沒有default.target，systemd如何啟動？

systemd 文件和各種線上資源都聲明 systemd 應該（預設情況下）啟動名為“default.target”的“特殊”目標。可以通過 systemctl set-default 更改此設置，這基本上是從 default.target 到所需目標的符號連結。

但是在我的系統中沒有這樣的文件。沒有/etc/systemd/system/default.target，沒有/usr/lib/systemd/system/default.target（實際上這個文件在整個系統中是不存在的）。系統仍在啟動。我的問題是怎麼可能？

（當然我可以自己做一個符號連結，我只是想知道如何在沒有 default.target 的情況下定義一個預設值）

附加資訊：

# systemctl get-default
graphical.target

所以我的預設目標是graphics.target。但它在哪裡（以及如何）定義？

# find /usr/lib -iname "default.target" # no results
# find /etc -iname "default.target" # no results
# lsb_release -a
LSB Version:    n/a
Distributor ID: Gentoo
Description:    Gentoo Base System release 2.4.1
Release:    2.4.1
Codename:   n/a

# systemctl list-units --type=target
UNIT                  LOAD   ACTIVE SUB    DESCRIPTION                  
basic.target          loaded active active Basic System                 
getty.target          loaded active active Login Prompts                
graphical.target      loaded active active Graphical Interface          
local-fs-pre.target   loaded active active Local File Systems (Pre)     
local-fs.target       loaded active active Local File Systems           
machines.target       loaded active active Containers                   
multi-user.target     loaded active active Multi-User System            
network-online.target loaded active active Network is Online            
network.target        loaded active active Network                      
nss-lookup.target     loaded active active Host and Network Name Lookups
paths.target          loaded active active Paths                        
remote-fs.target      loaded active active Remote File Systems          
slices.target         loaded active active Slices                       
sockets.target        loaded active active Sockets                      
sound.target          loaded active active Sound Card                   
swap.target           loaded active active Swap                         
sysinit.target        loaded active active System Initialization        
timers.target         loaded active active Timers                       

LOAD   = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB    = The low-level unit activation state, values depend on unit type.

18 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.

Gentoo將 systemd 文件的位置更改/lib/systemd為 2017 年 7 月。在我的系統上，我可以在以下位置看到預設符號連結/lib/systemd/system：

$ ls -l /lib/systemd/system/default.target
lrwxrwxrwx 1 root root 16 Apr  2 15:48 /lib/systemd/system/default.target -> graphical.target

如果您添加符號連結/etc/systemd/system（就像這樣systemctl set-default做），它將覆蓋分發的預設設置。

引用自：https://unix.stackexchange.com/questions/435324