1SYSTEMD.GENERATOR(7) systemd.generator SYSTEMD.GENERATOR(7)
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6 systemd.generator - systemd unit generators
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9 /path/to/generator normal-dir early-dir late-dir
10 /run/systemd/system-generators/*
12 /etc/systemd/system-generators/*
13 /usr/local/lib/systemd/system-generators/*
14 /usr/lib/systemd/system-generators/*
15 /run/systemd/user-generators/*
17 /etc/systemd/user-generators/*
18 /usr/local/lib/systemd/user-generators/*
19 /usr/lib/systemd/user-generators/*
20
23 Generators are small executables that live in
24 /usr/lib/systemd/system-generators/ and other directories listed above.
25 systemd(1) will execute those 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 into dynamically
28 generated unit files.
29 Each generator is called with three directory paths that are to be used
31 for generator output. In these three directories, generators may
32 dynamically generate unit files (regular ones, instances, as well as
33 templates), unit file .d/ drop-ins, and create symbolic links to unit
34 files to add additional dependencies, create aliases, or instantiate
35 existing templates. Those directories are included in the unit load
36 path of systemd(1), allowing generated configuration to extend or
37 override existing definitions.
38 Directory paths for generator output differ by priority:
40 .../generator.early has priority higher than the admin configuration in
41 /etc, while .../generator has lower priority than /etc but higher than
42 vendor configuration in /usr, and .../generator.late has priority lower
43 than all other configuration. See the next section and the discussion
44 of unit load paths and unit overriding in systemd.unit(5).
45 Generators are loaded from a set of paths determined during
47 compilation, as listed above. System and user generators are loaded
48 from directories with names ending in system-generators/ and
49 user-generators/, respectively. Generators found in directories listed
50 earlier override the ones with the same name in directories lower in
51 the list. A symlink to /dev/null or an empty file can be used to mask a
52 generator, thereby preventing it from running. Please note that the
53 order of the two directories with the highest priority is reversed with
54 respect to the unit load path, and generators in /run overwrite those
55 in /etc.
56 After installing new generators or updating the configuration,
58 systemctl daemon-reload may be executed. This will delete the previous
59 configuration created by generators, re-run all generators, and cause
60 systemd to reload units from disk. See systemctl(1) for more
61 information.
62
64 Generators are invoked with three arguments: paths to directories where
65 generators can place their generated unit files or symlinks. By default
66 those paths are runtime directories that are included in the search
67 path of systemd, but a generator may be called with different paths for
68 debugging purposes.
69 1. normal-dir
71 In normal use this is /run/systemd/generator in case of the system
73 generators and $XDG_RUNTIME_DIR/generator in case of the user
74 generators. Unit files placed in this directory take precedence
75 over vendor unit configuration but not over native
76 user/administrator unit configuration.
77 2. early-dir
79 In normal use this is /run/systemd/generator.early in case of the
81 system generators and $XDG_RUNTIME_DIR/generator.early in case of
82 the user generators. Unit files placed in this directory override
83 unit files in /usr, /run and /etc. This means that unit files
84 placed in this directory take precedence over all normal
85 configuration, both vendor and user/administrator.
86 3. late-dir
88 In normal use this is /run/systemd/generator.late in case of the
90 system generators and $XDG_RUNTIME_DIR/generator.late in case of
91 the user generators. This directory may be used to extend the unit
92 file tree without overriding any other unit files. Any native
93 configuration files supplied by the vendor or user/administrator
94 take precedence.
95
97 · All generators are executed in parallel. That means all executables
98 are started at the very same time and need to be able to cope with
99 this parallelism.
100 · Generators are run very early at boot and cannot rely on any
102 external services. They may not talk to any other process. That
103 includes simple things such as logging to syslog(3), or systemd
104 itself (this means: no systemctl(1))! Non-essential file systems
105 like /var and /home are mounted after generators have run.
106 Generators can however rely on the most basic kernel functionality
107 to be available, including a mounted /sys, /proc, /dev, /usr.
