In section 18.104.22.168 titled Double Quotes, the Bash manual says:
Enclosing characters in double quotes (‘"’) preserves the literal value of all characters within the quotes, with the exception of ‘$’, ‘`’, ‘\’, and, when history expansion is enabled, ‘!’.
At the moment I am concerned with the single quote(
It's special meaning, described in the preceding section, section 22.214.171.124 is:
Enclosing characters in single quotes (
') preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.
Combining the two expositions,
a is not defined (hence
$a = null string), should print
$a on the screen, as
'', having it's special meaning inside, would shield
$ from the special interpretation. Instead, it prints
''. Why so?