"IF" on Linux shell script

VALID_PASSWORD="secret" #this is our password.

echo "Please enter the password:"
read PASSWORD

if [ "$PASSWORD" == "$VALID_PASSWORD" ]; then
 echo "You have access!"
else
 echo "ACCESS DENIED!"
fi

Comparisons:

-eq	equal to
-ne	not equal to
-lt	less than
-le	less than or equal to
-gt	greater than
-ge	greater than or equal to

File Operations:

-s	file exists and is not empty
-f	file exists and is not a directory
-d	directory exists
-x	file is executable
-w	file is writable
-r	file is readable
-n	tests to see if the argument is non empty

X=""

# -n tests to see if the argument is non empty

if [ -n $X ]; then  
 echo "the variable X is not the empty string"
fi

if [ "$AGE" -lt 20 ] || [ "$AGE" -ge 50 ]; then
 echo "Sorry, you are out of the age range."
elif [ "$AGE" -ge 20 ] && [ "$AGE" -lt 30 ]; then
 echo "You are in your 20s"

fi 

if [[ $count -gt 0  &&  $somevar != $var ]]; then
...no brackets inside, only double [[  ]]
fi

if [ $count -gt 0 ] && [ $somevar != $var ]; then
...do something
fi