As seen in Advanced Bash Shell Scripting Guide - 7.3. Other Comparison Operators:
string comparison
<
is less than, in ASCII alphabetical order
if [[ "$a" < "$b" ]]
if [ "$a" \< "$b" ]
Note that the "<" needs to be escaped within a [ ] construct.
So the way to do it is with -lt
or, quoting again:
integer comparison
-lt is less than
if [ "$a" -lt "$b" ]
< is less than (within double parentheses)
(("$a" < "$b"))