name='caoxudong'
echo ${name:0:2}
echo ${name:2:2}
echo ${name:-1:2}
echo ${name:-100:2}

ca
ox
caoxudong
caoxudong

DEFINITIONS

    The following definitions are used throughout the rest of this document.
    blank  A space or tab.
    word   A sequence of characters considered as a single unit by the shell.  Also known as a  token.
    name   A  word  consisting  only of alphanumeric characters and underscores, and beginning with an
           alphabetic character or an underscore.  Also referred to as an identifier.
    metacharacter
           A character that, when unquoted, separates words.  One of the following:
           |  & ; ( ) < > space tab
    control operator
           A token that performs a control function.  It is one of the following symbols:
           || & && ; ;; ( ) | <newline>

PARAMETERS

    A parameter is an entity that stores values.  It can be a name, a number, or one  of  the  special
    characters listed below under Special Parameters.  A variable is a parameter denoted by a name.  A
    variable has a value and zero or more attributes.   Attributes  are  assigned  using  the  declare
    builtin command (see declare below in SHELL BUILTIN COMMANDS).

${parameter:offset:length}

    Substring Expansion.  Expands to up to length characters of parameter starting at the char-
    acter specified by offset.  If length is omitted, expands to  the  substring  of  parameter
    starting  at  the  character specified by offset.  length and offset are arithmetic expres-
    sions (see ARITHMETIC EVALUATION below).  length must evaluate to a number greater than  or
    equal  to  zero.   If  offset evaluates to a number less than zero, the value is used as an
    offset from the end of the value of parameter.  If parameter is @,  the  result  is  length
    positional  parameters  beginning at offset.  If parameter is an array name indexed by @ or
    *, the result is the length members of the array beginning  with  ${parameter[offset]}.   A
    negative  offset  is  taken relative to one greater than the maximum index of the specified
    array.  Note that a negative offset must be separated from the colon by at least one  space
    to avoid being confused with the :- expansion.  Substring indexing is zero-based unless the
    positional parameters are used, in which case the indexing starts at 1.