systemd 是linux操作系统的系统和服务管理器，作为boot启动后的第一个进程（PID=1），systemd扮演初始化系统，以及启动和维护用户空间服务。
为了兼容SysV，如果systemd初始化被调用并且进程id不是1，它将执行telinit，并且将同步参数到该应用，所以在正常系统登录状态下的init和telinit是等同的。更多信息通过man telinit 了解。
当作为系统实例启动，systemd通过system.conf 解析配置文件，如果作为用户进程启动，从user.conf解析配置文件,更多信息通过systemd-system.conf 了解。
系统同时存在sysV init脚本使能和systemd service，那么到底是生谁效的？
如果同时存在systemd 单元文件和sysVinit脚本，亦或者upstart配置文件，理论上systemd>sysv init>upstart.
如下路径的unit 关系，哪些unit是使能的?
参考 man systemctl->COMMANDS->Unit File Commands-> enable NAME...
在/lib/systemd/system 保存了所有unit file，其中部分.service 文件包含[install] 部分，当调用systemctl enable watchdog.service 会在 /etc/systemd/systemd下面创建软连接。开机通过查找该路径下面的软连接启动对应服务。
$ cat watchdog.service
[Install]
WantedBy=multi-user.target$ tree /etc/systemd/system
/etc/systemd/system
├── multi-user.target.wants
│ ├── watchdog.service -> /lib/systemd/system/watchdog.service可以通过systemctl list-unit-files |grep enable查找所有使能的unit文件，同时可以通过systemctl preset-all复位所有使能的unit file。
systemd如何控制sysV init脚本?
/lib/systemd/system/supervisor.service
/etc/systemd/system/multi-user.target.wants/supervisor.service #软连接如果同时存在supervisor.service 和对应软连接，此时启动由systemd控制，并且状态为enable。
$ systemctl list-unit-files |grep supervisor
supervisor.service enabled如果存在superviosr.service不存在对应的软连接，此时启动有systemd控制，状态为disable,开机不启动supervisor服务。
$ systemctl list-unit-files |grep supervisor
supervisor.service disabled/etc/init.d/supervisor
/etc/rc5.d/S01supervisor如果同时存在supervisor和S01supervisor 并且不存在如上的systemd 单元文件，那么此时启动由sysVinit控制。
如上问题还是只有从man systemctl 当中找答案哦。
systemctl (1) - Control the systemd system and service manager
systemd (1) - systemd system and service manager
systemd-activate (8) - Test socket activation of daemons
systemd-analyze (1) - Analyze system boot-up performance
systemd-ask-password (1) - Query the user for a system password
systemd-ask-password-console.path (8) - Query the user for system passwords on the console and via wall
systemd-ask-password-console.service (8) - Query the user for system passwords on the console and via wall
systemd-ask-password-wall.path (8) - Query the user for system passwords on the console and via wall
systemd-ask-password-wall.service (8) - Query the user for system passwords on the console and via wall
systemd-backlight (8) - Load and save the display backlight brightness at boot and shutdown
systemd-backlight@.service (8) - Load and save the display backlight brightness at boot and shutdown
systemd-binfmt (8) - Configure additional binary formats for executables at boot
systemd-binfmt.service (8) - Configure additional binary formats for executables at boot
systemd-bootchart (1) - Boot performance graphing tool
systemd-cat (1) - Connect a pipeline or program's output with the journal
systemd-cgls (1) - Recursively show control group contents
systemd-cgtop (1) - Show top control groups by their resource usage
systemd-cryptsetup (8) - Full disk decryption logic
systemd-cryptsetup-generator (8) - Unit generator for /etc/crypttab
systemd-cryptsetup@.service (8) - Full disk decryption logic
systemd-debug-generator (8) - Generator for enabling a runtime debug shell and masking specific units at boot
systemd-delta (1) - Find overridden configuration files
systemd-detect-virt (1) - Detect execution in a virtualized environment
systemd-efi-boot-generator (8) - Generator for automatically mounting the EFI System Partition used by the current boot to /boot
systemd-escape (1) - Escape strings for usage in system unit names
systemd-fsck (8) - File system checker logic
systemd-fsck-root.service (8) - File system checker logic
systemd-fsck@.service (8) - File system checker logic
systemd-fstab-generator (8) - Unit generator for /etc/fstab
systemd-getty-generator (8) - Generator for enabling getty instances on the console
systemd-gpt-auto-generator (8) - Generator for automatically discovering and mounting root, /home and /srv partitions, as well as discovering and enabling swap partitions, based on GPT par...
