Move from [ ] to [[ ]] #27

The [[ ]] test syntax is both more flexible and less error-prone:

• Does not require parameter expansions on the left-hand side to be quoted.
• Always treats expansions to empty strings as "" rather than string-splitting them out of existence (and thus potentially causing syntax errors).
• Allows pattern tests by using an unquoted glob expression on the right-hand side of an = test. ([[ $foo = 'Hello '* ]])
• Allows regexp tests by using an unquoted regular expression on the right-hand side of an =~ test. For maximum flexibility and compatibility across versions, the regex should be in a variable: re='foo.*bar'; [[ $var =~ $re ]]. Stores groups within matched regexes to the ${BASH_REMATCH[@]} array.
• Allows substring tests by using a quoted value on the right-hand side of an =~ operator: [[ $foo =~ "$substring" ]] determines whether the literal contents of the $substring variable exist anywhere within $foo.
• Allows standard grouping operators and short-circuiting boolean logic within the test itself, as so: [[ a = b && ( c = d || e = f ) ]]