服务「装了却起不来」、开机偶发失败、改完 unit 不生效，多数落在 依赖顺序、Type 类型、环境变量与 drop-in 覆盖 。本文把 systemd 服务管理从 unit 结构讲到 systemctl 排障，便于对照本机验证。
源码锚点
| 路径 / 手册 | 作用 |
|---|---|
man systemd.service |
Service 单元字段语义 |
man systemd.unit |
通用单元、依赖、条件 |
man systemctl |
启停、状态、编辑 |
/etc/systemd/system/ |
管理员单元与 drop-in |
/lib/systemd/system/ |
软件包默认单元 |
最小 service 示例：
ini
[Unit]
Description=Example daemon
After=network-online.target
Wants=network-online.target
[Service]
Type=simple
ExecStart=/usr/local/bin/mydaemon
Restart=on-failure
[Install]
WantedBy=multi-user.target调用链
#mermaid-svg-uiQxtSiPbxS1XlEI{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;fill:#333;}@keyframes edge-animation-frame{from{stroke-dashoffset:0;}}@keyframes dash{to{stroke-dashoffset:0;}}#mermaid-svg-uiQxtSiPbxS1XlEI .edge-animation-slow{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 50s linear infinite;stroke-linecap:round;}#mermaid-svg-uiQxtSiPbxS1XlEI .edge-animation-fast{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 20s linear infinite;stroke-linecap:round;}#mermaid-svg-uiQxtSiPbxS1XlEI .error-icon{fill:#552222;}#mermaid-svg-uiQxtSiPbxS1XlEI .error-text{fill:#552222;stroke:#552222;}#mermaid-svg-uiQxtSiPbxS1XlEI .edge-thickness-normal{stroke-width:1px;}#mermaid-svg-uiQxtSiPbxS1XlEI .edge-thickness-thick{stroke-width:3.5px;}#mermaid-svg-uiQxtSiPbxS1XlEI .edge-pattern-solid{stroke-dasharray:0;}#mermaid-svg-uiQxtSiPbxS1XlEI .edge-thickness-invisible{stroke-width:0;fill:none;}#mermaid-svg-uiQxtSiPbxS1XlEI .edge-pattern-dashed{stroke-dasharray:3;}#mermaid-svg-uiQxtSiPbxS1XlEI .edge-pattern-dotted{stroke-dasharray:2;}#mermaid-svg-uiQxtSiPbxS1XlEI .marker{fill:#333333;stroke:#333333;}#mermaid-svg-uiQxtSiPbxS1XlEI .marker.cross{stroke:#333333;}#mermaid-svg-uiQxtSiPbxS1XlEI svg{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;}#mermaid-svg-uiQxtSiPbxS1XlEI p{margin:0;}#mermaid-svg-uiQxtSiPbxS1XlEI .label{font-family:"trebuchet ms",verdana,arial,sans-serif;color:#333;}#mermaid-svg-uiQxtSiPbxS1XlEI .cluster-label text{fill:#333;}#mermaid-svg-uiQxtSiPbxS1XlEI .cluster-label span{color:#333;}#mermaid-svg-uiQxtSiPbxS1XlEI .cluster-label span p{background-color:transparent;}#mermaid-svg-uiQxtSiPbxS1XlEI .label text,#mermaid-svg-uiQxtSiPbxS1XlEI span{fill:#333;color:#333;}#mermaid-svg-uiQxtSiPbxS1XlEI .node rect,#mermaid-svg-uiQxtSiPbxS1XlEI .node circle,#mermaid-svg-uiQxtSiPbxS1XlEI .node ellipse,#mermaid-svg-uiQxtSiPbxS1XlEI .node polygon,#mermaid-svg-uiQxtSiPbxS1XlEI .node path{fill:#ECECFF;stroke:#9370DB;stroke-width:1px;}#mermaid-svg-uiQxtSiPbxS1XlEI .rough-node .label text,#mermaid-svg-uiQxtSiPbxS1XlEI .node .label text,#mermaid-svg-uiQxtSiPbxS1XlEI .image-shape .label,#mermaid-svg-uiQxtSiPbxS1XlEI .icon-shape .label{text-anchor:middle;}#mermaid-svg-uiQxtSiPbxS1XlEI .node .katex path{fill:#000;stroke:#000;stroke-width:1px;}#mermaid-svg-uiQxtSiPbxS1XlEI .rough-node .label,#mermaid-svg-uiQxtSiPbxS1XlEI .node .label,#mermaid-svg-uiQxtSiPbxS1XlEI .image-shape .label,#mermaid-svg-uiQxtSiPbxS1XlEI .icon-shape .label{text-align:center;}#mermaid-svg-uiQxtSiPbxS1XlEI .node.clickable{cursor:pointer;}#mermaid-svg-uiQxtSiPbxS1XlEI .root .anchor