bash pour boucle avec des décimales

you cannot do that, bash do not support floting arithmatics.
use bash sequences indead
bash for loos do not start from fractions like 0.5 0.25 .. only support intigers 

for i in $(seq 3.0 0.1 4.5); 
do 
  echo $i; 
done
Hutch Polecat