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71 lines
2.4 KiB
Markdown
71 lines
2.4 KiB
Markdown
# The Init Daemon
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## Initial Ram Disk to Rootfs transition
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After mounting the root filesystem, either the kernel or the initial ram disk
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startup process is expected to exec the init program from the root filesystem.
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At the current time, there is no support for re-scanning the service files
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*yet*, so when init is started, the final configuration in `/etc/init.d` has to
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be present. As a result, we currently cannot perform mounting of `/etc/` or
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packing init into the initial ram disk and doing the rootfs transition.
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Also, as a result of this, changing the service configuration requires a system
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reboot to be effective.
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This _will_ change in the future.
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## Service Types and Targets
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A service file has a *type*, specifying whether the service should be run once
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or restarted when it terminates and a *target*, specifying when the service
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should be run.
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The *target* is similar to a runlevel in System V init. The init daemon
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currently knows about the following targets:
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* boot
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* reboot
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* shutdown
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When `init` is run, it starts all the services for the `boot` target. From the
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`boot` target it can transition to any other target and execute the services
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for the specified target.
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The `reboot` and `shutdown` targets cannot transition to any other target and
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when invoked, cause initd to drop everything else it intended to do.
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For the `reboot` and `shutdown` targets, respawn type processes are no longer
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restarted when they terminate and once all services have been executed, the
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`init` program performs a hard system reboot or power off.
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The init program tries to capture the `CTRL+ALT+DELETE` key sequence (or its
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local equivalent) and transitions to the reboot target if pressed.
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## Service Configuration Rescan
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TBD
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## Control Socket and Signals
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The `init` program catches the following signals:
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* `SIGCHLD`
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* `SIGINT`
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* `SIGTERM`
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The `SIGCHLD` handler implements standard process reaping. If a terminated
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process belongs to one of the supervised services, the configured action is
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taken (e.g. restarting it).
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When `SIGINT` is caugth, `init` transitions to the `reboot` target. Similarly,
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`SIGTERM` causes `init` to transition to the `shutdown` target.
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For more complex tasks, `init` creates a control socket that the command line
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tools included in this package can use. For the time being, the control socket
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can only tell the init daemon to transition to the `reboot` or `shutdown`
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target.
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