假如有以下环境
a=apple # 一个变量
arr=(apple) # 一个只有一个元素的数组
然后使用echo来打印第二列的表达式，然后会出现第三列的结果，原因是第四列
| # | 表达式 | 结果 | 原因 |
|---|---|---|---|
| 1 | "$a" |
apple |
在""之间的变量会被替换为变量的值，因为$表示一个变量 |
| 2 | '$a' |
$a |
在‘’之间的变量不会被替换为变量的值 |
| 3 | "'$a'" |
'apple' |
""中如果包含了'' ，那么‘’会失去他的作用，没有任何意义，只是单纯的符号,但是变量依然会被替换成值。 |
| 4 | '"$a"' |
"$a" |
''中如果包含了"" ，那么""会失去他的作用，没有任何意义，只是单纯的符号。 |
| 5 | '\'' |
invalid | can not escape a ' within ''; use "'" or $'\'' (ANSI-C quoting) |
| 6 | "red$arocks" |
red |
$arocks does not expand $a; use ${a}rocks to preserve $a |
| 7 | "redapple$" |
redapple$ |
$ followed by no variable name evaluates to $ |
| 8 | '\"' |
\" |
\ has no special meaning inside '' |
| 9 | "\'" |
\' |
shell程序认为，\' 在 "" 之中没有意义所以不会被转义 |
| 10 | "\"" |
" |
\" is interpreted inside "" |
| 11 | "*" |
* |
glob does not work inside "" or '' |
| 12 | "\t\n" |
\t\n |
\t and \n have no special meaning inside "" or ''; use ANSI-C quoting |
| 13 | "`echo hi`" |
hi |
`` and $() are evaluated inside "" (backquotes are retained in actual output) |
| 14 | '`echo hi`' |
echo hi | `` and $() are not evaluated inside '' (backquotes are retained in actual output) |
| 15 | '${arr[0]}' |
${arr[0]} |
array access not possible inside '' |
| 16 | "${arr[0]}" |
apple |
array access works inside "" |
| 17 | $'$a\'' |
$a' |
single quotes can be escaped inside ANSI-C quoting |
| 18 | "$'\t'" |
$'\t' |
ANSI-C quoting is not interpreted inside "" |
| 19 | '!cmd' |
!cmd |
history expansion character '!' is ignored inside '' |
| 20 | "!cmd" |
cmd args |
expands to the most recent command matching "cmd" |
| 21 | $'!cmd' |
!cmd |
history expansion character '!' is ignored inside ANSI-C quotes |
来源：https://stackoverflow.com/questions/6697753/difference-between-single-and-double-quotes-in-bash
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