How does bash interpret the equal operator with no surrounding spaces in a conditional?

Question

The following script does not behave as I would have expected. Adding spaces around the '=' in the conditional made it perform how I wanted, but it got me thinking, what is it actually doing inside the conditional?

#!/bin/bash
S1='foo'
S2='bar'
if [ $S1=$S2 ];
then
    echo "S1('$S1') is equal to S2('$S2')
fi
echo $S1
echo $S2

The output is:

S1('foo') is equal to S2('bar')
foo
bar

The contents of S1 and S2 don't change from what they are assigned, so the = doesn't perform an assignment.

Answer 1

It's helpful to remember that [ is actually a command, usually also available as test. In bash, it's a builtin, so you can see the documentation with man builtin.

In that documentaiton:

          test and [ evaluate conditional expressions using a set of rules
          based on the number of arguments.

          0 arguments
                 The expression is false.
          1 argument
                 The expression is true if and only if the argument is not
                 null.
          2 arguments
                 [...]
          3 arguments
                 [...]

The two-argument rules are various tests, and the three-argument ones are generally comparisons. When you put a space around the =, you get three arguments. But when you put it all together, you get one argument, and as you can see, if that argument isn't null, it returns true.

Answer 2

The equals operator does nothing in this case.

The expression $S1=$S2 evaluates to an actual string, with the values of S1 and S2 in place, effectively the string literal "foo=bar".

Since this string literal is not null, the statement

if [ "foo=bar" ];

evaluates to true, and the body of the if statement is executed.