# Short answer:
# Single quotes don't allow interpolation, but double quotes do

# Long answer:
# From wikipedia:
# In computer programming, string interpolation (or variable interpolation,
# 	variable substitution, or variable expansion) is the process of evaluating
# 	a string literal containing one or more placeholders, yielding a result in
# 	which the placeholders are replaced with their corresponding values.

# Concrete example:
echo "$(echo 'word')" # here, "$(echo 'word')" gets interpolated (interpreted)
--> word

echo '$(echo "word")' # here, '$(echo "word")' is echoed literally (no interpolation)
--> $(echo "word")

# Note, this link also has some good info:
https://stackoverflow.com/questions/10067266/when-should-i-wrap-quotes-around-a-shell-variable/42104627#42104627