  Attosecond pulses are fundamental for the investigation of valence and
core-electron dynamics on their natural timescale. At present the reproducible
generation and characterisation of attosecond waveforms has been demonstrated
only through the process of high-order harmonic generation. Several methods for
the shaping of attosecond waveforms have been proposed, including metallic
filters, multilayer mirrors and manipulation of the driving field. However,
none of these approaches allow for the flexible manipulation of the temporal
characteristics of the attosecond waveforms, and they suffer from the low
conversion efficiency of the high-order harmonic generation process. Free
Electron Lasers, on the contrary, deliver femtosecond, extreme ultraviolet and
X-ray pulses with energies ranging from tens of $\mathrm{\mu}$J to a few mJ.
Recent experiments have shown that they can generate sub-fs spikes, but with
temporal characteristics that change shot-to-shot. Here we show the first
demonstration of reproducible generation of high energy ($\mathrm{\mu}$J level)
attosecond waveforms using a seeded Free Electron Laser. We demonstrate
amplitude and phase manipulation of the harmonic components of an attosecond
pulse train in combination with a novel approach for its temporal
reconstruction. The results presented here open the way to perform attosecond
time-resolved experiments with Free Electron Lasers.
