import sys

input_file = sys.argv[1]
output_file = sys.argv[2]
val = sys.argv[3]

print("update_poscar.py:")
print(f"  input : {input_file}")
print(f"  output: {output_file}")
print(f"  kc    : {val}")
with open(input_file, 'r') as f:
    lines = f.readlines()

lines[1] = f"{val}\n"

with open(output_file, 'w') as f:
    f.writelines(lines)