Notes from last couple of weeks
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---
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title: 'Buses'
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---
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DBus - https://www.freedesktop.org/wiki/Software/dbus/
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ubus (openwrt) - https://oldwiki.archive.openwrt.org/doc/techref/ubus
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- ACLs
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---
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title: 'Criticism of systemd'
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---
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https://skarnet.org/software/systemd.html
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http://ewontfix.com/14/
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- keep supervision and system state management out of PID 1
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- there's a sample PID 1 implementation there
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---
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title: 'Survey: Process Supervision'
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---
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# {{ page.title }}
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RedoxOS "fired" -- can't find code or a homepage?
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daemontools
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- https://cr.yp.to/daemontools.html
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- services should be symlinked into a directory monitored by svscan
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- programs:
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- svscanboot - starts svscan for /service, plus readproctitle for errors via ps
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- svscan - "starts and monitors a collection of services."
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- starts one `supervise` per service in a service directory (cwd)
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- designed to run forever
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- if `s` is a service, and `s/log` is a service, creates *two*
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`supervise`s, with a pipe between them
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- if any `supervise` terminates, it restarts it
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- reuses the *same* pipe if restarting one end of a connected
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pair of `supervise`s; this way no log messages are lost
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- supervise - (re)starts ./run for a given service. Writes status to ./supervise/*
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- svc - talks to a `supervise`
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- svok - predicate: is `supervise` running for a service?
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- svstat - list service status information for zero or more services (given explicitly)
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- fghack - vile hack for antibackgrounding
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- pgrphack - wrap a child in a new process group
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- readproctitle - takes stdin and puts it into its own command-line, to show up in ps output
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- multilog - scriptable filterable actions on each line of stdin; e.g. append to log, replace contents of file etc.
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- a "log" is a directory full of files with a special format
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- tai64n - puts hex TAI timestamps on each line of stdin
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- tai64nlocal - rewrites tai64n timestamps to human-readable
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- setuidgid - command wrapper for setting uid/gid
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- envuidgid - command wrapper for setting environment variables UID and GID
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- envdir - command wrapper for setting environment based on files in a directory
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- softlimit - rlimit
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- setlock - command wrapper for holding a "locked ofile" during the lifetime of the command
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- what is an "ofile"?
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- hax to get daemontools svscan as PID 1: https://code.dogmap.org/svscan-1/
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- "For a clean shutdown, we want to kill each service and ensure
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that its logger has written all the logs before killing the
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logger."
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See "Artistic considerations" on https://skarnet.org/software/s6/why.html
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systemd
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- sd_listen_fds() - LISTEN_PID, LISTEN_FDS, LISTEN_FDNAMES; https://www.freedesktop.org/software/systemd/man/sd_listen_fds.html#
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- sd_notify() - NOTIFY_SOCKET; https://www.freedesktop.org/software/systemd/man/sd_notify.html#
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- sd_booted() - checks for /run/systemd/system/
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- sd_watchdog_enabled() - WATCHDOG_USEC, WATCHDOG_PID; https://www.freedesktop.org/software/systemd/man/sd_watchdog_enabled.html#
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Dinit https://github.com/davmac314/dinit
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- startup notification lets you signal when the process is actually ready
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- has something akin to dpkg's automatic/manual installation of
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packages, but for service startedness. You can "release" a service
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which is like "stop" but doesn't stop it if some dependent service
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is using it.
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- can "pin" a service in stopped or started state to prevent it from
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starting/stopping.
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s6 has startup notification - is it compatible with systemd?
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Reasons why DBUS is the way it is (2015): https://lwn.net/Articles/641277/
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- "Message passing or IPC isn't really the most important part of
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dbus. Process lifecycle tracking and discovery are more important.
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However, by integrating the IPC system with the lifecycle tracking
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you can simplify the overall system and avoid race conditions."
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- "dbus has a lot of semantic guarantees, such as message ordering,
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that reduce application complexity and therefore reduce code and
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reduce bugs." - sounds familiar!
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- "dbus names are directly modeled on X selections (see ICCCM)" - huh
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Horust
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- https://news.ycombinator.com/item?id=22657301
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- https://horust.dev/ (broken?!?!)
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- https://github.com/FedericoPonzi/Horust
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Integrate monit with syndicate-system?
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s6 - https://skarnet.org/software/s6/ - Laurent Bercot
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- see also https://skarnet.com/projects/service-manager.html
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- and https://archive.fosdem.org/2017/schedule/event/s6_supervision/
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- oh! and this! https://skarnet.org/software/s6-rc/overview.html
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- and https://skarnet.org/software/s6-rc/faq.html
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- redirfd trickery to get FIFOs set up for dependency resolution for
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early logging: https://skarnet.org/software/s6/s6-svscan-1.html#log
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-- in principle, could Syndicate dependency tracking take the place
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of this?
