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