Bash Shell中单引号和双引号的区别小结

屈盛

目录

• 单引号
  
• 双引号
  
• 单引号 (') - 双引号 (")
  

在Bash Shell中，单引号(

1. '

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)和双引号(

1. "

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)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，

1. $

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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。

1. echo '$HOME'

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上面的命令将输出

1. $HOME

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，而不是展开为环境变量

1. HOME

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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：

1. echo 'It'\''s a test'

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这将输出

1. It's a test

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。
单引号 (

1. '

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) 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

1. *

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、

1. $

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、反斜杠 (

1. \

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) 等。

1. (base) yongqiang@yongqiang:~/bash_work$ ls -l
   
2. total 0
   
3. -rwxr-xr-x 1 yongqiang yongqiang 231 May 13 23:24 bash_script.sh
   
4. -rw-r--r-- 1 yongqiang yongqiang   0 May 15 09:31 yongqiang.txt
   
5. (base) yongqiang@yongqiang:~/bash_work$
   
6. (base) yongqiang@yongqiang:~/bash_work$ echo *
   
7. bash_script.sh yongqiang.txt
   
8. (base) yongqiang@yongqiang:~/bash_work$
   
9. (base) yongqiang@yongqiang:~/bash_work$ echo '*'
   
10. *
    
11. (base) yongqiang@yongqiang:~/bash_work$
    
12. (base) yongqiang@yongqiang:~/bash_work$ echo "*"
    
13. *
    
14. (base) yongqiang@yongqiang:~/bash_work$

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1. (base) yongqiang@yongqiang:~/bash_work$ echo $
   
2. $
   
3. (base) yongqiang@yongqiang:~/bash_work$
   
4. (base) yongqiang@yongqiang:~/bash_work$ echo '$'
   
5. $
   
6. (base) yongqiang@yongqiang:~/bash_work$
   
7. (base) yongqiang@yongqiang:~/bash_work$ echo "$"
   
8. $
   
9. (base) yongqiang@yongqiang:~/bash_work$
   
10. 
    
11. (base) yongqiang@yongqiang:~/bash_work$ echo $PATH
    
12. /home/yongqiang/bin:/home/yongqiang/miniconda3/bin:/home/yongqiang/miniconda3/condabin:......:/home/yongqiang/software/platform-tools/
    
13. (base) yongqiang@yongqiang:~/bash_work$
    
14. (base) yongqiang@yongqiang:~/bash_work$ echo '$PATH'
    
15. $PATH
    
16. (base) yongqiang@yongqiang:~/bash_work$
    
17. (base) yongqiang@yongqiang:~/bash_work$ echo $PATH
    
18. /home/yongqiang/bin:/home/yongqiang/miniconda3/bin:/home/yongqiang/miniconda3/condabin:......:/home/yongqiang/software/platform-tools/
    
19. (base) yongqiang@yongqiang:~/bash_work$
    
20. 
    
21. (base) yongqiang@yongqiang:~/bash_work$ echo $USER
    
22. yongqiang
    
23. (base) yongqiang@yongqiang:~/bash_work$
    
24. (base) yongqiang@yongqiang:~/bash_work$ echo '$USER'
    
25. $USER
    
26. (base) yongqiang@yongqiang:~/bash_work$
    
27. (base) yongqiang@yongqiang:~/bash_work$ echo "$USER"
    
28. yongqiang
    
29. (base) yongqiang@yongqiang:~/bash_work$
    
30. 
    
31. (base) yongqiang@yongqiang:~/bash_work$ echo $((5 + 1))
    
32. 6
    
33. (base) yongqiang@yongqiang:~/bash_work$
    
34. (base) yongqiang@yongqiang:~/bash_work$ echo '$((5 + 1))'
    
35. $((5 + 1))
    
36. (base) yongqiang@yongqiang:~/bash_work$
    
37. (base) yongqiang@yongqiang:~/bash_work$ echo "$((5 + 1))"
    
38. 6
    
39. (base) yongqiang@yongqiang:~/bash_work$
    
40. 
    
