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