Extract macOS release and build versions from a macOS Big Sur installer. Pass it a path to an InstallAssistant app, like so: ./get-macos-build.sh "/Applications/Install macOS Big Sur.app/"

get-macos-build.sh

#!/bin/sh

readonly installAssistant="$1"

# Display error messages and exit script
error() {
  echo "[ERROR] $*"
  exit 1
}

# Check operating system
isOs() {
  if [ ! "$(/usr/bin/uname)" = "$1" ]; then return 1; fi
}

# Check if parameter $1 has been passed in to script
checkParams() {
  if [ ! "$installAssistant" ] ; then return 1; fi
}

# Attach SharedSupport.dmg
attachDmg() {
  dmgPath="${installAssistant}/Contents/SharedSupport/SharedSupport.dmg"

  if [ ! -f "$dmgPath" ]; then
    error "Could not find '$(basename "$dmgPath")'. Is this a macOS Big Sur InstallAssistant app?"
  fi

  /usr/bin/hdiutil attach \
    -nobrowse \
    -noverify \
    -quiet \
    "$dmgPath"
}

# Extract "OSVersion" and "Build" keys from com_apple_MobileAsset_MacSoftwareUpdate.xml
parsePlist() {
  path="/Volumes/Shared Support/com_apple_MobileAsset_MacSoftwareUpdate/com_apple_MobileAsset_MacSoftwareUpdate.xml"

  if [ ! -f "$path" ]; then
    error "Could not parse 'com_apple_MobileAsset_MacSoftwareUpdate.xml'"
  fi

  osVersion=$(/usr/libexec/PlistBuddy \
    -c "Print :Assets:0:OSVersion" \
    "$path")

  buildVersion=$(/usr/libexec/PlistBuddy \
    -c "Print :Assets:0:Build" \
    "$path")
}

cleanUp() {
  # Detach SharedSupport volume
  volume="/Volumes/Shared Support/"
  /usr/bin/hdiutil detach \
    -quiet \
    "$volume"
}

trap cleanUp INT TERM EXIT
isOs Darwin || error "It seems you are not running macOS. Exiting …"
checkParams || error "Please provide path to InstallAssistant app, e.g. $0 \"/Applications/Install macOS Big Sur.app/\""
attachDmg
parsePlist
echo "macOS version:" "$osVersion"
echo "Build version:" "$buildVersion"