41. (base) yongqiang@yongqiang:~/bash_work$ echo $(echo $USER)
    
42. yongqiang
    
43. (base) yongqiang@yongqiang:~/bash_work$
    
44. (base) yongqiang@yongqiang:~/bash_work$ echo '$(echo $USER)'
    
45. $(echo $USER)
    
46. (base) yongqiang@yongqiang:~/bash_work$
    
47. (base) yongqiang@yongqiang:~/bash_work$ echo "$(echo $USER)"
    
48. yongqiang
    
49. (base) yongqiang@yongqiang:~/bash_work$

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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(

1. $

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)，然后再对里层的单引号转义。

1. (base) yongqiang@yongqiang:~/bash_work$ echo '\'
   
2. \
   
3. (base) yongqiang@yongqiang:~/bash_work$
   
4. (base) yongqiang@yongqiang:~/bash_work$ echo 'It's me!'
   
5. bash: !': event not found
   
6. (base) yongqiang@yongqiang:~/bash_work$
   
7. (base) yongqiang@yongqiang:~/bash_work$ echo 'It\'s me!'
   
8. bash: !': event not found
   
9. (base) yongqiang@yongqiang:~/bash_work$
   
10. (base) yongqiang@yongqiang:~/bash_work$ echo $'It\'s me!'
    
11. It's me!
    
12. (base) yongqiang@yongqiang:~/bash_work$

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在双引号之中使用单引号。

1. (base) yongqiang@yongqiang:~/bash_work$ echo "It's me!"
   
2. It's me!
   
3. (base) yongqiang@yongqiang:~/bash_work$

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双引号

变量扩展：双引号中的变量会被扩展，即

1. $var

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会被替换为变量

1. var

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的值。

1. name="John"
   
2. echo "Hello, $name"

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上面的命令将输出

1. Hello, John

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。
特殊字符解释：双引号中的特殊字符，如

1. \n

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（换行）和

1. \t

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（制表符），会被解释。

1. echo "Line 1\nLine 2"

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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

1. !

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可能会引起历史扩展问题，这需要特别注意。
双引号 (

1. "

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) 比单引号 (

1. '

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) 宽松。在双引号中，反引号 (

1. `

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)、

1. $

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、反斜杠 (

1. \

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) 三个特殊字符会被 Bash 自动扩展。

1. $

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符号用于引用变量，反引号 (

1. `

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) 用于执行子命令。反斜杠 (

1. \

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) 在双引号之中用来转义。

1. (base) yongqiang@yongqiang:~/bash_work$ echo ?
   
2. ?
   
3. (base) yongqiang@yongqiang:~/bash_work$
   
4. (base) yongqiang@yongqiang:~/bash_work$ echo '?'
   
5. ?
   
6. (base) yongqiang@yongqiang:~/bash_work$
   
7. (base) yongqiang@yongqiang:~/bash_work$ echo "?"
   
8. ?
   
9. (base) yongqiang@yongqiang:~/bash_work$

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1. (base) yongqiang@yongqiang:~/bash_work$ echo $SHELL
   
2. /bin/bash
   
3. (base) yongqiang@yongqiang:~/bash_work$
   
4. (base) yongqiang@yongqiang:~/bash_work$ echo '$SHELL'
   
5. $SHELL
   
6. (base) yongqiang@yongqiang:~/bash_work$
   
7. (base) yongqiang@yongqiang:~/bash_work$ echo "$SHELL"
   
8. /bin/bash
   
9. (base) yongqiang@yongqiang:~/bash_work$

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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。

1. (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!"
   
2. It's me!
   
3. (base) yongqiang@yongqiang:~/bash_work$

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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，

1. echo

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命令会将换行符原样输出，显示的时候正常解释为换行。

1. (base) yongqiang@yongqiang:~/bash_work$ echo "yong
   
2. > qiang"
   
3. yong
   
4. qiang
   
5. (base) yongqiang@yongqiang:~/bash_work$

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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。

1. (base) yongqiang@yongqiang:~/bash_work$ ls -l
   
2. total 0
   
3. -rwxr-xr-x 1 yongqiang yongqiang 106 May 13 20:11 bash_script.sh
   
4. (base) yongqiang@yongqiang:~/bash_work$
   
5. (base) yongqiang@yongqiang:~/bash_work$ touch "yong qiang.txt"
   
6. (base) yongqiang@yongqiang:~/bash_work$
   
7. (base) yongqiang@yongqiang:~/bash_work$ ls -l
   
8. total 0
   
9. -rwxr-xr-x 1 yongqiang yongqiang 106 May 13 20:11  bash_script.sh
   
10. -rw-r--r-- 1 yongqiang yongqiang   0 May 13 21:27 'yong qiang.txt'
    
