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     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>$XDG_RUNTIME_DIR/systemd/generator</filename> in case of the user
 107         generators. Unit files placed in this directory take precedence over vendor unit configuration but
 108         not over native user/administrator unit configuration.
 109         </para>
 110       </listitem>
 111
 112       <listitem>
 113         <para><parameter>early-dir</parameter></para>
 114         <para>In normal use this is <filename>/run/systemd/generator.early</filename> in case of the system
 115         generators and <filename>$XDG_RUNTIME_DIR/systemd/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>$XDG_RUNTIME_DIR/systemd/generator.late</filename> in case of the user
 125         generators. This directory may be used to extend the unit file tree without overriding any other unit
 126         files. Any native configuration files supplied by the vendor or user/administrator take
 127         precedence.</para>
 128       </listitem>
 129     </orderedlist>
 130
 131     <para>Note: generators <emphasis>must not</emphasis> write to other locations or otherwise make changes
 132     to system state. Generator output is supposed to last only until the next
 133     <command>daemon-reload</command> or <command>daemon-reexec</command>; if the generator is replaced
 134     or masked, its effects should vanish.</para>
 135   </refsect1>
 136
 137   <refsect1>
 138     <title>Environment</title>
 139
 140     <para>The service manager sets a number of environment variables when invoking generator
 141     executables. They carry information about the execution context of the generator, in order to simplify
 142     conditionalizing generators to specific environments. The following environment variables are set:</para>
 143
 144     <variablelist class='environment-variables'>
 145       <varlistentry>
 146         <term><varname>$SYSTEMD_SCOPE</varname></term>
 147
 148         <listitem><para>If the generator is invoked from the system service manager this variable is set to
 149         <literal>system</literal>; if invoked from the per-user service manager it is set to
 150         <literal>user</literal>.</para></listitem>
 151       </varlistentry>
 152
 153       <varlistentry>
 154         <term><varname>$SYSTEMD_IN_INITRD</varname></term>
 155
 156         <listitem><para>If the generator is run as part of an initrd this is set to <literal>1</literal>. If
 157         it is run from the regular host (i.e. after the transition from initrd to host) it is set to
 158         <literal>0</literal>. This environment variable is only set for system generators.</para></listitem>
 159       </varlistentry>
 160
 161       <varlistentry>
 162         <term><varname>$SYSTEMD_FIRST_BOOT</varname></term>
 163
 164         <listitem><para>If this boot-up cycle is considered a "first boot", this is set to
 165         <literal>1</literal>; if it is a subsequent, regular boot it is set to <literal>0</literal>. For
 166         details see the documentation of <varname>ConditionFirstBoot=</varname> in
 167         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>. This
 168         environment variable is only set for system generators.</para></listitem>
 169       </varlistentry>
 170
 171       <varlistentry>
 172         <term><varname>$SYSTEMD_VIRTUALIZATION</varname></term>
 173
 174         <listitem><para>If the service manager is run in a virtualized environment,
 175         <varname>$SYSTEMD_VIRTUALIZATION</varname> is set to a pair of strings, separated by a colon. The
 176         first string is either <literal>vm</literal> or <literal>container</literal>, categorizing the type
 177         of virtualization. The second string identifies the implementation of the virtualization
 178         technology. If no virtualization is detected this variable will not be set. This data is identical to
 179         what
 180         <citerefentry><refentrytitle>systemd-detect-virt</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 181         detects and reports, and uses the same vocabulary of virtualization implementation
 182         identifiers.</para></listitem>
 183       </varlistentry>
 184
 185       <varlistentry>
 186         <term><varname>$SYSTEMD_ARCHITECTURE</varname></term>
 187
 188         <listitem><para>This variable is set to a short identifier of the reported architecture of the
 189         system. For details about defined values, see documentation of
 190         <varname>ConditionArchitecture=</varname> in
 191         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 192       </varlistentry>
 193     </variablelist>
 194   </refsect1>
 195
 196   <refsect1>
 197     <title>Notes about writing generators</title>
 198
 199     <itemizedlist>
 200       <listitem>
 201         <para>All generators are executed in parallel. That means all executables are started at the very
 202         same time and need to be able to cope with this parallelism.
 203         </para>
 204       </listitem>
 205
 206       <listitem>
 207         <para>Generators are run very early at boot and cannot rely on any external services. They may not
 208         talk to any other process. That includes simple things such as logging to <citerefentry
 209         project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>, or
 210         <command>systemd</command> itself (this means: no
 211         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>)!
 212         Non-essential file systems like <filename>/var/</filename> and <filename>/home/</filename> are
 213         mounted after generators have run. Generators can however rely on the most basic kernel functionality
 214         to be available, as well as mounted <filename>/sys/</filename>, <filename>/proc/</filename>,
 215         <filename>/dev/</filename>, <filename>/usr/</filename> and <filename>/run/</filename> file systems.
