man/systemd.generator.xml

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   8
   9 <refentry id="systemd.generator" xmlns:xi="http://www.w3.org/2001/XInclude">
  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><simplelist>
  34       <member><filename>/run/systemd/system-generators/*</filename></member>
  35       <member><filename>/etc/systemd/system-generators/*</filename></member>
  36       <member><filename>/usr/local/lib/systemd/system-generators/*</filename></member>
  37       <member><filename>&SYSTEM_GENERATOR_DIR;/*</filename></member>
  38     </simplelist></para>
  39
  40     <para><simplelist>
  41       <member><filename>/run/systemd/user-generators/*</filename></member>
  42       <member><filename>/etc/systemd/user-generators/*</filename></member>
  43       <member><filename>/usr/local/lib/systemd/user-generators/*</filename></member>
  44       <member><filename>&USER_GENERATOR_DIR;/*</filename></member>
  45     </simplelist></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>
 151
 152         <xi:include href="version-info.xml" xpointer="v251"/></listitem>
 153       </varlistentry>
 154
 155       <varlistentry>
 156         <term><varname>$SYSTEMD_IN_INITRD</varname></term>
 157
 158         <listitem><para>If the generator is run as part of an initrd this is set to <literal>1</literal>. If
 159         it is run from the regular host (i.e. after the transition from initrd to host) it is set to
 160         <literal>0</literal>. This environment variable is only set for system generators.</para>
 161
 162         <xi:include href="version-info.xml" xpointer="v251"/></listitem>
 163       </varlistentry>
 164
 165       <varlistentry>
 166         <term><varname>$SYSTEMD_FIRST_BOOT</varname></term>
 167
 168         <listitem><para>If this boot-up cycle is considered a "first boot", this is set to
 169         <literal>1</literal>; if it is a subsequent, regular boot it is set to <literal>0</literal>. For
 170         details see the documentation of <varname>ConditionFirstBoot=</varname> in
 171         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>. This
 172         environment variable is only set for system generators.</para>
 173
 174         <xi:include href="version-info.xml" xpointer="v251"/></listitem>
 175       </varlistentry>
 176
 177       <varlistentry>
 178         <term><varname>$SYSTEMD_VIRTUALIZATION</varname></term>
 179
 180         <listitem><para>If the service manager is run in a virtualized environment,
 181         <varname>$SYSTEMD_VIRTUALIZATION</varname> is set to a pair of strings, separated by a colon. The
 182         first string is either <literal>vm</literal> or <literal>container</literal>, categorizing the type
 183         of virtualization. The second string identifies the implementation of the virtualization
 184         technology. If no virtualization is detected this variable will not be set. This data is identical to
 185         what
 186         <citerefentry><refentrytitle>systemd-detect-virt</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 187         detects and reports, and uses the same vocabulary of virtualization implementation
 188         identifiers.</para>
 189
 190         <xi:include href="version-info.xml" xpointer="v251"/></listitem>
 191       </varlistentry>
 192
 193       <varlistentry>
 194         <term><varname>$SYSTEMD_ARCHITECTURE</varname></term>
 195
 196         <listitem><para>This variable is set to a short identifier of the reported architecture of the
 197         system. For details about defined values, see documentation of
 198         <varname>ConditionArchitecture=</varname> in
 199         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
 200
 201         <xi:include href="version-info.xml" xpointer="v251"/></listitem>
 202       </varlistentry>
 203
 204       <varlistentry>
 205         <term><varname>$CREDENTIALS_DIRECTORY</varname></term>
 206         <term><varname>$ENCRYPTED_CREDENTIALS_DIRECTORY</varname></term>
 207
 208         <listitem><para>If set, refers to the directory system credentials have been placed in. Credentials
 209         passed into the system in plaintext form will be placed in <varname>$CREDENTIALS_DIRECTORY</varname>,
 210         and those passed in in encrypted form will be placed in
 211         <varname>$ENCRYPTED_CREDENTIALS_DIRECTORY</varname>. Use the
 212         <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 213         command to automatically decrypt/authenticate credentials passed in, if needed. Specifically, use the
 214         <command>systemd-creds --system cat</command> command.</para>
 215
 216         <xi:include href="version-info.xml" xpointer="v254"/></listitem>
 217       </varlistentry>
 218
 219       <varlistentry>
 220         <term><varname>$SYSTEMD_CONFIDENTIAL_VIRTUALIZATION</varname></term>
 221
 222         <listitem><para>If the service manager is run in a confidential virtualized environment,
 223         <varname>$SYSTEMD_CONFIDENTIAL_VIRTUALIZATION</varname> is set to a string that identifies
 224         the confidential virtualization hardware technology. If no confidential virtualization is
 225         detected this variable will not be set. This data is identical to what
 226         <citerefentry><refentrytitle>systemd-detect-virt</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 227         detects and reports, and uses the same vocabulary of confidential virtualization
 228         technology identifiers.</para>
 229
 230         <xi:include href="version-info.xml" xpointer="v254"/></listitem>
 231       </varlistentry>
 232     </variablelist>
 233   </refsect1>
 234
 235   <refsect1>
 236     <title>Notes about writing generators</title>
 237
 238     <itemizedlist>
 239       <listitem>
 240         <para>All generators are executed in parallel. That means all executables are started at the very
 241         same time and need to be able to cope with this parallelism.
