"grep -Ff " doesn't highlighted pattern with colour in bash script

Question

while running below command in terminal it's working fine with highlighted red and white colour matching with the pattern as per image below,

$ grep -Ff file1.txt file2.txt

Output:

But when I put the same command in script as pl.sh file and run it it's not highlighting at all as above image. I am not sure what I am doing wrong! Do I need to change the script?

#!/bin/bash

# Main file:
echo -n "Choose the Main Assignment File : "
read mainfile

# Compare a file
echo -n "Choose a file to compare with : "
read comparefile

# Compare two files and highlight differences 
sudo grep -Ff "$mainfile" "$comparefile"

Answer 1

Your interactive shell likely has an alias that re-defines grep to use color when outputing to a terminal device ex. alias grep='grep --color=auto' (perhaps defined in the default ~/.bashrc file).

Bash non-interactive scripts don't expand aliases by default - and neither does sudo. So you'd need to add the color option to grep explicitly ex.

grep --color=auto -Ff "$mainfile" -- "$comparefile"