sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
This article provides step-by-step instructions to obtain the official installer and create a portable DMG file for backup or multiple installations. mac os high sierra download dmg
How to download the complete installation… - Apple Community sudo /Applications/Install\ macOS\ High\ Sierra
If you manage to download the macOS High Sierra installer from the App Store, you can create a bootable installer on a USB drive or obtain the DMG file through other means (ensure through legitimate sources). mac os high sierra download dmg