I once noticed that by default there is no /etc/rc.local file in Ubuntu 18, but it can be created.
To do this, switch to the root user:
sudo su
Create a file /etc/rc.local for example with the nano editor (Ctrl+X to exit the editor, y/n to save or cancel changes):
nano /etc/rc.local
Add content to it like in Ubuntu 16:
#!/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
Let’s make the file executable:
chmod +x /etc/rc.local
Done, now you can write your own commands and they will be executed when the operating system starts, the last line must remain “exit 0”.
See also my articles:
Run application at startup in Ubuntu
Using and configuring CRON