       added to <filename>fstab</filename>, <command>systemctl start
 302       <replaceable>/path/to/mountpoint</replaceable></command> or
 303       <command>systemctl start local-fs.target</command> may be used.
 304       </para>
 305     </example>
 306
 307     <example>
 308       <title>systemd-system-update-generator</title>
 309
 310       <para><citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 311       temporarily redirects <filename>default.target</filename> to
 312       <filename>system-update.target</filename>, if a system update is
 313       scheduled. Since this needs to override the default user
 314       configuration for <filename>default.target</filename>, it uses
 315       argv[2]. For details about this logic, see
 316       <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 317       </para>
 318     </example>
 319
 320     <example>
 321       <title>Debugging a generator</title>
 322
 323       <programlisting>dir=$(mktemp -d)
 324 SYSTEMD_LOG_LEVEL=debug &systemgeneratordir;/systemd-fstab-generator \
 325         "$dir" "$dir" "$dir"
 326 find $dir</programlisting>
 327     </example>
 328   </refsect1>
 329
 330   <refsect1>
 331     <title>See also</title>
 332
 333     <para>
 334       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 335       <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 336       <citerefentry><refentrytitle>systemd-debug-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 337       <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 338       <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 339       <citerefentry><refentrytitle>systemd-getty-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 340       <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 341       <citerefentry><refentrytitle>systemd-hibernate-resume-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 342       <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 343       <citerefentry><refentrytitle>systemd-sysv-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 344       <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 345       <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 346       <citerefentry><refentrytitle>systemd.environment-generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 347     </para>
 348   </refsect1>
 349 </refentry>