list installed casks that have newer versions available https://caskroom.github.io/

brew-cask-outdated

#!/usr/bin/env bash
set -e

contains() {
    local needle=$1
    for x in "${@:2}"; do
        [[ "$x" == "$needle" ]] && return 0
    done
    return 1
}

main() {
    brew cask list --versions | while read -r -a versions; do
        cask=${versions[0]}
        unset versions[0]
        version=$(brew cask _stanza version "${cask}")
        if ! contains "${version}" "${versions[@]}"; then
            latest=${versions[*]: -1}
            if [[ -t 1 ]]; then
                echo "${cask}: ${latest} -> ${version}"
            else
                echo "${cask}"
            fi
        fi
    done
}

main "$@"