11. (base) yongqiang@yongqiang:~/bash_work$
    
12. (base) yongqiang@yongqiang:~/bash_work$ ls -l "yong qiang.txt"
    
13. -rw-r--r-- 1 yongqiang yongqiang 0 May 13 21:27 'yong qiang.txt'
    
14. (base) yongqiang@yongqiang:~/bash_work$
    
15. (base) yongqiang@yongqiang:~/bash_work$ ls -l 'yong qiang.txt'
    
16. -rw-r--r-- 1 yongqiang yongqiang 0 May 13 21:27 'yong qiang.txt'
    
17. (base) yongqiang@yongqiang:~/bash_work$

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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。

1. (base) yongqiang@yongqiang:~/bash_work$ echo "yongqiang"
   
2. yongqiang
   
3. (base) yongqiang@yongqiang:~/bash_work$
   
4. (base) yongqiang@yongqiang:~/bash_work$ echo 'yongqiang'
   
5. yongqiang
   
6. (base) yongqiang@yongqiang:~/bash_work$

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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。

1. (base) yongqiang@yongqiang:~/bash_work$ NAME="yongqiang"
   
2. (base) yongqiang@yongqiang:~/bash_work$
   
3. (base) yongqiang@yongqiang:~/bash_work$ echo $NAME
   
4. yongqiang
   
5. (base) yongqiang@yongqiang:~/bash_work$
   
6. (base) yongqiang@yongqiang:~/bash_work$ echo '$NAME'
   
7. $NAME
   
8. (base) yongqiang@yongqiang:~/bash_work$
   
9. (base) yongqiang@yongqiang:~/bash_work$ echo "$NAME"
   
10. yongqiang
    
11. (base) yongqiang@yongqiang:~/bash_work$
    
12. 
    
13. (base) yongqiang@yongqiang:~/bash_work$ NAME='yongqiang'
    
14. (base) yongqiang@yongqiang:~/bash_work$
    
15. (base) yongqiang@yongqiang:~/bash_work$ echo $NAME
    
16. yongqiang
    
17. (base) yongqiang@yongqiang:~/bash_work$
    
18. (base) yongqiang@yongqiang:~/bash_work$ echo '$NAME'
    
19. $NAME
    
20. (base) yongqiang@yongqiang:~/bash_work$
    
21. (base) yongqiang@yongqiang:~/bash_work$ echo "$NAME"
    
22. yongqiang
    
23. (base) yongqiang@yongqiang:~/bash_work$

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单引号 (

1. '

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) 和双引号 (

1. "

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) 可以保存多个连续的空格。

1. (base) yongqiang@yongqiang:~/bash_work$ echo "yong    qiang"
   
2. yong    qiang
   
3. (base) yongqiang@yongqiang:~/bash_work$
   
4. (base) yongqiang@yongqiang:~/bash_work$ echo 'yong    qiang'
   
5. yong    qiang
   
6. (base) yongqiang@yongqiang:~/bash_work$

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1. (base) yongqiang@yongqiang:~/bash_work$ cat ./bash_script.sh
   
2. #!/bin/bash
   
3. 
   
4. echo "start"
   
5. echo
   
6. echo "Hello, world!"
   
7. 
   
8. NAME="yongqiang!"
   
9. echo $NAME
   
10. echo
    
11. echo "111!"
    
12. 
    
13. COMMENT="Hello, $NAME"
    
14. echo $COMMENT
    
15. echo
    
16. echo "222!"
    
17. 
    
18. COMMENT='Hello, $NAME'
    
19. echo $COMMENT
    
20. 
    
21. echo "Hello, world!"
    
22. echo
    
23. echo "end"
    
24. 
    
25. (base) yongqiang@yongqiang:~/bash_work$
    
26. (base) yongqiang@yongqiang:~/bash_work$ ./bash_script.sh
    
27. start
    
28. 
    
29. Hello, world!
    
30. yongqiang!
    
31. 
    
32. 111!
    
33. Hello, yongqiang!
    
34. 
    
35. 222!
    
36. Hello, $NAME
    
37. Hello, world!
    
38. 
    
39. end
    
40. (base) yongqiang@yongqiang:~/bash_work$

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mmaggie

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