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- Maybe not because: "No logs are ever lost." (from https://skarnet.org/software/s6-linux-init/)
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- On the other hand, maybe, if the actual mechanism for log
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collection is a simple FIFO rather than full Syndicate (which
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would be used for dependency tracking but not communication, for
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this specific subtask). Keeps things UNIXy, keeps things
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accessible, relies on the kernel for buffering stuff...?
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Logging in daemontools:
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- https://cr.yp.to/daemontools/faq/create.html#runlog
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- stdout is piped to stdin of the logger program
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runit - see https://docs.voidlinux.org/config/services/index.html
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---
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title: 'What does systemd do?'
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---
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- ac-power: reports whether A/C power is connected
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- activate: do socket activation (setup sockets, exec() on activity)
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- analyze: "profile systemd", various renderings and checks of system state and config
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- ask-password: secure password entry for e.g. cryptsetup
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- backlight: saves/restores backlight brightness at shutdown/boot
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- basic
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- architecture: show some abstraction over current architecture
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- escape: string escaping for e.g. shell (blearghhh)
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- log: various kinds of logging (incl. syslog, journal, etc)
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- ratelimit: generic rate limiting datastructure and utilities
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- reboot: system call
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- util
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- in_initrd() predicate
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- virt: detects whether, and what kind of, virtualization is in effect
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- binfmt: registers binfmts with the kernel
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- boot: bootctl etc: communicate with EFI (?) about successful/failed boots
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- boot/efi: an actual EFI implementation?? can't be? maybe it's an EFI boot program. I don't know enough about this
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- busctl: introspect contents of D-Bus
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- cgls: "Recursively show control group contents"
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- cgroups-agent: sends a message to unix socket /run/systemd/cgroups-agent
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- cgtop: "Show top control groups by their resource usage"
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- core
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- automount
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- dbus-*, lots of dbus things, wonder what they all are
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- dynamic-user: dynamic user/group ID allocation for, like, temporary use?
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- efi-random: incorporate any random seed passed down from EFI
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- execute: execute programs. lots of code here!
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- job: a job/worker manager system
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- detect-virt: detect virtualization
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- firstboot
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- fsck: runs fsck
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- fstab-generator
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- getty-generator
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- hibernate-resume: a little initrd program that runs when restoring from hibernation!
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- home: manage "home areas"
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- hostname management
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- hwdb: a hardware database
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- id128: various identifiers
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- machine ID
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- boot ID
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- invocation ID
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- import: image importing for containers presumably
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- initctl: like telinit I guess
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- journal: journald, etc
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- kernel-install: when would this be used?
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- libsystemd: ?
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- libsystemd-network: implementations of network protocols?
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- dhcp, dhcp6
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- lldp
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- libudev: ?
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- login
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- machine: machined: "Manage registrations of local VMs and containers"
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- machine-id-setup: gives a machine a random persistent ID
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- modules-load: load configured modules (for explicit loading, rather than on demand)
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- mount: transient mounts
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- network: various network services e.g. dhcp, lldp, route management, wifi management
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- notify: "Notify the init system about service status updates"
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- nspawn: "Spawn a command or OS in a light-weight container"
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- nss-*
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- oom: userspace OOM killer
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- partition: partitioning tools
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- pstore: ?
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- random-seed: load/save random seed at boot/shutdown
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- resolve: a full DNS resolver
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- rfkill: radio power control
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- run: "Run the specified command in a transient scope or service"
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- shared
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- shutdown
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- sleep: enter sleep, hibernate, or both (?)
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- sysctl: "Applies kernel sysctl settings"
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- sysusers: creates system user accounts
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- timedate, timesync: clock management, NTP etc
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- tmpfiles: creates hierarchy under volatile/tmpfs areas of the filesystem (!) eg /run, /tmp etc
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- udev (!!)
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- vconsole: setup virtual consoles for e.g. utf8 mode
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Unit names: "plain", "template", "instance" (of template).
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Unit files:
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- have a "scope" - "system", "global", "user". What does it mean?
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- have a "type":
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UNIT_SERVICE
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UNIT_MOUNT
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UNIT_SWAP
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UNIT_SOCKET
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UNIT_TARGET
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UNIT_DEVICE
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UNIT_AUTOMOUNT
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UNIT_TIMER
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UNIT_PATH
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UNIT_SLICE
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UNIT_SCOPE
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- have a "state":
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UNIT_ACTIVE
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UNIT_RELOADING
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UNIT_INACTIVE
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UNIT_FAILED
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UNIT_ACTIVATING
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UNIT_DEACTIVATING
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UNIT_MAINTENANCE
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Lots of code spent on argv parsing
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