How to compare a variable to a text string ?

s1="hi"

if [ "hi" == "$s1" ]
then

   echo "match"

fi

INVALID EXAMPLES

1. No spaces between variable/value and brackets OR before/after operator 

if ["hi" == "$s1"]

2. Using -eq operator (used for number comparison)

if [ "hi" -eq "$s1" ]

4. Not using double quotes for string variable

if [ "hi" == $s1 ]

Note: If there is no value or empty string assigned to string variable, it could be read as : 

if [ "hi" == ]

3. Avoid accidental assignment to variable. 

if [ "$s1" == "hi" ]

Note: Always keep value first.