Bash == operator in [[ ]] is too smart!

Question

Case in point. I want to know if some set of files have as a first line '------'.

So,

for file in *.txt
do
    if [[ `head -1 "$file"` == "------" ]]
    then
        echo "$file starts with dashes"
    fi
done

Thing is, head returns the content with a newline, but "------" does not have a newline.

Why does it work?

Answer 1

The backticks strip the trailing newline. For example:

foo=`echo bar`
echo "<$foo>"

prints

<bar>

even though that first echo printed out "bar" followed by a newline.

Answer 2

Bash performs word splitting on the result of command substitution i.e. head -1 "$file"

Word splitting will remove newlines among other things.