I can't get working this conditional expression
<!--#if expr="$DOCUMENT_NAME!=index.html || $DOCUMENT_NAME!=links.html" -->
while this one without ! before = works perfect
<!--#if expr="$DOCUMENT_NAME=index.html || $DOCUMENT_NAME=links.html" -->
What's the problem? I get no error simply != doesn't work though || with other condition but works for single condition.
This is because = and != are hardly the same operator. Note that, by De Morgan's law (which I also explained in this old post),
a != b || c != d
is equivalent to
a = b && c = d
which is never true for x = a && x = b where a != b.
Changing the binary operator requires changing the conditionals as well to be equivalent.
Thus, by the above logic,
$DOCUMENT_NAME!=index.html || $DOCUMENT_NAME!=links.html
is equivalent to
$DOCUMENT_NAME=index.html && $DOCUMENT_NAME=links.html
which cannot be true as $DOCUMENT_NAME can be "index.html" or "links.html" but not both.
However, the 2nd snippet,
$DOCUMENT_NAME=index.html || $DOCUMENT_NAME=links.html
"works" because there is not the logical never-true fallacy mentioned above. It will be true when $DOCUMENT_NAME is either "index.html" or "links.html".
Some languages/values will violate the above equivalency .. but that is another topic.
来源：https://stackoverflow.com/questions/12169639/why-ssi-condition-doesnt-work