How to append a character at the end of string?

I have a string foo: I want to append a character at the end of a string [ So the final output should be foo: [

I tried the following code.

puts 'foo: ' << '[' & puts 'foo: ' + '['

But both are appending the character in a new line & giving me the following output.

foo: [

However, I want to append the character in the same line & want the below output:

foo: [