@Stefan Schippers - my spice file looks like below :
** sch_path: /home/ratul619/IIT_study_VM/xschem/sim_1t1r_IrefRcell.sch
**.subckt sim_1t1r_IrefRcell
R1 BL net1 5k m=1
XM1 net1 WL GND GND sky130_fd_pr__nfet_g5v0d10v5 L=0.5 W=1 nf=1 ad='int((nf+1)/2) * W/nf * 0.29' as='int((nf+2)/2) * W/nf * 0.29'
+ pd='2*int((nf+1)/2) * (W/nf + 0.29)' ps='2*int((nf+2)/2) * (W/nf + 0.29)' nrd='0.29 / W' nrs='0.29 / W'
+ sa=0 sb=0 sd=0 mult=1 m=1
C1 GND GND 1p m=1
V1 WL GND 3.3
V2 BL GND 0.2
**.ends
parameter sweep
* resistive divider, R1 swept from start_r to stop_r
* replaces .STEP R1 1k 10k 1k
* chapter 16.13.4.2
R1 1 2 1k
.control
let start_r = 1k
let stop_r = 10k
let delta_r = 1k
let r_act = start_r
* loop
while r_act le stop_r
alter r1 r_act
run
write dc-sweep.out v(2)
set appendwrite
let r_act = r_act + delta_r
end
*plot dc1.v(2) dc2.v(2) dc3.v(2) dc4.v(2) dc5.v(2)
*+ dc6.v(2) dc7.v(2) dc8.v(2) dc9.v(2) dc10.v(2)
.endc
.tran 0.01n 100n
.save all
.option savecurrents
.GLOBAL GND
.end
But ngspice is saying "parameter sweep - model name not found"
Now i tried to integrate the function into my spice file. The standalone function may seen working fine