Set update channel to receive developer beta update

sudo softwareupdate --set-catalog https://swscan.apple.com/content/catalogs/others/index-10.11seed-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz

Set update channel to receive public beta update

sudo softwareupdate --set-catalog https://swscan.apple.com/content/catalogs/others/index-10.11beta-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz

List available updates

sudo softwareupdate --list

Set update channel to receive default, stable updates

sudo softwareupdate --clear-catalog

Show current settings

defaults read /Library/Preferences/com.apple.SoftwareUpdate.plist

Write setting manually

defaults write /Library/Preferences/com.apple.SoftwareUpdate CatalogURL https://swscan.apple.com/content/catalogs/others/index-10.11beta-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz