Systemd进程启动

分析Linux系统进程何启动起来

参考文档

• systemd (简体中文)
• udev (简体中文)
• https://www.raspberrypi.org/documentation/linux/usage/systemd.md
• https://www.raspberrypi.org/documentation/linux/usage/rc-local.md

init真相

• stat /sbin/init

    File: /sbin/init -> /lib/systemd/systemd
    Size: 20              Blocks: 0          IO Block: 4096   symbolic link
  Device: b302h/45826d    Inode: 286014      Links: 1
  Access: (0777/lrwxrwxrwx)  Uid: (    0/    root)   Gid: (    0/    root)
  Access: 2021-01-11 13:31:33.086942487 +0000
  Modify: 2020-12-10 00:18:16.000000000 +0000
  Change: 2021-01-11 13:31:33.086942487 +0000
   Birth: -

Systemd应用原理

在sysvinit中有明确定义的运行级别（如：0、1、3、5、6）与systemd中特定的 目标 存在一一对应的关系。然而，对于用户自定义运行级别（2、4）却没有。如需要同样功能，建议你以原有运行级别所对应的systemd目标为基础，新建一个/etc/systemd/system/<目标名>.target（可参考/usr/lib/systemd/system/graphical.target）, 然后创建目录/etc/systemd/system/<目标名>.wants，并向其中加入需启用的服务链接（指向/ur/lib/systemd/system/）。

"SysV 运行级别" 与 "systemd 目标" 对照表

• systemctl get-default

  graphical.target
• cd /etc/systemd
• find * -iname default.target

  system/default.target
• ls -al system/default.target

  lrwxrwxrwx 1 root root 36 Jan 11 13:01 system/default.target -> /lib/systemd/system/graphical.target
• cat /lib/systemd/system/graphical.target

  # ...省略
  
  [Unit]
  Description=Graphical Interface
  Documentation=man:systemd.special(7)
  Requires=multi-user.target
  Wants=display-manager.service
  Conflicts=rescue.service rescue.target
  After=multi-user.target rescue.service rescue.target display-manager.service
  AllowIsolate=yes
  • Requires=multi-user.target

rc.local启动分析

• systemctl status rc.local

  Warning: The unit file, source configuration file or drop-ins of rc-local.service changed on disk. Run 'systemctl daemon-reload' to reload units.
  ● rc-local.service - /etc/rc.local Compatibility
     Loaded: loaded (/lib/systemd/system/rc-local.service; enabled-runtime; vendor preset: enabled)
    Drop-In: /usr/lib/systemd/system/rc-local.service.d
             └─debian.conf
             /etc/systemd/system/rc-local.service.d
             └─ttyoutput.conf
     Active: active (exited) since Thu 2021-02-11 08:08:35 GMT; 6h ago
       Docs: man:systemd-rc-local-generator(8)
    Process: 461 ExecStart=/etc/rc.local start (code=exited, status=0/SUCCESS)
  
  Feb 11 08:08:35 raspberrypi systemd[1]: Starting /etc/rc.local Compatibility...
  Feb 11 08:08:35 raspberrypi systemd[1]: Started /etc/rc.local Compatibility.
  • Loaded: loaded (/lib/systemd/system/rc-local.service; enabled-runtime; vendor preset: enabled)
    • /lib/systemd/system/rc-local.service
  • Active: active (exited) since Thu 2021-02-11 08:08:35 GMT; 6h ago
• 

Wifi配置启动分析

• /etc/systemd/system/multi-user.target.wants/raspberrypi-net-mods.service

  [Unit]
  Description=Copy user wpa_supplicant.conf
  ConditionPathExists=/boot/wpa_supplicant.conf
  Before=dhcpcd.service
  After=systemd-rfkill.service
  
  [Service]
  Type=oneshot
  RemainAfterExit=yes
  ExecStart=/bin/mv /boot/wpa_supplicant.conf /etc/wpa_supplicant/wpa_supplicant.conf
  ExecStartPost=/bin/chmod 600 /etc/wpa_supplicant/wpa_supplicant.conf
  ExecStartPost=/usr/sbin/rfkill unblock wifi
  
  [Install]
  WantedBy=multi-user.target

SSH开启设置分析

• /etc/systemd/system/multi-user.target.wants/sshswitch.service

  [Unit]
  Description=Turn on SSH if /boot/ssh is present
  ConditionPathExistsGlob=/boot/ssh{,.txt}
  After=regenerate_ssh_host_keys.service
  
  [Service]
  Type=oneshot
  ExecStart=/bin/sh -c "systemctl enable --now ssh && rm -f /boot/ssh ; rm -f /boot/ssh.txt"
  
  [Install]
  WantedBy=multi-user.target