NAME
    [[ - test a logical expression

SYNOPSIS
    [[ <expression> ]]

DESCRIPTION
    The expression is evaluated using logical operators,
    and returns a boolean value indicating whether the expression
    was true or false. The set of operators are as follows:
        -a <filename>           - return TRUE if file exists
        -e <filename>           - same as -a
        -d <dirname>            - TRUE if arg is a directory
        -f <filename>           - TRUE if file is a regular file
        -x <filename>           - TRUE if file is executable
        -r <filename>           - TRUE if file is readable by user
        -w <filename>           - TRUE if file is writable by user
        -z <string>             - TRUE if string is 0 length
        -n <string>             - TRUE if string is not 0 length
        <string> <= <string>    - TRUE if first is lexographically less than second
        <string> == <regex>     - TRUE if regex matches ALL of string
        <string> != <regex>     - TRUE if regex does not match string
        <string> =~ <regex>     - TRUE if regex matches anything in string
                                  vars $v0-$v9 are set to contain matches in parens
        <expr> && <expr>        - TRUE if both expressions are true
        <expr> || <expr>        - TRUE if either expression is true

        <varname>++             - postincrement
        <varname>--             - postdecrement
        <expr> * <expr>         - TRUE if multiplication result is non-zero
        <expr> / <expr>         - TRUE if division is non-zero, and denominator is non-zero
        <expr> + <expr>         - TRUE if sum is non-zero
        <expr> - <expr>         - TRUE if difference is non-zero
        <nexpr> -lt <nexpr>     - TRUE if first numeric expr is < second
        <nexpr> -le <nexpr>     - TRUE if first numeric expr is <= second
        <nexpr> -eq <nexpr>     - TRUE if numeric expressions are equal
        <nexpr> -ge <nexpr>     - TRUE if first numeric expr is >= second
        <nexpr> -gt <nexpr>     - TRUE if first numeric expr is > second