Systemd unit generators unit

systemd.generator(7) - Linux manual page http://man7.org/linux/man-pages/man7/systemd.generator.7.html

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

NAME top

       systemd.generator - Systemd unit generators

SYNOPSIS top

       /path/to/generator normal-dir early-dir late-dir

       /run/systemd/system-generators/*

       /etc/systemd/system-generators/*

       /usr/local/lib/systemd/system-generators/*

       /usr/lib/systemd/system-generators/*

       /run/systemd/user-generators/*

       /etc/systemd/user-generators/*

       /usr/local/lib/systemd/user-generators/*

       /usr/lib/systemd/user-generators/*

DESCRIPTION top

       Generators are small binaries that live in

       /usr/lib/systemd/user-generators/ and other directories listed above.

       systemd(1) will execute those binaries very early at bootup and at

       configuration reload time — before unit files are loaded. Generators

       can dynamically generate unit files or create symbolic links to unit

       files to add additional dependencies, thus extending or overriding

       existing definitions. Their main purpose is to convert configuration

       files that are not native unit files dynamically into native unit

       files.

       Generators are loaded from a set of paths determined during

       compilation, as listed above. System and user generators are loaded

       from directories with names ending in system-generators/ and

       user-generators/, respectively. Generators found in directories

       listed earlier override the ones with the same name in directories

       lower in the list. A symlink to /dev/null or an empty file can be

       used to mask a generator, thereby preventing it from running. Please

       note that the order of the two directories with the highest priority

       is reversed with respect to the unit load path, and generators in

       /run overwrite those in /etc.

       After installing new generators or updating the configuration,

       systemctl daemon-reload may be executed. This will delete the

       previous configuration created by generators, re-run all generators,

       and cause systemd to reload units from disk. See systemctl(1) for

       more information.

WRITING GENERATORS top

       Generators are invoked with three arguments: paths to runtime

       directories where generators can place their generated unit files or

       symlinks.

        1. normal-dir

           argv[1] may be used to override unit files in /usr, but not those

           in /etc. This means that unit files placed in this directory take

           precedence over vendor unit configuration but not over native

           user/administrator unit configuration.

        2. early-dir

           argv[2] may be used to override unit files in /usr and in /etc.

           This means that unit files placed in this directory take

           precedence over all configuration, both vendor and

           user/administrator.

        3. late-dir

           argv[3] may be used to extend the unit file tree without

           overriding any other unit files. Any native configuration files

           supplied by the vendor or user/administrator take precedence over

           the generated ones placed in this directory.

   Notes

       ·   All generators are executed in parallel. That means all

           executables are started at the very same time and need to be able

           to cope with this parallelism.

       ·   Generators are run very early at boot and cannot rely on any

           external services. They may not talk to any other process. That

           includes simple things such as logging to syslog(3), or systemd

           itself (this means: no systemctl(1))! Non-essential file systems

           like /var and /home are mounted after generators have run.

           Generators can however rely on the most basic kernel

           functionality to be available, including a mounted /sys, /proc,

           /dev, /usr.

       ·   Units written by generators are removed when the configuration is

           reloaded. That means the lifetime of the generated units is

           closely bound to the reload cycles of systemd itself.

       ·   Generators should only be used to generate unit files, not any

           other kind of configuration. Due to the lifecycle logic mentioned

           above, generators are not a good fit to generate dynamic

           configuration for other services. If you need to generate dynamic

           configuration for other services, do so in normal services you

           order before the service in question.

       ·   Since syslog(3) is not available (see above), log messages have

           to be written to /dev/kmsg instead.

       ·   It is a good idea to use the SourcePath= directive in generated

           unit files to specify the source configuration file you are

           generating the unit from. This makes things more easily

           understood by the user and also has the benefit that systemd can

           warn the user about configuration files that changed on disk but

           have not been read yet by systemd.

       ·   Generators may write out dynamic unit files or just hook unit

           files into other units with the usual .wants/ or .requires/

           symlinks. Often, it is nicer to simply instantiate a template

           unit file from /usr with a generator instead of writing out

           entirely dynamic unit files. Of course, this works only if a

           single parameter is to be used.

