How can i compare strings of two files in shell script?

Question

The first field is the username, the second is the password, and the last indicates the action (login/register)

File 1: User input

hello,world,register

File 2: Plain text database

admin,123
user,321
foo,bar

How can i compare the user input with the database to check if the username already exists?

I´ve tried the following code, but it´s not working:

if cut -d "," -f1 user_input | grep -wf user_input database; then
    echo "This username is already in use, try again!"
else
    echo "Successfully registered!"
fi

PS: i need a solution without awk,sed or find, just grep and cut

Answer 1

grep + cut approach:

if grep -qwf <(cut -d, -f1 user_input) database; then 
    echo "This username is already in use, try again!"
else
    echo "Successfully registered!"
fi

Answer 2

A solution that only uses bash built-in and grep.

while IFS=',' read -r username
do
    if grep -wq $username database
    then
        echo "This username is already in use, try again!"
    else
        echo "Successfully registered!"
    fi
done < user_input

Answer 3

I done by using cut command,for loop and if condition

#!/bin/bash
 i=`cut -d "," -f1 user_input.txt`

#echo $i
for j in `cut -d "," -f1 database.txt`
do
if [[ $i == $j ]]
then
echo $j "already exsists"
else
echo $j "doesn't exsists"
fi
done