 242         </para>
 243       </listitem>
 244
 245       <listitem>
 246         <para>Generators are run very early at boot and cannot rely on any external services. They may not
 247         talk to any other process. That includes simple things such as logging to <citerefentry
 248         project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>, or
 249         <command>systemd</command> itself (this means: no
 250         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>)!
 251         Non-essential file systems like <filename>/var/</filename> and <filename>/home/</filename> are
 252         mounted after generators have run. Generators can however rely on the most basic kernel functionality
 253         to be available, as well as mounted <filename>/sys/</filename>, <filename>/proc/</filename>,
 254         <filename>/dev/</filename>, <filename>/usr/</filename> and <filename>/run/</filename> file systems.
 255         </para>
 256       </listitem>
 257
 258       <listitem>
 259         <para>Units written by generators are removed when the configuration is reloaded. That means the
 260         lifetime of the generated units is closely bound to the reload cycles of <command>systemd</command>
 261         itself.</para>
 262       </listitem>
 263
 264       <listitem>
 265         <para>Generators should only be used to generate unit files, <filename>.d/*.conf</filename> drop-ins
 266         for them and symlinks to them, not any other kind of non-unit related configuration. Due to the
 267         lifecycle logic mentioned above, generators are not a good fit to generate dynamic configuration for
 268         other services. If you need to generate dynamic configuration for other services, do so in normal
 269         services you order before the service in question.</para>
 270
 271         <para>Note that using the <varname>StandardInputData=</varname>/<varname>StandardInputText=</varname>
 272         settings of service unit files (see
 273         <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>), it
 274         is possible to make arbitrary input data (including daemon-specific configuration) part of the unit
 275         definitions, which often might be sufficient to embed data or configuration for other programs into
 276         unit files in a native fashion.</para>
 277       </listitem>
 278
 279       <listitem>
 280         <para>Since
 281         <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 282
 283         is not available (see above), log messages have to be written to <filename>/dev/kmsg</filename>
 284         instead.</para>
 285       </listitem>
 286
 287       <listitem>
 288         <para>The generator should always include its own name in a comment at the top of the generated file,
 289         so that the user can easily figure out which component created or amended a particular unit.</para>
 290
 291         <para>The <varname>SourcePath=</varname> directive should be used in generated files to specify the
 292         source configuration file they are generated from. This makes things more easily understood by the
 293         user and also has the benefit that systemd can warn the user about configuration files that changed
 294         on disk but have not been read yet by systemd. The <varname>SourcePath=</varname> value does not have
 295         to be a file in a physical filesystem. For example, in the common case of the generator looking at
 296         the kernel command line, <option>SourcePath=/proc/cmdline</option> should be used.</para>
 297       </listitem>
 298
 299       <listitem>
 300         <para>Generators may write out dynamic unit files or just hook unit files into other units with the
 301         usual <filename>.wants/</filename> or <filename>.requires/</filename> symlinks. Often, it is nicer to
 302         simply instantiate a template unit file from <filename>/usr/</filename> with a generator instead of
 303         writing out entirely dynamic unit files. Of course, this works only if a single parameter is to be
 304         used.</para>
 305       </listitem>
 306
 307       <listitem>
 308         <para>If you are careful, you can implement generators in shell scripts. We do recommend C code
 309         however, since generators are executed synchronously and hence delay the entire boot if they are
 310         slow.</para>
 311       </listitem>
 312
 313       <listitem>
 314         <para>Regarding overriding semantics: there are two rules we try to follow when thinking about the
 315         overriding semantics:</para>
 316
 317         <orderedlist numeration="lowerroman">
 318           <listitem>
 319             <para>User configuration should override vendor configuration. This (mostly) means that stuff
