Linux Shell - 单引号 ( `'` )、双引号 ( `"` ) 和反引号 (`)

Bash 只有一种数据类型，即字符串。用户输入的数据，Bash 都视为字符串。

1. 单引号 (`'`)

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$

2. 双引号 (`"`)

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$

3. 反引号 (`)

MARKDOWN 编辑反引号 (`)

MARKDOWN 编辑反引号 (`` ` ``)

调用命令或者将命令的输出赋予变量，则必须使用反引号包含命令，这样命令才会执行。反引号的作用和 $(命令) 是一样的，反引号容易和单引号混淆，推荐使用 $(命令) 的方式调用命令。

(base) yongqiang@yongqiang:~/bash_work$ echo ls
ls
(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo 'ls'
ls
(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo "ls"
ls
(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo `ls`
bash_script.sh yongqiang.txt
(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo '`ls`'
`ls`
(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo "`ls`"
bash_script.sh
yongqiang.txt
(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo $(ls)
bash_script.sh yongqiang.txt
(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo "$(ls)"
bash_script.sh
yongqiang.txt
(base) yongqiang@yongqiang:~/bash_work$

反引号 (`) 括起来的命令又被单引号 (') 括起来，则命令不会执行，被当成普通字符输出。
反引号 (`) 括起来的命令又被双引号 ( " ) 括起来，则命令会执行。

(base) yongqiang@yongqiang:~/bash_work$ echo date
date
(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo 'date'
date
(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo "date"
date
(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo `date`
Sun May 15 10:00:31 CST 2022
(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo '`date`'
`date`
(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo "`date`"
Sun May 15 10:00:48 CST 2022
(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo $(date)
Sun May 15 10:28:34 CST 2022
(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo "$(date)"
Sun May 15 10:34:07 CST 2022
(base) yongqiang@yongqiang:~/bash_work$

# 调用命令 echo `cal` 和 echo $(cal) 输出单行
(base) yongqiang@yongqiang:~/bash_work$ echo `cal`
May 2022 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ echo $(cal)
May 2022 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
(base) yongqiang@yongqiang:~/bash_work$# 调用命令 echo "$(cal)" 输出原始格式
(base) yongqiang@yongqiang:~/bash_work$ echo "$(cal)"May 2022
Su Mo Tu We Th Fr Sa1  2  3  4  5  6  78  9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28
29 30 31(base) yongqiang@yongqiang:~/bash_work$

4. 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/bashecho "start"
echo
echo "Hello, world!"NAME="yongqiang!"
echo $NAME
echo
echo "111!"COMMENT="Hello, $NAME"
echo $COMMENT
echo
echo "222!"COMMENT='Hello, $NAME'
echo $COMMENTecho "Hello, world!"
echo
echo "end"(base) yongqiang@yongqiang:~/bash_work$
(base) yongqiang@yongqiang:~/bash_work$ ./bash_script.sh
startHello, world!
yongqiang!111!
Hello, yongqiang!222!
Hello, $NAME
Hello, world!end
(base) yongqiang@yongqiang:~/bash_work$

References

https://yongqiang.blog.csdn.net/
https://www.w3cschool.cn/bashshell/
https://wangdoc.com/bash/index.html