path{fill:#333333!important;stroke-width:0;stroke:#333333;}#mermaid-svg-uiQxtSiPbxS1XlEI .arrowheadPath{fill:#333333;}#mermaid-svg-uiQxtSiPbxS1XlEI .edgePath .path{stroke:#333333;stroke-width:2.0px;}#mermaid-svg-uiQxtSiPbxS1XlEI .flowchart-link{stroke:#333333;fill:none;}#mermaid-svg-uiQxtSiPbxS1XlEI .edgeLabel{background-color:rgba(232,232,232, 0.8);text-align:center;}#mermaid-svg-uiQxtSiPbxS1XlEI .edgeLabel p{background-color:rgba(232,232,232, 0.8);}#mermaid-svg-uiQxtSiPbxS1XlEI .edgeLabel rect{opacity:0.5;background-color:rgba(232,232,232, 0.8);fill:rgba(232,232,232, 0.8);}#mermaid-svg-uiQxtSiPbxS1XlEI .labelBkg{background-color:rgba(232, 232, 232, 0.5);}#mermaid-svg-uiQxtSiPbxS1XlEI .cluster rect{fill:#ffffde;stroke:#aaaa33;stroke-width:1px;}#mermaid-svg-uiQxtSiPbxS1XlEI .cluster text{fill:#333;}#mermaid-svg-uiQxtSiPbxS1XlEI .cluster span{color:#333;}#mermaid-svg-uiQxtSiPbxS1XlEI div.mermaidTooltip{position:absolute;text-align:center;max-width:200px;padding:2px;font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:12px;background:hsl(80, 100%, 96.2745098039%);border:1px solid #aaaa33;border-radius:2px;pointer-events:none;z-index:100;}#mermaid-svg-uiQxtSiPbxS1XlEI .flowchartTitleText{text-anchor:middle;font-size:18px;fill:#333;}#mermaid-svg-uiQxtSiPbxS1XlEI rect.text{fill:none;stroke-width:0;}#mermaid-svg-uiQxtSiPbxS1XlEI .icon-shape,#mermaid-svg-uiQxtSiPbxS1XlEI .image-shape{background-color:rgba(232,232,232, 0.8);text-align:center;}#mermaid-svg-uiQxtSiPbxS1XlEI .icon-shape p,#mermaid-svg-uiQxtSiPbxS1XlEI .image-shape p{background-color:rgba(232,232,232, 0.8);padding:2px;}#mermaid-svg-uiQxtSiPbxS1XlEI .icon-shape .label rect,#mermaid-svg-uiQxtSiPbxS1XlEI .image-shape .label rect{opacity:0.5;background-color:rgba(232,232,232, 0.8);fill:rgba(232,232,232, 0.8);}#mermaid-svg-uiQxtSiPbxS1XlEI .label-icon{display:inline-block;height:1em;overflow:visible;vertical-align:-0.125em;}#mermaid-svg-uiQxtSiPbxS1XlEI .node .label-icon path{fill:currentColor;stroke:revert;stroke-width:revert;}#mermaid-svg-uiQxtSiPbxS1XlEI :root{--mermaid-font-family:"trebuchet ms",verdana,arial,sans-serif;} systemctl start foo
systemd PID1
加载 unit / drop-in
满足 After/Requires
执行 ExecStart
按 Type 判定就绪
journal 记录
text
enable → 写 want 链接
start → 解析依赖 → fork/exec → 状态 active/failed
reload / daemon-reload → 重新加载单元定义重点知识
Type 决定「成功」含义
simple：ExecStart 一拉起即认为启动（默认常见）。forking：旧 daemon 双 fork，需PIDFile=。oneshot：跑完即结束；常配RemainAfterExit=yes。notify：就绪后经sd_notify告知，适合真正要等初始化完成的服务。
选错 Type 会出现「systemctl 显示 active 但端口未听」或「一直 activating」。
依赖与启动顺序
After= 只排序不强制依赖；Requires=/Wants= 才表达依赖强弱。网络类服务优先 After=network-online.target，并理解有的镜像没有真正的 online。
drop-in 覆盖
bash
systemctl edit foo.service
# 生成 /etc/systemd/system/foo.service.d/override.conf
systemctl daemon-reload
systemctl restart foo不要直接改 /lib/systemd/system/ 里的包文件，升级会被覆盖。
排障命令
bash
systemctl status foo.service -l
systemctl show foo -p FragmentPath -p DropInPaths -p ActiveState
journalctl -u foo -b --no-pager | tail -100
systemctl cat foo
systemd-analyze verify /etc/systemd/system/foo.service失败常见原因：ExecStart 路径错、权限不足、缺环境变量、Type=forking 无 PIDFile、依赖的 socket/网络未好。
嵌入式注意
只读根上 unit 放可写覆盖目录；精简系统若无 network-online.target，避免死等。容器里 PID1 不一定是 systemd，命令集合不同。