字符串的特殊形式：$'string'

$'string'这种形式用于特殊的转义字符串，manual中的规定是：

3.1.2.4 ANSI-C Quoting

Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows:
\a
alert (bell)
\b
backspace
\e
an escape character (not ANSI C)
\f
form feed
\n
newline
\r
carriage return
\t
horizontal tab
\v
vertical tab
\\
backslash
\'
single quote
\nnn
the eight-bit character whose value is the octal value nnn (one to three digits)
\xHH
the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
\cx
a control-x character

The expanded result is single-quoted, as if the dollar sign had not been present.

ABS中的例子：

#!/bin/bash

# String expansion.
# Introduced with version 2 of Bash.

#  Strings of the form $'xxx'
#+ have the standard escaped characters interpreted. 

echo $'Ringing bell 3 times \a \a \a'
     # May only ring once with certain terminals.
     # Or ...
     # May not ring at all, depending on terminal settings.
echo $'Three form feeds \f \f \f'
echo $'10 newlines \n\n\n\n\n\n\n\n\n\n'
echo $'\102\141\163\150'
     #   B   a   s   h
     # Octal equivalent of characters.

exit