108 · Units written by generators are removed when the configuration is
110 reloaded. That means the lifetime of the generated units is closely
111 bound to the reload cycles of systemd itself.
112 · Generators should only be used to generate unit files and symlinks
114 to them, not any other kind of configuration. Due to the lifecycle
115 logic mentioned above, generators are not a good fit to generate
116 dynamic configuration for other services. If you need to generate
117 dynamic configuration for other services, do so in normal services
118 you order before the service in question.
119 · Since syslog(3) is not available (see above), log messages have to
121 be written to /dev/kmsg instead.
122 · The generator should always include its own name in a comment at
124 the top of the generated file, so that the user can easily figure
125 out which component created or amended a particular unit.
126 The SourcePath= directive should be used in generated files to
128 specify the source configuration file they are generated from. This
129 makes things more easily understood by the user and also has the
130 benefit that systemd can warn the user about configuration files
131 that changed on disk but have not been read yet by systemd. The
132 SourcePath= value does not have to be a file in a physical
133 filesystem. For example, in the common case of the generator
134 looking at the kernel command line, SourcePath=/proc/cmdline should
135 be used.
136 · Generators may write out dynamic unit files or just hook unit files
138 into other units with the usual .wants/ or .requires/ symlinks.
139 Often, it is nicer to simply instantiate a template unit file from
140 /usr with a generator instead of writing out entirely dynamic unit
141 files. Of course, this works only if a single parameter is to be
142 used.
143 · If you are careful, you can implement generators in shell scripts.
145 We do recommend C code however, since generators are executed
146 synchronously and hence delay the entire boot if they are slow.
147 · Regarding overriding semantics: there are two rules we try to
149 follow when thinking about the overriding semantics:
150 1. User configuration should override vendor configuration. This
152 (mostly) means that stuff from /etc should override stuff from
153 /usr.
154 2. Native configuration should override non-native configuration.
156 This (mostly) means that stuff you generate should never
157 override native unit files for the same purpose.
158 Of these two rules the first rule is probably the more important
160 one and breaks the second one sometimes. Hence, when deciding
161 whether to use argv[1], argv[2], or argv[3], your default choice
162 should probably be argv[1].
163 · Instead of heading off now and writing all kind of generators for
165 legacy configuration file formats, please think twice! It is often
166 a better idea to just deprecate old stuff instead of keeping it
167 artificially alive.
168
170 Example 1. systemd-fstab-generator
171 systemd-fstab-generator(8) converts /etc/fstab into native mount units.
173 It uses argv[1] as location to place the generated unit files in order
174 to allow the user to override /etc/fstab with their own native unit
175 files, but also to ensure that /etc/fstab overrides any vendor default
176 from /usr.
177 After editing /etc/fstab, the user should invoke systemctl
179 daemon-reload. This will re-run all generators and cause systemd to
180 reload units from disk. To actually mount new directories added to
181 fstab, systemctl start /path/to/mountpoint or systemctl start
182 local-fs.target may be used.
183 Example 2. systemd-system-update-generator
185 systemd-system-update-generator(8) temporarily redirects default.target
187 to system-update.target, if a system update is scheduled. Since this
188 needs to override the default user configuration for default.target, it
189 uses argv[2]. For details about this logic, see systemd.offline-
190 updates(7).
191 Example 3. Debugging a generator
193 dir=$(mktemp -d)
195 SYSTEMD_LOG_LEVEL=debug /usr/lib/systemd/system-generators/systemd-fstab-generator \
196 "$dir" "$dir" "$dir"
197 find $dir
198
200 systemd(1), systemd-cryptsetup-generator(8), systemd-debug-
201 generator(8), systemd-fstab-generator(8), fstab(5), systemd-getty-
202 generator(8), systemd-gpt-auto-generator(8), systemd-hibernate-resume-
203 generator(8), systemd-rc-local-generator(8), systemd-system-update-
204 generator(8), systemd-sysv-generator(8), systemd.unit(5), systemctl(1),
205 systemd.environment-generator(7)
206systemd 245 SYSTEMD.GENERATOR(7)