Shell Programming and Scripting

${parameter:offset:length}
   ${parameter:offset}
          Expands to the portion of the value of parameter starting at
          the character (counting from 0) determined by expanding offset
          as an arithmetic expression and consisting of the number of
          characters determined by the arithmetic expression defined by
          length. In the second form, the remainder of the value is used.
          If parameter is * or @, or is an array name indexed by * or @,
          then offset and length refer to the array index and number of
          elements respectively.