Bash?Shell中單引號和雙引號的區(qū)別小結(jié)

更新時(shí)間：2024年11月01日 10:36:10 作者：黑風(fēng)風(fēng)

在Bash中,單引號和雙引號都能定義字符串,但它們處理變量擴(kuò)展、特殊字符的方式不同,本文就來介紹一下Bash?Shell中單引號和雙引號的區(qū)別小結(jié),感興趣的可以了解一下

單引號

字面值：在單引號中的所有字符都會被視為字面值，即它們的特殊含義不會被解釋。例如，$用于變量擴(kuò)展，但在單引號中它只會被視為普通的美元符號。

echo '$HOME'

上面的命令將輸出$HOME，而不是展開為環(huán)境變量HOME的值。

不允許嵌套單引號：單引號內(nèi)不能包含其他單引號，即使它們被轉(zhuǎn)義。要在單引號字符串中包含單引號，你需要使用一種特殊的語法：

echo 'It'\''s a test'

這將輸出It's a test。

單引號 (') 用于保留字符的字面含義，特殊字符在單引號里面，都會變?yōu)槠胀ㄗ址?，例?nbsp;*、$、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 $USER)"
yongqiang
(base) yongqiang@yongqiang:~/bash_work$

反斜杠在單引號中變成了普通字符。如果在單引號之中，還要使用單引號，不能使用轉(zhuǎn)義，需要在外層的單引號前面加上一個(gè)($)，然后再對里層的單引號轉(zhuǎn)義。

(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 $'It\'s me!'
It's me!
(base) yongqiang@yongqiang:~/bash_work$

在雙引號之中使用單引號。

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

雙引號

變量擴(kuò)展：雙引號中的變量會被擴(kuò)展，即$var會被替換為變量var的值。

name="John"
echo "Hello, $name"

上面的命令將輸出Hello, John。

特殊字符解釋：雙引號中的特殊字符，如\n（換行）和\t（制表符），會被解釋。

echo "Line 1\nLine 2"

這將輸出兩行文本，而不是一行。

感嘆號問題：雙引號中的感嘆號!可能會引起歷史擴(kuò)展問題，這需要特別注意。

雙引號 (") 比單引號 (') 寬松。在雙引號中，反引號 (`)、$、反斜杠 (\) 三個(gè)特殊字符會被 Bash 自動擴(kuò)展。$ 符號用于引用變量，反引號 (`) 用于執(zhí)行子命令。反斜杠 (\) 在雙引號之中用來轉(zhuǎn)義。

(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，在雙引號之中插入雙引號，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 正常情況下會將換行符解釋為命令結(jié)束，但是換行符在雙引號之中就失去了這種特殊作用，只用來換行。可以輸入多行，echo 命令會將換行符原樣輸出，顯示的時(shí)候正常解釋為換行。

(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，需要使用雙引號或單引號，將文件名放在里面。

(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$

單引號 (') - 雙引號 (")

對于普通字符串或文本，單引號和雙引號的作用相同。

(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$

當(dāng)打印已定義的變量時(shí)，需要使用雙引號，使用單引號將被視為普通字符。

(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 "$NAME"
yongqiang
(base) yongqiang@yongqiang:~/bash_work$

單引號 ( ' ) 和雙引號 ( " ) 可以保存多個(gè)連續(xù)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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