Debian Stretch uses the rc.local service for executing commands on system startup. To use this service we need to run the following commands:
[Unit]
Description=/etc/rc.local Compatibility
ConditionPathExists=/etc/rc.local
[Service]
Type=forking
ExecStart=/etc/rc.local start
TimeoutSec=0
StandardOutput=tty
RemainAfterExit=yes
SysVStartPriority=99
[Install]
WantedBy=multi-user.target
sudo chmod +x /etc/systemd/system/rc-local.service
sudo chmod +x /etc/rc.local
sudo systemctl enable rc-local
sudo systemctl start rc-local.service
sudo systemctl status rc-local.service
#!/bin/sh -e
#
# rc.local
#
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
#
# In order to enable or disable this script just change the execution
# bits.
#
# By default this script does nothing.
exit 0
See the discussion How to Enable /etc/rc.local with Systemd for more information on getting /etc/rc.local to work on Debian 9