我要评论在bash shell中，单引号(')和双引号(")都用于定义字符串，但它们之间有一些重要的区别。这些区别影响字符串的解释方式，变量的扩展，以及特殊字符的处理。
单引号
字面值：在单引号中的所有字符都会被视为字面值，即它们的特殊含义不会被解释。例如，$用于变量扩展，但在单引号中它只会被视为普通的美元符号。
echo '$home'
上面的命令将输出$home，而不是展开为环境变量home的值。
不允许嵌套单引号：单引号内不能包含其他单引号，即使它们被转义。要在单引号字符串中包含单引号，你需要使用一种特殊的语法：
echo 'it'\''s a test'
这将输出it's a test。
单引号 (') 用于保留字符的字面含义，特殊字符在单引号里面，都会变为普通字符，例如 *、$、反斜杠 (\) 等。
(base) yongqiang@yongqiang:~/bash_work$ ls -l total 0 -rwxr-xr-x 1 yongqiang yongqiang 231 may 13 23:24 bash_script.sh -rw-r--r-- 1 yongqiang yongqiang 0 may 15 09:31 yongqiang.txt (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo * bash_script.sh yongqiang.txt (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo '*' * (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo "*" * (base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo $ $ (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo '$' $ (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo "$" $ (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo $path /home/yongqiang/bin:/home/yongqiang/miniconda3/bin:/home/yongqiang/miniconda3/condabin:......:/home/yongqiang/software/platform-tools/ (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo '$path' $path (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo $path /home/yongqiang/bin:/home/yongqiang/miniconda3/bin:/home/yongqiang/miniconda3/condabin:......:/home/yongqiang/software/platform-tools/ (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo $user yongqiang (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo '$user' $user (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo "$user" yongqiang (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo $((5 + 1)) 6 (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo '$((5 + 1))' $((5 + 1)) (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo "$((5 + 1))" 6 (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo $(echo $user) yongqiang (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo '$(echo $user)' $(echo $user) (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo "$(echo $user)" yongqiang (base) yongqiang@yongqiang:~/bash_work$
反斜杠在单引号中变成了普通字符。如果在单引号之中，还要使用单引号，不能使用转义，需要在外层的单引号前面加上一个($)，然后再对里层的单引号转义。
(base) yongqiang@yongqiang:~/bash_work$ echo '\' \ (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo 'it's me!' bash: !': event not found (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo 'it\'s me!' bash: !': event not found (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo $'it\'s me!' it's me! (base) yongqiang@yongqiang:~/bash_work$
在双引号之中使用单引号。
(base) yongqiang@yongqiang:~/bash_work$ echo "it's me!" it's me! (base) yongqiang@yongqiang:~/bash_work$
双引号
变量扩展：双引号中的变量会被扩展，即$var会被替换为变量var的值。
name="john" echo "hello, $name"
上面的命令将输出hello, john。
特殊字符解释：双引号中的特殊字符，如\n（换行）和\t（制表符），会被解释。
echo "line 1\nline 2"
这将输出两行文本，而不是一行。
感叹号问题：双引号中的感叹号!可能会引起历史扩展问题，这需要特别注意。
双引号 (") 比单引号 (') 宽松。在双引号中，反引号 (`)、$、反斜杠 (\) 三个特殊字符会被 bash 自动扩展。$ 符号用于引用变量，反引号 (`) 用于执行子命令。反斜杠 (\) 在双引号之中用来转义。
(base) yongqiang@yongqiang:~/bash_work$ echo ? ? (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo '?' ? (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo "?" ? (base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo $shell /bin/bash (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo '$shell' $shell (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo "$shell" /bin/bash (base) yongqiang@yongqiang:~/bash_work$
使用反斜杠，在双引号之中插入双引号，或者插入反斜杠本身。
(base) yongqiang@yongqiang:~/bash_work$ echo '\\' \\ (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo "\\" \ (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo ""yongqiang"" yongqiang (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo "\"yongqiang\"" "yongqiang" (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo "it's me!" it's me! (base) yongqiang@yongqiang:~/bash_work$
bash 正常情况下会将换行符解释为命令结束，但是换行符在双引号之中就失去了这种特殊作用，只用来换行。可以输入多行，echo 命令会将换行符原样输出，显示的时候正常解释为换行。
(base) yongqiang@yongqiang:~/bash_work$ echo "yong > qiang" yong qiang (base) yongqiang@yongqiang:~/bash_work$
如果文件名中包含空格，需要使用双引号或单引号，将文件名放在里面。
(base) yongqiang@yongqiang:~/bash_work$ ls -l total 0 -rwxr-xr-x 1 yongqiang yongqiang 106 may 13 20:11 bash_script.sh (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ touch "yong qiang.txt" (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ ls -l total 0 -rwxr-xr-x 1 yongqiang yongqiang 106 may 13 20:11 bash_script.sh -rw-r--r-- 1 yongqiang yongqiang 0 may 13 21:27 'yong qiang.txt' (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ ls -l "yong qiang.txt" -rw-r--r-- 1 yongqiang yongqiang 0 may 13 21:27 'yong qiang.txt' (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ ls -l 'yong qiang.txt' -rw-r--r-- 1 yongqiang yongqiang 0 may 13 21:27 'yong qiang.txt' (base) yongqiang@yongqiang:~/bash_work$
单引号 (') - 双引号 (")
对于普通字符串或文本，单引号和双引号的作用相同。
(base) yongqiang@yongqiang:~/bash_work$ echo "yongqiang" yongqiang (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo 'yongqiang' yongqiang (base) yongqiang@yongqiang:~/bash_work$
当打印已定义的变量时，需要使用双引号，使用单引号将被视为普通字符。
(base) yongqiang@yongqiang:~/bash_work$ name="yongqiang" (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo $name yongqiang (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo '$name' $name (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo "$name" yongqiang (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ name='yongqiang' (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo $name yongqiang (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo '$name' $name (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo "$name" yongqiang (base) yongqiang@yongqiang:~/bash_work$
单引号 ( ' ) 和双引号 ( " ) 可以保存多个连续的空格。
(base) yongqiang@yongqiang:~/bash_work$ echo "yong qiang" yong qiang (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ echo 'yong qiang' yong qiang (base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ cat ./bash_script.sh #!/bin/bash echo "start" echo echo "hello, world!" name="yongqiang!" echo $name echo echo "111!" comment="hello, $name" echo $comment echo echo "222!" comment='hello, $name' echo $comment echo "hello, world!" echo echo "end" (base) yongqiang@yongqiang:~/bash_work$ (base) yongqiang@yongqiang:~/bash_work$ ./bash_script.sh start hello, world! yongqiang! 111! hello, yongqiang! 222! hello, $name hello, world! end (base) yongqiang@yongqiang:~/bash_work$
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