My contribution to bootableinstaller.com

create-bootable-installer-for-macos-big-sur.sh

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# bootableinstaller.com #
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set -e
set -u

# Create an empty
hdiutil create -o /tmp/empty -size 12800m -volname "Big Sur" -layout SPUD -fs HFS+J

# Rename to .img since this isn't a compressed disk image
mv /tmp/empty.dmg /tmp/bigsur.img

# Mount the empty disk image
hdiutil attach /tmp/bigsur.img -noverify -nobrowse -mountpoint '/Volumes/Install macOS Big Sur/'

# Create the installer image
sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --nointeraction --volume '/Volumes/Install macOS Big Sur/'

# Detach the installer
hdiutil detach '/Volumes/Install macOS Big Sur/'

# This is actually already in the correct format
mv /tmp/bigsur.img bigsur.iso

# Many thanks to https://support.apple.com/en-us/HT201372