1SYSTEMD.GENERATOR(7)           systemd.generator          SYSTEMD.GENERATOR(7)
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NAME

6       systemd.generator - systemd unit generators
7

SYNOPSIS

9       /path/to/generator normal-dir [early-dir] [late-dir]
10
11       /run/systemd/system-generators/*
12       /etc/systemd/system-generators/*
13       /usr/local/lib/systemd/system-generators/*
14       /usr/lib/systemd/system-generators/*
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16       /run/systemd/user-generators/*
17       /etc/systemd/user-generators/*
18       /usr/local/lib/systemd/user-generators/*
19       /usr/lib/systemd/user-generators/*
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21

DESCRIPTION

23       Generators are small executables placed in
24       /usr/lib/systemd/system-generators/ and other directories listed above.
25       systemd(1) will execute these binaries very early at bootup and at
26       configuration reload time — before unit files are loaded. Their main
27       purpose is to convert configuration and execution context parameters
28       that are not native to the service manager into dynamically generated
29       unit files, symlinks or unit file drop-ins, so that they can extend the
30       unit file hierarchy the service manager subsequently loads and operates
31       on.
32
33       systemd will call each generator with three directory paths that are to
34       be used for generator output. In these three directories, generators
35       may dynamically generate unit files (regular ones, instances, as well
36       as templates), unit file .d/ drop-ins, and create symbolic links to
37       unit files to add additional dependencies, create aliases, or
38       instantiate existing templates. Those directories are included in the
39       unit load path, allowing generated configuration to extend or override
40       existing definitions. For tests, generators may be called with just one
41       argument; the generator should assume that all three paths are the same
42       in that case.
43
44       Directory paths for generator output differ by priority:
45       .../generator.early has priority higher than the admin configuration in
46       /etc/, while .../generator has lower priority than /etc/ but higher
47       than vendor configuration in /usr/, and .../generator.late has priority
48       lower than all other configuration. See the next section and the
49       discussion of unit load paths and unit overriding in systemd.unit(5).
50
51       Generators are loaded from a set of paths determined during
52       compilation, as listed above. System and user generators are loaded
53       from directories with names ending in system-generators/ and
54       user-generators/, respectively. Generators found in directories listed
55       earlier override the ones with the same name in directories lower in
56       the list. A symlink to /dev/null or an empty file can be used to mask a
57       generator, thereby preventing it from running. Please note that the
58       order of the two directories with the highest priority is reversed with
59       respect to the unit load path, and generators in /run/ overwrite those
60       in /etc/.
61
62       After installing new generators or updating the configuration,
63       systemctl daemon-reload may be executed. This will delete the previous
64       configuration created by generators, re-run all generators, and cause
65       systemd to reload units from disk. See systemctl(1) for more
66       information.
67

OUTPUT DIRECTORIES

69       Generators are invoked with three arguments: paths to directories where
70       generators can place their generated unit files or symlinks. By default
71       those paths are runtime directories that are included in the search
72       path of systemd, but a generator may be called with different paths for
73       debugging purposes. If only one argument is provided, the generator
74       should use the same directory as the the three output paths.
75
76        1. normal-dir
77
78           In normal use this is /run/systemd/generator in case of the system
79           generators and $XDG_RUNTIME_DIR/systemd/generator in case of the
80           user generators. Unit files placed in this directory take
81           precedence over vendor unit configuration but not over native
82           user/administrator unit configuration.
83
84        2. early-dir
85
86           In normal use this is /run/systemd/generator.early in case of the
87           system generators and $XDG_RUNTIME_DIR/systemd/generator.early in
88           case of the user generators. Unit files placed in this directory
89           override unit files in /usr/, /run/ and /etc/. This means that unit
90           files placed in this directory take precedence over all normal
91           configuration, both vendor and user/administrator.
92
93        3. late-dir
94
95           In normal use this is /run/systemd/generator.late in case of the
96           system generators and $XDG_RUNTIME_DIR/systemd/generator.late in
97           case of the user generators. This directory may be used to extend
98           the unit file tree without overriding any other unit files. Any
99           native configuration files supplied by the vendor or
100           user/administrator take precedence.
101
102       Note: generators must not write to other locations or otherwise make
103       changes to system state. Generator output is supposed to last only
104       until the next daemon-reload or daemon-reexec; if the generator is
105       replaced or masked, its effects should vanish.
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ENVIRONMENT

108       The service manager sets a number of environment variables when
109       invoking generator executables. They carry information about the
110       execution context of the generator, in order to simplify
111       conditionalizing generators to specific environments. The following
112       environment variables are set:
113
114       $SYSTEMD_SCOPE
115           If the generator is invoked from the system service manager this
116           variable is set to "system"; if invoked from the per-user service
117           manager it is set to "user".
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119       $SYSTEMD_IN_INITRD
120           If the generator is run as part of an initrd this is set to "1". If
121           it is run from the regular host (i.e. after the transition from
122           initrd to host) it is set to "0". This environment variable is only
123           set for system generators.
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125       $SYSTEMD_FIRST_BOOT
126           If this boot-up cycle is considered a "first boot", this is set to
127           "1"; if it is a subsequent, regular boot it is set to "0". For
128           details see the documentation of ConditionFirstBoot= in
129           systemd.unit(5). This environment variable is only set for system
130           generators.
131
132       $SYSTEMD_VIRTUALIZATION
133           If the service manager is run in a virtualized environment,
134           $SYSTEMD_VIRTUALIZATION is set to a pair of strings, separated by a
135           colon. The first string is either "vm" or "container", categorizing
136           the type of virtualization. The second string identifies the
137           implementation of the virtualization technology. If no
138           virtualization is detected this variable will not be set. This data
139           is identical to what systemd-detect-virt(1) detects and reports,
140           and uses the same vocabulary of virtualization implementation
141           identifiers.
142
143       $SYSTEMD_ARCHITECTURE
144           This variable is set to a short identifier of the reported
145           architecture of the system. For details about defined values, see
146           documentation of ConditionArchitecture= in systemd.unit(5).
147

