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

6       systemd.generator - systemd unit generators
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SYNOPSIS

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

66       Generators are invoked with three arguments: paths to directories where
67       generators can place their generated unit files or symlinks. By default
68       those paths are runtime directories that are included in the search
69       path of systemd, but a generator may be called with different paths for
70       debugging purposes.
71
72        1. normal-dir
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74           In normal use this is /run/systemd/generator in case of the system
75           generators and $XDG_RUNTIME_DIR/generator in case of the user
76           generators. Unit files placed in this directory take precedence
77           over vendor unit configuration but not over native
78           user/administrator unit configuration.
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80        2. early-dir
81
82           In normal use this is /run/systemd/generator.early in case of the
83           system generators and $XDG_RUNTIME_DIR/generator.early in case of
84           the user generators. Unit files placed in this directory override
85           unit files in /usr/, /run/ and /etc/. This means that unit files
86           placed in this directory take precedence over all normal
87           configuration, both vendor and user/administrator.
88
89        3. late-dir
90
91           In normal use this is /run/systemd/generator.late in case of the
92           system generators and $XDG_RUNTIME_DIR/generator.late in case of
93           the user generators. This directory may be used to extend the unit
94           file tree without overriding any other unit files. Any native
95           configuration files supplied by the vendor or user/administrator
96           take precedence.
97

NOTES ABOUT WRITING GENERATORS

99       •   All generators are executed in parallel. That means all executables
100           are started at the very same time and need to be able to cope with
101           this parallelism.
102
103       •   Generators are run very early at boot and cannot rely on any
104           external services. They may not talk to any other process. That
105           includes simple things such as logging to syslog(3), or systemd
106           itself (this means: no systemctl(1))! Non-essential file systems
107           like /var/ and /home/ are mounted after generators have run.
108           Generators can however rely on the most basic kernel functionality
109           to be available, as well as mounted /sys/, /proc/, /dev/, /usr/ and
110           /run/ file systems.
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112       •   Units written by generators are removed when the configuration is
113           reloaded. That means the lifetime of the generated units is closely
114           bound to the reload cycles of systemd itself.
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116       •   Generators should only be used to generate unit files, .d/*.conf
117           drop-ins for them and symlinks to them, not any other kind of
118           non-unit related configuration. Due to the lifecycle logic
119           mentioned above, generators are not a good fit to generate dynamic
120           configuration for other services. If you need to generate dynamic
121           configuration for other services, do so in normal services you
122           order before the service in question.
123
124           Note that using the StandardInputData=/StandardInputText= settings
125           of service unit files (see systemd.exec(5)), it is possible to make
126           arbitrary input data (including daemon-specific configuration) part
127           of the unit definitions, which often might be sufficient to embed
128           data or configuration for other programs into unit files in a
129           native fashion.
130
131       •   Since syslog(3) is not available (see above), log messages have to
132           be written to /dev/kmsg instead.
133
134       •   The generator should always include its own name in a comment at
135           the top of the generated file, so that the user can easily figure
136           out which component created or amended a particular unit.
137
138           The SourcePath= directive should be used in generated files to
139           specify the source configuration file they are generated from. This
140           makes things more easily understood by the user and also has the
141           benefit that systemd can warn the user about configuration files
142           that changed on disk but have not been read yet by systemd. The
143           SourcePath= value does not have to be a file in a physical
144           filesystem. For example, in the common case of the generator
145           looking at the kernel command line, SourcePath=/proc/cmdline should
146           be used.
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148       •   Generators may write out dynamic unit files or just hook unit files
149           into other units with the usual .wants/ or .requires/ symlinks.
150           Often, it is nicer to simply instantiate a template unit file from
151           /usr/ with a generator instead of writing out entirely dynamic unit
152           files. Of course, this works only if a single parameter is to be
153           used.
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155       •   If you are careful, you can implement generators in shell scripts.
156           We do recommend C code however, since generators are executed
157           synchronously and hence delay the entire boot if they are slow.
158
159       •   Regarding overriding semantics: there are two rules we try to
160           follow when thinking about the overriding semantics:
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162            1. User configuration should override vendor configuration. This
163               (mostly) means that stuff from /etc/ should override stuff from
164               /usr/.
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166            2. Native configuration should override non-native configuration.
167               This (mostly) means that stuff you generate should never
168               override native unit files for the same purpose.
169
170           Of these two rules the first rule is probably the more important
171           one and breaks the second one sometimes. Hence, when deciding
172           whether to use argv[1], argv[2], or argv[3], your default choice
173           should probably be argv[1].
174
175       •   Instead of heading off now and writing all kind of generators for
176           legacy configuration file formats, please think twice! It is often
177           a better idea to just deprecate old stuff instead of keeping it
178           artificially alive.
179

EXAMPLES

181       Example 1. systemd-fstab-generator
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183       systemd-fstab-generator(8) converts /etc/fstab into native mount units.
184       It uses argv[1] as location to place the generated unit files in order
185       to allow the user to override /etc/fstab with their own native unit
186       files, but also to ensure that /etc/fstab overrides any vendor default
187       from /usr/.
188
189       After editing /etc/fstab, the user should invoke systemctl
190       daemon-reload. This will re-run all generators and cause systemd to
191       reload units from disk. To actually mount new directories added to
192       fstab, systemctl start /path/to/mountpoint or systemctl start
193       local-fs.target may be used.
194
195       Example 2. systemd-system-update-generator
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197       systemd-system-update-generator(8) temporarily redirects default.target
198       to system-update.target, if a system update is scheduled. Since this
199       needs to override the default user configuration for default.target, it
200       uses argv[2]. For details about this logic, see systemd.offline-
201       updates(7).
202
203       Example 3. Debugging a generator
204
205           dir=$(mktemp -d)
206           SYSTEMD_LOG_LEVEL=debug /usr/lib/systemd/system-generators/systemd-fstab-generator \
207                   "$dir" "$dir" "$dir"
208           find $dir
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SEE ALSO

211       systemd(1), systemd-cryptsetup-generator(8), systemd-debug-
212       generator(8), systemd-fstab-generator(8), fstab(5), systemd-getty-
213       generator(8), systemd-gpt-auto-generator(8), systemd-hibernate-resume-
214       generator(8), systemd-rc-local-generator(8), systemd-system-update-
215       generator(8), systemd-sysv-generator(8), systemd-xdg-autostart-
216       generator(8), systemd.unit(5), systemctl(1), systemd.environment-
217       generator(7)
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221systemd 249                                               SYSTEMD.GENERATOR(7)
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