1、systemd默认读取/etc/systemd/system下的配置文件，该目录下的文件会链接/lib/systemd/system/下的文件。一般系统安装完/lib/systemd/system/下会有rc-local.service文件，即我们需要的配置文件。
创建软链接：
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ln -fs /lib/systemd/system/rc-local.service /etc/systemd/system/rc-local.service
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cd /etc/systemd/system/
cat rc-local.service
rc-local.service内容如下
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# SPDX-License-Identifier: LGPL-2.1+
#
# This file is part of systemd.
#
# systemd is free software; you can redistribute it and/or modify it
# under the terms of the GNU Lesser General Public License as published by
# the Free Software Foundation; either version 2.1 of the License, or
# (at your option) any later version.
# This unit gets pulled automatically into multi-user.target by
# systemd-rc-local-generator if /etc/rc.local is executable.
[Unit]
Description=/etc/rc.local Compatibility
Documentation=man:systemd-rc-local-generator(8)
ConditionFileIsExecutable=/etc/rc.local
After=network.target
[Service]
Type=forking
ExecStart=/etc/rc.local start
TimeoutSec=0
RemainAfterExit=yes
GuessMainPID=no
[Install]
WantedBy=multi-user.target Alias=rc-local.service
区块解释：
[Unit]：启动顺序与依赖关系。
[Service]：启动行为,如何启动，启动类型。
[Install] ：定义如何安装这个配置文件，即怎样做到开机启动。
2、创建/etc/rc.local文件，赋可执行权限
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sudo touch /etc/rc.local
sudo chmod 755 /etc/rc.local
3、编辑/etc/rc.local，添加需要开机启动的任务
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#!/bin/bash
echo "test " > /var/test.log
4、重启系统，然后查看test.log
