Shell Programming and Scripting

The bracketed expression consists of characters, emphasis on characters. [10] is not the number ten; it is the digit one followed by the digit zero. The range within the bracketed expression [9-10] does not mean nine to ten. It means nine to one along with an unrelated zero. The range [9-1], at best, will be empty and will match nothing. In that case, [9-10] is equivalent to [0]. At worst, [9-1] will yield an invalid range error message.

For all the details, consult your shell's manual page, specifically the section on pattern matching.

Regards,
Alister

Just to nit pick on alister's in depth comment, square brackets will always yield one single character only, either from the enumerated list of characters within, or from the (locale depending) char ranges within: So [10] will match either 0 or 1 , nothing else.