 216         </para>
 217       </listitem>
 218
 219       <listitem>
 220         <para>Units written by generators are removed when the configuration is reloaded. That means the
 221         lifetime of the generated units is closely bound to the reload cycles of <command>systemd</command>
 222         itself.</para>
 223       </listitem>
 224
 225       <listitem>
 226         <para>Generators should only be used to generate unit files, <filename>.d/*.conf</filename> drop-ins
 227         for them and symlinks to them, not any other kind of non-unit related configuration. Due to the
 228         lifecycle logic mentioned above, generators are not a good fit to generate dynamic configuration for
 229         other services. If you need to generate dynamic configuration for other services, do so in normal
 230         services you order before the service in question.</para>
 231
 232         <para>Note that using the <varname>StandardInputData=</varname>/<varname>StandardInputText=</varname>
 233         settings of service unit files (see
 234         <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>), it
 235         is possible to make arbitrary input data (including daemon-specific configuration) part of the unit
 236         definitions, which often might be sufficient to embed data or configuration for other programs into
 237         unit files in a native fashion.</para>
 238       </listitem>
 239
 240       <listitem>
 241         <para>Since
 242         <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 243
 244         is not available (see above), log messages have to be written to <filename>/dev/kmsg</filename>
 245         instead.</para>
 246       </listitem>
 247
 248       <listitem>
 249         <para>The generator should always include its own name in a comment at the top of the generated file,
 250         so that the user can easily figure out which component created or amended a particular unit.</para>
 251
 252         <para>The <varname>SourcePath=</varname> directive should be used in generated files to specify the
 253         source configuration file they are generated from. This makes things more easily understood by the
 254         user and also has the benefit that systemd can warn the user about configuration files that changed
 255         on disk but have not been read yet by systemd. The <varname>SourcePath=</varname> value does not have
 256         to be a file in a physical filesystem. For example, in the common case of the generator looking at
 257         the kernel command line, <option>SourcePath=/proc/cmdline</option> should be used.</para>
 258       </listitem>
 259
 260       <listitem>
 261         <para>Generators may write out dynamic unit files or just hook unit files into other units with the
 262         usual <filename>.wants/</filename> or <filename>.requires/</filename> symlinks. Often, it is nicer to
 263         simply instantiate a template unit file from <filename>/usr/</filename> with a generator instead of
 264         writing out entirely dynamic unit files. Of course, this works only if a single parameter is to be
 265         used.</para>
 266       </listitem>
 267
 268       <listitem>
 269         <para>If you are careful, you can implement generators in shell scripts. We do recommend C code
 270         however, since generators are executed synchronously and hence delay the entire boot if they are
 271         slow.</para>
 272       </listitem>
 273
 274       <listitem>
 275         <para>Regarding overriding semantics: there are two rules we try to follow when thinking about the
 276         overriding semantics:</para>
 277
 278         <orderedlist numeration="lowerroman">
 279           <listitem>
 280             <para>User configuration should override vendor configuration. This (mostly) means that stuff
 281             from <filename>/etc/</filename> should override stuff from <filename>/usr/</filename>.</para>
 282           </listitem>
 283
 284           <listitem>
 285             <para>Native configuration should override non-native configuration. This (mostly) means that
 286             stuff you generate should never override native unit files for the same purpose.</para>
 287           </listitem>
 288         </orderedlist>
 289
 290         <para>Of these two rules the first rule is probably the more important one and breaks the second one
 291         sometimes. Hence, when deciding whether to use argv[1], argv[2], or argv[3], your default choice
 292         should probably be argv[1].</para>
 293       </listitem>
 294
 295       <listitem>
 296         <para>Instead of heading off now and writing all kind of generators for legacy configuration file
 297         formats, please think twice! It is often a better idea to just deprecate old stuff instead of keeping
 298         it artificially alive.
 299         </para>
 300       </listitem>
 301     </itemizedlist>
 302   </refsect1>
 303
 304   <refsect1>
 305     <title>Examples</title>
 306     <example>
 307       <title>systemd-fstab-generator</title>
 308
 309       <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 310       converts <filename>/etc/fstab</filename> into native mount units. It uses argv[1] as location to place
 311       the generated unit files in order to allow the user to override <filename>/etc/fstab</filename> with
 312       their own native unit files, but also to ensure that <filename>/etc/fstab</filename> overrides any
 313       vendor default from <filename>/usr/</filename>.</para>
 314
 315       <para>After editing <filename>/etc/fstab</filename>, the user should invoke <command>systemctl
 316       daemon-reload</command>. This will re-run all generators and cause <command>systemd</command> to reload
 317       units from disk. To actually mount new directories added to <filename>fstab</filename>,
 318       <command>systemctl start <replaceable>/path/to/mountpoint</replaceable></command> or <command>systemctl
 319       start local-fs.target</command> may be used.</para>
 320     </example>
 321
 322     <example>
 323       <title>systemd-system-update-generator</title>
 324
 325       <para><citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 326       temporarily redirects <filename>default.target</filename> to <filename>system-update.target</filename>,
 327       if a system update is scheduled. Since this needs to override the default user configuration for
 328       <filename>default.target</filename>, it uses argv[2]. For details about this logic, see
 329       <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 330       </para>
 331     </example>
 332
 333     <example>
 334       <title>Debugging a generator</title>
 335
 336       <programlisting>dir=$(mktemp -d)
 337 SYSTEMD_LOG_LEVEL=debug &SYSTEM_GENERATOR_DIR;/systemd-fstab-generator \
 338         "$dir" "$dir" "$dir"
 339 find $dir</programlisting>
 340     </example>
 341   </refsect1>
 342
 343   <refsect1>
 344     <title>See also</title>
 345
 346     <para>
 347       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 348       <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 349       <citerefentry><refentrytitle>systemd-debug-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 350       <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 351       <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 352       <citerefentry><refentrytitle>systemd-getty-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 353       <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 354       <citerefentry><refentrytitle>systemd-hibernate-resume-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 355       <citerefentry><refentrytitle>systemd-rc-local-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 356       <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 357       <citerefentry><refentrytitle>systemd-sysv-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 358       <citerefentry><refentrytitle>systemd-xdg-autostart-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 359       <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 360       <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 361       <citerefentry><refentrytitle>systemd.environment-generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 362     </para>
 363   </refsect1>
 364 </refentry>