systemd-halt.service (8) - System shutdown logic
systemd-hibernate.service (8) - System sleep state logic
systemd-hostnamed (8) - Host name bus mechanism
systemd-hostnamed.service (8) - Host name bus mechanism
systemd-hybrid-sleep.service (8) - System sleep state logic
systemd-inhibit (1) - Execute a program with an inhibition lock taken
systemd-initctl (8) - /dev/initctl compatibility
systemd-initctl.service (8) - /dev/initctl compatibility
systemd-initctl.socket (8) - /dev/initctl compatibility
systemd-journald (8) - Journal service
systemd-journald-dev-log.socket (8) - Journal service
systemd-journald.service (8) - Journal service
systemd-journald.socket (8) - Journal service
systemd-kexec.service (8) - System shutdown logic
systemd-localed (8) - Locale bus mechanism
systemd-localed.service (8) - Locale bus mechanism
systemd-logind (8) - Login manager
systemd-logind.service (8) - Login manager
systemd-machine-id-setup (1) - Initialize the machine ID in /etc/machine-id
systemd-machined (8) - Virtual machine and container registration manager
systemd-machined.service (8) - Virtual machine and container registration manager
systemd-modules-load (8) - Configure kernel modules to load at boot
systemd-modules-load.service (8) - Configure kernel modules to load at boot
systemd-networkd (8) - Network manager
systemd-networkd-wait-online (8) - Wait for network to come online
systemd-networkd-wait-online.service (8) - Wait for network to come online
systemd-networkd.service (8) - Network manager
systemd-notify (1) - Notify service manager about start-up completion and other daemon status changes
systemd-nspawn (1) - Spawn a namespace container for debugging, testing and building
systemd-path (1) - List and query system and user paths
systemd-poweroff.service (8) - System shutdown logic
systemd-quotacheck (8) - File system quota checker logic
systemd-quotacheck.service (8) - File system quota checker logic
systemd-random-seed (8) - Load and save the system random seed at boot and shutdown
systemd-random-seed.service (8) - Load and save the system random seed at boot and shutdown
systemd-readahead (8) - Disk read ahead logic
systemd-readahead-collect.service (8) - Disk read ahead logic
systemd-readahead-done.service (8) - Disk read ahead logic
systemd-readahead-done.timer (8) - Disk read ahead logic
systemd-readahead-replay.service (8) - Disk read ahead logic
systemd-reboot.service (8) - System shutdown logic
systemd-remount-fs (8) - Remount root and kernel file systems
systemd-remount-fs.service (8) - Remount root and kernel file systems
systemd-resolved (8) - Network Name Resolution manager
systemd-resolved.service (8) - Network Name Resolution manager
systemd-rfkill (8) - Load and save the RF kill switch state at boot and shutdown
systemd-rfkill@.service (8) - Load and save the RF kill switch state at boot and shutdown
systemd-run (1) - Run programs in transient scope or service units
systemd-shutdown (8) - System shutdown logic
systemd-shutdownd (8) - Scheduled shutdown service
systemd-shutdownd.service (8) - Scheduled shutdown service
systemd-shutdownd.socket (8) - Scheduled shutdown service
systemd-sleep (8) - System sleep state logic
systemd-sleep.conf (5) - Suspend and hibernation configuration file
systemd-socket-proxyd (8) - Bidirectionally proxy local sockets to another (possibly remote) socket.
systemd-suspend.service (8) - System sleep state logic
systemd-sysctl (8) - Configure kernel parameters at boot
systemd-sysctl.service (8) - Configure kernel parameters at boot
systemd-system-update-generator (8) - Generator for redirecting boot to offline update mode
systemd-system.conf (5) - System and session service manager configuration file
systemd-sysusers (8) - Allocate system users and groups
systemd-sysusers.service (8) - Allocate system users and groups
systemd-timedated (8) - Time and date bus mechanism
systemd-timedated.service (8) - Time and date bus mechanism
systemd-timesyncd (8) - Network Time Synchronization
systemd-timesyncd.service (8) - Network Time Synchronization
systemd-tmpfiles (8) - Creates, deletes and cleans up volatile and temporary files and directories
systemd-tmpfiles-clean.service (8) - Creates, deletes and cleans up volatile and temporary files and directories
systemd-tmpfiles-clean.timer (8) - Creates, deletes and cleans up volatile and temporary files and directories
systemd-tmpfiles-setup-dev.service (8) - Creates, deletes and cleans up volatile and temporary files and directories
systemd-tmpfiles-setup.service (8) - Creates, deletes and cleans up volatile and temporary files and directories
systemd-tty-ask-password-agent (1) - List or process pending systemd password requests
systemd-udevd (8) - Device event managing daemon
systemd-udevd-control.socket (8) - Device event managing daemon
systemd-udevd-kernel.socket (8) - Device event managing daemon
systemd-udevd.service (8) - Device event managing daemon
systemd-update-utmp (8) - Write audit and utmp updates at bootup, runlevel changes and shutdown
systemd-update-utmp-runlevel.service (8) - Write audit and utmp updates at bootup, runlevel changes and shutdown
systemd-update-utmp.service (8) - Write audit and utmp updates at bootup, runlevel changes and shutdown
systemd-user-sessions (8) - Permit user logins after boot, prohibit user logins at shutdown
systemd-user-sessions.service (8) - Permit user logins after boot, prohibit user logins at shutdown
systemd-user.conf (5) - System and session service manager configuration file
systemd.automount (5) - Automount unit configuration
systemd.device (5) - Device unit configuration
systemd.directives (7) - Index of configuration directives
systemd.exec (5) - Execution environment configuration
systemd.index (7) - List all manpages from the systemd project
systemd.journal-fields (7) - Special journal fields
systemd.kill (5) - Process killing procedure configuration
systemd.link (5) - Network device configuration
systemd.mount (5) - Mount unit configuration
systemd.netdev (5) - Virtual Network Device configuration
systemd.network (5) - Network configuration
systemd.path (5) - Path unit configuration
systemd.preset (5) - Service enablement presets
systemd.resource-control (5) - Resource control unit settings
systemd.scope (5) - Scope unit configuration
systemd.service (5) - Service unit configuration
systemd.slice (5) - Slice unit configuration
systemd.snapshot (5) - Snapshot unit configuration
systemd.socket (5) - Socket unit configuration
systemd.special (7) - Special systemd units
systemd.swap (5) - Swap unit configuration
systemd.target (5) - Target unit configuration
systemd.time (7) - Time and date specifications
systemd.timer (5) - Timer unit configuration
systemd.unit (5) - Unit configuration