Toward a direct measurement of the cosmic acceleration: roadmap and
  forecast on FAST
  HI absorption systems are great targets for direct measurement of the
Sandage-Loeb (SL) effect throughout a wide range of redshift for ground-based
radio telescopes. We demonstrate the significance of improving the accuracy of
SL effect measurement in cosmological model selection. With its wide sky
coverage and high sensitivity, we forecast that for 1 year of the upcoming
commensal survey (CRAFTS) the FAST telescope is capable of discovering about
800 HI absorption systems thereby improving the SL measurement accuracy. Aiming
to measurement the cosmic redshift drift rate at the precision of $\dot{z} \sim
10^{-10} \mathrm{decade^{-1}}$, we propose combined observation mode with
blind-searching and targeted observation. For a decade of consecutive targeted
spectroscopic observation with the frequency resolution at a level of sub-$0.1\
\rm Hz$, we could detect the first-order derivative of the cosmological
redshift with the expected precision.
