There is a growing interest in the generation and characterization of femtosecond and sub- femtosecond pulses from linac-based free-electron lasers (FELs). In this report, we present a simple, single-shot method for measuring the longitudinal bunch pro_le of ultrashort electron bunches. The method uses a chicane followed by an rf linac and is based on a longitudinal transformation of K. Ricci and T. Smith [Phys. Rev. ST-AB 3, 032801 (2000)]. We show that this method can be applied in a straightforward manner at x-ray FEL facilities such as the Linac Coherent Light Source by slightly adjusting the second bunch compressor followed by running the bunch on an rf zero-crossing phase of the final linac. After taking into account the linac wakefield, we find the condition under which the final beam energy spread corresponds directly to the compressed bunch length. When used in conjunction with a high-resolution electron spectrometer, this method potentially reveals the temporal pro_le of the electron beam down to the femtosecond and sub-femotsecond scale. Show Partial Abstract