shell中$后加引号有什么用($"string"和$'string')
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有些时候在某些服务管理脚本中看到$"$string"或$"string"，经过一些测试，又发现引号外面的$有和没有是一样的。一直也没去找究竟，刚才有人问了我，于是就去翻了下man bash，找到了解释。
(1).如果没有特殊定制bash环境或有特殊需求，$"string"和"string"是完全等价的，使用$""只是为了保证本地化。
以下是man bash关于$""的解释：
A double-quoted string preceded by a dollar sign ($"string") will cause the string to be translated according to the current locale. If
the current locale is C or POSIX, the dollar sign is ignored. If the string is translated and replaced, the replacement is double-quoted.
(2).还有$后接单引号的$‘string‘，这在bash中被特殊对待：会将某些反斜线序列(如
， ，"，‘等)继续转义，而不认为它是字面符号(如果没有$符号，单引号会强制将string翻译为字面符号，包括反斜线)。简单的例子：
[root@localhost ~]# echo ‘a
b‘
a
b
[root@localhost ~]# echo $‘a
b‘
a
b
[root@localhost ~]# echo -e ‘a
b‘
a
b
[root@localhost ~]# echo -e "a
b"
a
b
以下是man bash里关于$‘的说明：
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)
backspace
e
E an escape character
f form feed
new line
carriage return
horizontal tab
v vertical tab
\ backslash
‘ single quote
" double quote
nn 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)
uHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits)
UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits)
cx a control-x character
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