NOTES ABOUT WRITING GENERATORS

149       •   All generators are executed in parallel. That means all executables
150           are started at the very same time and need to be able to cope with
151           this parallelism.
152
153       •   Generators are run very early at boot and cannot rely on any
154           external services. They may not talk to any other process. That
155           includes simple things such as logging to syslog(3), or systemd
156           itself (this means: no systemctl(1))! Non-essential file systems
157           like /var/ and /home/ are mounted after generators have run.
158           Generators can however rely on the most basic kernel functionality
159           to be available, as well as mounted /sys/, /proc/, /dev/, /usr/ and
160           /run/ file systems.
161
162       •   Units written by generators are removed when the configuration is
163           reloaded. That means the lifetime of the generated units is closely
164           bound to the reload cycles of systemd itself.
165
166       •   Generators should only be used to generate unit files, .d/*.conf
167           drop-ins for them and symlinks to them, not any other kind of
168           non-unit related configuration. Due to the lifecycle logic
169           mentioned above, generators are not a good fit to generate dynamic
170           configuration for other services. If you need to generate dynamic
171           configuration for other services, do so in normal services you
172           order before the service in question.
173
174           Note that using the StandardInputData=/StandardInputText= settings
175           of service unit files (see systemd.exec(5)), it is possible to make
176           arbitrary input data (including daemon-specific configuration) part
177           of the unit definitions, which often might be sufficient to embed
178           data or configuration for other programs into unit files in a
179           native fashion.
180
181       •   Since syslog(3) is not available (see above), log messages have to
182           be written to /dev/kmsg instead.
183
184       •   The generator should always include its own name in a comment at
185           the top of the generated file, so that the user can easily figure
186           out which component created or amended a particular unit.
187
188           The SourcePath= directive should be used in generated files to
189           specify the source configuration file they are generated from. This
190           makes things more easily understood by the user and also has the
191           benefit that systemd can warn the user about configuration files
192           that changed on disk but have not been read yet by systemd. The
193           SourcePath= value does not have to be a file in a physical
194           filesystem. For example, in the common case of the generator
195           looking at the kernel command line, SourcePath=/proc/cmdline should
196           be used.
197
198       •   Generators may write out dynamic unit files or just hook unit files
199           into other units with the usual .wants/ or .requires/ symlinks.
200           Often, it is nicer to simply instantiate a template unit file from
201           /usr/ with a generator instead of writing out entirely dynamic unit
202           files. Of course, this works only if a single parameter is to be
203           used.
204
205       •   If you are careful, you can implement generators in shell scripts.
206           We do recommend C code however, since generators are executed
207           synchronously and hence delay the entire boot if they are slow.
208
209       •   Regarding overriding semantics: there are two rules we try to
210           follow when thinking about the overriding semantics:
211
212            1. User configuration should override vendor configuration. This
213               (mostly) means that stuff from /etc/ should override stuff from
214               /usr/.
215
216            2. Native configuration should override non-native configuration.
217               This (mostly) means that stuff you generate should never
218               override native unit files for the same purpose.
219
220           Of these two rules the first rule is probably the more important
221           one and breaks the second one sometimes. Hence, when deciding
222           whether to use argv[1], argv[2], or argv[3], your default choice
223           should probably be argv[1].
224
225       •   Instead of heading off now and writing all kind of generators for
226           legacy configuration file formats, please think twice! It is often
227           a better idea to just deprecate old stuff instead of keeping it
228           artificially alive.
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EXAMPLES

231       Example 1. systemd-fstab-generator
232
233       systemd-fstab-generator(8) converts /etc/fstab into native mount units.
234       It uses argv[1] as location to place the generated unit files in order
235       to allow the user to override /etc/fstab with their own native unit
236       files, but also to ensure that /etc/fstab overrides any vendor default
237       from /usr/.
238
239       After editing /etc/fstab, the user should invoke systemctl
240       daemon-reload. This will re-run all generators and cause systemd to
241       reload units from disk. To actually mount new directories added to
242       fstab, systemctl start /path/to/mountpoint or systemctl start
243       local-fs.target may be used.
244
245       Example 2. systemd-system-update-generator
246
247       systemd-system-update-generator(8) temporarily redirects default.target
248       to system-update.target, if a system update is scheduled. Since this
249       needs to override the default user configuration for default.target, it
250       uses argv[2]. For details about this logic, see systemd.offline-
251       updates(7).
252
253       Example 3. Debugging a generator
254
255           dir=$(mktemp -d)
256           SYSTEMD_LOG_LEVEL=debug /usr/lib/systemd/system-generators/systemd-fstab-generator \
257                   "$dir" "$dir" "$dir"
258           find $dir
259

SEE ALSO

261       systemd(1), systemd-cryptsetup-generator(8), systemd-debug-
262       generator(8), systemd-fstab-generator(8), fstab(5), systemd-getty-
263       generator(8), systemd-gpt-auto-generator(8), systemd-hibernate-resume-
264       generator(8), systemd-rc-local-generator(8), systemd-system-update-
265       generator(8), systemd-sysv-generator(8), systemd-xdg-autostart-
266       generator(8), systemd.unit(5), systemctl(1), systemd.environment-
267       generator(7)
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271systemd 253                                               SYSTEMD.GENERATOR(7)
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