       ·   If you are careful, you can implement generators in shell

           scripts. We do recommend C code however, since generators are

           executed synchronously and hence delay the entire boot if they

           are slow.

       ·   Regarding overriding semantics: there are two rules we try to

           follow when thinking about the overriding semantics:

            1. User configuration should override vendor configuration. This

               (mostly) means that stuff from /etc should override stuff

               from /usr.

            2. Native configuration should override non-native

               configuration. This (mostly) means that stuff you generate

               should never override native unit files for the same purpose.

           Of these two rules the first rule is probably the more important

           one and breaks the second one sometimes. Hence, when deciding

           whether to user argv[1], argv[2], or argv[3], your default choice

           should probably be argv[1].

       ·   Instead of heading off now and writing all kind of generators for

           legacy configuration file formats, please think twice! It is

           often a better idea to just deprecate old stuff instead of

           keeping it artificially alive.

EXAMPLES top

       Example 1. systemd-fstab-generator

       systemd-fstab-generator(8) converts /etc/fstab into native mount

       units. It uses argv[1] as location to place the generated unit files

       in order to allow the user to override /etc/fstab with her own native

       unit files, but also to ensure that /etc/fstab overrides any vendor

       default from /usr.

       After editing /etc/fstab, the user should invoke systemctl

       daemon-reload. This will re-run all generators and cause systemd to

       reload units from disk. To actually mount new directories added to

       fstab, systemctl start /path/to/mountpoint or systemctl start

       local-fs.target may be used.

       Example 2. systemd-system-update-generator

       systemd-system-update-generator(8) temporarily redirects

       default.target to system-update.target, if a system update is

       scheduled. Since this needs to override the default user

       configuration for default.target, it uses argv[2]. For details about

       this logic, see systemd.offline-updates(7).

       Example 3. Debugging a generator

           dir=$(mktemp -d)

           SYSTEMD_LOG_LEVEL=debug /usr/lib/systemd/system-generators/systemd-fstab-generator \

                   "$dir" "$dir" "$dir"

           find $dir

COLOPHON top

       This page is part of the systemd (systemd system and service manager)

       project.  Information about the project can be found at

       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have a bug

       report for this manual page, see

       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.  This

       page was obtained from the project's upstream Git repository

       ⟨https://github.com/systemd/systemd.git⟩ on 2018-10-29.  (At that

       time, the date of the most recent commit that was found in the repos‐

       itory was 2018-10-29.)  If you discover any rendering problems in

       this HTML version of the page, or you believe there is a better or

       more up-to-date source for the page, or you have corrections or

       improvements to the information in this COLOPHON (which is not part

       of the original manual page), send a mail to man-pages@man7.org

systemd 234                                             SYSTEMD.GENERATOR(7)

unit

[root@d ~]# service mysqd start
Redirecting to /bin/systemctl start mysqd.service
Failed to start mysqd.service: Unit not found.
[root@d ~]# service mysqld start
Redirecting to /bin/systemctl start mysqld.service
[root@d ~]# service mysqld status
Redirecting to /bin/systemctl status mysqld.service
● mysqld.service - MySQL Server
Loaded: loaded (/usr/lib/systemd/system/mysqld.service; enabled; vendor preset: disabled)
Active: active (running) since Fri 2018-12-21 16:54:24 CST; 15s ago
Docs: man:mysqld(8)
http://dev.mysql.com/doc/refman/en/using-systemd.html
Process: 7433 ExecStartPre=/usr/bin/mysqld_pre_systemd (code=exited, status=0/SUCCESS)
Main PID: 7453 (mysqld)
Status: "SERVER_OPERATING"
CGroup: /system.slice/mysqld.service
└─7453 /usr/sbin/mysqld

Dec 21 16:54:22 d systemd[1]: Starting MySQL Server...
Dec 21 16:54:24 d systemd[1]: Started MySQL Server.
[root@d ~]#