 320             from <filename>/etc/</filename> should override stuff from <filename>/usr/</filename>.</para>
 321           </listitem>
 322
 323           <listitem>
 324             <para>Native configuration should override non-native configuration. This (mostly) means that
 325             stuff you generate should never override native unit files for the same purpose.</para>
 326           </listitem>
 327         </orderedlist>
 328
 329         <para>Of these two rules the first rule is probably the more important one and breaks the second one
 330         sometimes. Hence, when deciding whether to use argv[1], argv[2], or argv[3], your default choice
 331         should probably be argv[1].</para>
 332       </listitem>
 333
 334       <listitem>
 335         <para>Instead of heading off now and writing all kind of generators for legacy configuration file
 336         formats, please think twice! It is often a better idea to just deprecate old stuff instead of keeping
 337         it artificially alive.
 338         </para>
 339       </listitem>
 340     </itemizedlist>
 341   </refsect1>
 342
 343   <refsect1>
 344     <title>Examples</title>
 345     <example>
 346       <title>systemd-fstab-generator</title>
 347
 348       <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 349       converts <filename>/etc/fstab</filename> into native mount units. It uses argv[1] as location to place
 350       the generated unit files in order to allow the user to override <filename>/etc/fstab</filename> with
 351       their own native unit files, but also to ensure that <filename>/etc/fstab</filename> overrides any
 352       vendor default from <filename>/usr/</filename>.</para>
 353
 354       <para>After editing <filename>/etc/fstab</filename>, the user should invoke <command>systemctl
 355       daemon-reload</command>. This will re-run all generators and cause <command>systemd</command> to reload
 356       units from disk. To actually mount new directories added to <filename>fstab</filename>,
 357       <command>systemctl start <replaceable>/path/to/mountpoint</replaceable></command> or <command>systemctl
 358       start local-fs.target</command> may be used.</para>
 359     </example>
 360
 361     <example>
 362       <title>systemd-system-update-generator</title>
 363
 364       <para><citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 365       temporarily redirects <filename>default.target</filename> to <filename>system-update.target</filename>,
 366       if a system update is scheduled. Since this needs to override the default user configuration for
 367       <filename>default.target</filename>, it uses argv[2]. For details about this logic, see
 368       <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 369       </para>
 370     </example>
 371
 372     <example>
 373       <title>Debugging a generator</title>
 374
 375       <programlisting>dir=$(mktemp -d)
 376 SYSTEMD_LOG_LEVEL=debug &SYSTEM_GENERATOR_DIR;/systemd-fstab-generator \
 377         "$dir" "$dir" "$dir"
 378 find $dir</programlisting>
 379     </example>
 380   </refsect1>
 381
 382   <refsect1>
 383     <title>See also</title>
 384
 385     <para><simplelist type="inline">
 386       <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 387       <member><citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 388       <member><citerefentry><refentrytitle>systemd-debug-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 389       <member><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 390       <member><citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
 391       <member><citerefentry><refentrytitle>systemd-getty-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 392       <member><citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 393       <member><citerefentry><refentrytitle>systemd-hibernate-resume-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 394       <member><citerefentry><refentrytitle>systemd-rc-local-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 395       <member><citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 396       <member><citerefentry><refentrytitle>systemd-sysv-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 397       <member><citerefentry><refentrytitle>systemd-xdg-autostart-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 398       <member><citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
 399       <member><citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 400       <member><citerefentry><refentrytitle>systemd.environment-generator</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
 401     </simplelist></para>
 402   </refsect1>
 403 </refentry>