# Complete group names for first non-option chgrp(1) argument
_chgrp() {
    local i
    for ((i = 1; i < COMP_CWORD; i++)) ; do
        case ${COMP_WORDS[i]} in
            -*) ;;
            *) return 1 ;;
        esac
    done
    while read -r group ; do
        COMPREPLY[${#COMPREPLY[@]}]=$group
    done < <(compgen -A group -- "$2")
}
complete -F _chgrp -o bashdefault -o default chgrp