# In Terminal: cd /Applications hdiutil create -o ~/Desktop/Catalina10.15.7.dmg -srcfolder "Install macOS Catalina.app" -format UDZO This generates a compressed DMG file on your Desktop, which is a perfect backup for offline installations. For advanced users or those who cannot access the App Store (e.g., running a newer Mac or in a managed environment), Apple’s command-line tool is the safest bet.
softwareupdate --list-full-installers You should see output like: Mac Os Catalina 10.15 7 Dmg Download
To verify your DMG: