Bash Shell中单引号和双引号的区别小结
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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