#!/bin/sh

echo Username
read i
echo Password
read j

Username=$i":"$j

valid=`grep -n $Username /$HOME/PhaseTest/userinfo | wc -l`

# -q means "quiet", preventing grep from printing anything.
# The ! is a boolean 'not', so the if statement only runs if grep fails to find it.
if ! grep -q "^${Username}\$" ~/PhraseTest/userinfo
then
        echo "Invalid user"
        exit 1
fi

echo "********Welcome to Grounoch Marx University********"
...