They are generally abbreviated with the symbol for any organyl group, R, although Alk is sometimes used to specifically symbolize an alkyl group (as opposed to an alkenyl group or aryl group).
ðeɪ ɑːɹ dʒˈɛnɚɹəli ɐbɹˈiːvɪˌeɪɾᵻd wɪððə sˈɪmbəl fɔːɹ ˌɛni ˈɔːɹɡɐnˌaɪl ɡɹˈuːp, ˈɑːɹ, ɔːlðˈoʊ ˈælk ɪz sˈʌmtaɪmz jˈuːzd tə spəsˈɪfɪkli sˈɪmbəlˌaɪz ɐn ˈælkaɪl ɡɹˈuːp æz əpˈoʊzd tʊ ɐn ˈælkənˌaɪl ɡɹˈuːp ɔːɹ ˈɑːɹɹɪl ɡɹˈuːp.