unary operator expected in shell script when comparing null value with string

Question

I have two variables

var=\"\"
var1=abcd

Here is my shell script code

if [ $var == $var1 ]; then
  do something
else
  do so         


        

Answer 1

Since the value of $var is the empty string, this:

if [ $var == $var1 ]; then

expands to this:

if [ == abcd ]; then

which is a syntax error.

You need to quote the arguments:

if [ "$var" == "$var1" ]; then

You can also use = rather than ==; that's the original syntax, and it's a bit more portable.

If you're using bash, you can use the [[ syntax, which doesn't require the quotes:

if [[ $var = $var1 ]]; then

Even then, it doesn't hurt to quote the variable reference, and adding quotes:

if [[ "$var" = "$var1" ]]; then

might save a future reader a moment trying to remember whether [[ ... ]] requires them.

Answer 2

Why all people want to use '==' instead of simple '=' ? It is bad habit! It used only in [[ ]] expression. And in (( )) too. But you may use just = too! It work well in any case. If you use numbers, not strings use not parcing to strings and then compare like strings but compare numbers. like that

let -i i=5 # garantee that i is nubmber
test $i -eq 5 && echo "$i is equal 5" || echo "$i not equal 5"

It's match better and quicker. I'm expert in C/C++, Java, JavaScript. But if I use bash i never use '==' instead '='. Why you do so?