How SYSTEMd uses / etc / init D script

1. With apport For example , take /etc/init.d/apport Copy and generate new myapport, The contents are as follows ：

#! /bin/sh

### BEGIN INIT INFO
# Provides:        myapport
# Required-Start:  $local_fs $remote_fs
# Required-Stop:   $local_fs $remote_fs
# Default-Start:   2 3 4 5
# Default-Stop:
# Short-Description: myapport init.d with systemd test"
### END INIT INFO

DESC="myapport init.d with systemd test"
NAME=myapport
SCRIPTNAME=/etc/init.d/$NAME


# Define LSB log_* functions.
# Depend on lsb-base (>= 3.0-6) to ensure that this file is present.
. /lib/lsb/init-functions

#
# Function that starts the daemon/service
#
do_start()
{
    # Return
    #   0 if daemon has been started
    #   1 if daemon was already running
    #   2 if daemon could not be started

    echo "myapport started"
    return 0
}

#
# Function that stops the daemon/service
#
do_stop()
{
    # Return
    #   0 if daemon has been stopped
    #   1 if daemon was already stopped
    #   2 if daemon could not be stopped
    #   other if a failure occurred

    echo "myapport stopped"
    return 0
}

case "$1" in
	start)
    [ "$VERBOSE" != no ] && log_daemon_msg "Starting $DESC:" "$NAME"
    do_start
    case "$?" in
        0|1) [ "$VERBOSE" != no ] && log_end_msg 0 ;;
        2) [ "$VERBOSE" != no ] && log_end_msg 1 ;;
    esac
    ;;
  stop)
    [ "$VERBOSE" != no ] && log_daemon_msg "Stopping $DESC:" "$NAME"
    do_stop
    case "$?" in
        0|1) [ "$VERBOSE" != no ] && log_end_msg 0 ;;
        2) [ "$VERBOSE" != no ] && log_end_msg 1 ;;
    esac
    ;;
  restart|force-reload)
    $0 stop || true
    $0 start
    ;;
  *)
    echo "Usage: $SCRIPTNAME {start|stop|restart|force-reload}" >&2
    exit 3
    ;;
esac

2. stay /etc/rc?.d In any one (? Value 2 3 4 5), Establish symbolic links , Point to /etc/init.d/myapport

lrwxrwxrwx 1 root root 18 9 month   29 16:43 S01myapport -> ../init.d/myapport

notes ： If you don't establish relevant symbolic links , It means that SysV The service is not enabled , Then... Will not be executed when the system starts .

3.  Restart the machine and check myapport Service related information

$ systemctl status myapport
● myapport.service - LSB: myapport init.d with systemd test"
   Loaded: loaded (/etc/init.d/myapport; generated)
   Active: inactive (dead) since Wed 2021-09-29 16:56:23 CST; 6s ago
     Docs: man:systemd-sysv-generator(8)
  Process: 2771 ExecStop=/etc/init.d/myapport stop (code=exited, status=0/SUCCESS)
  Process: 979 ExecStart=/etc/init.d/myapport start (code=exited, status=0/SUCCESS)

9 month  29 16:54:25 eric-vm-dev systemd[1]: Starting LSB: myapport init.d with systemd test"...
9 month  29 16:54:29 eric-vm-dev myapport[979]:  * Starting myapport init.d with systemd test: myapport
9 month  29 16:54:29 eric-vm-dev myapport[979]: myapport started
9 month  29 16:54:29 eric-vm-dev myapport[979]:    ...done.
9 month  29 16:54:29 eric-vm-dev systemd[1]: Started LSB: myapport init.d with systemd test".

$ systemctl list-unit-files | grep myapport
myapport.service                           generated

notes ：generated Indicates that the service unit is composed of systemd-sysv-generator Application generation .

$ systemctl cat myapport
# /run/systemd/generator.late/myapport.service
# Automatically generated by systemd-sysv-generator

[Unit]
Documentation=man:systemd-sysv-generator(8)
SourcePath=/etc/init.d/myapport
Description=LSB: myapport init.d with systemd test"
Before=graphical.target
After=remote-fs.target

[Service]
Type=forking
Restart=no
TimeoutSec=5min
IgnoreSIGPIPE=no
KillMode=process
GuessMainPID=no
RemainAfterExit=yes
SuccessExitStatus=5 6
ExecStart=/etc/init.d/myapport start
ExecStop=/etc/init.d/myapport stop

Reference resources ：

How does systemd use /etc/init.d scripts? - Unix & Linux Stack Exchangehttps://unix.stackexchange.com/questions/233468/how-does-systemd-use-etc-init-d-scripts

https://qastack.cn/unix/233468/how-does-systemd-use-etc-init-d-scriptshttps://qastack.cn/unix/233468/how-does-systemd-use-etc-init-d-scripts