为什么echo命令的控制字符只在“”中生效？

biangbiang_cat

我是一个新人小白，最近开始学习shell的，今天遇到一个问题实在是想不通了。问题如下：
#!/bin/sh
DATE=$(date)
echo -e Today is $DATE\n\n

echo -e "Today is $DATE\n\n"

echo -e "Today is $DATE"\n\n

执行结果：

marmot@sles11xen:~> sh t1.sh
Today is Fri Jul 8 14:00:47 CST 2016nn
Today is Fri Jul  8 14:00:47 CST 2016


Today is Fri Jul  8 14:00:47 CST 2016nn

在echo命令中 双引号“”到底是一个怎样的存在？ 为什么反斜线控制符，必须放在双引号中，才生效？   我看了一点十三问的帖子，\这种meta在双引号中不是应该被关闭的嘛？

reyleon

先记住 echo 与 echo -e 是有区别的, 后面再说, 先说shell

首先要了解bash shell中相关龟腚:

1. bash shell中龟腚, 没有被引用的反斜杠(\)是转义字符, 所以没有被引用的 \n 实际上是反斜杠\后面的字符n的字面意思. 就是n.
如 \', \`,\\, \$, 都是输出反斜杠后面的那个字符

1. $ echo \`
   
2. `
   
3. $ echo \'
   
4. '
   
5. $ echo \\
   
6. \
   
7. $ echo \$
   
8. $
   
9. $ echo \n
   
10. n

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2.  在双引用中, 反斜杠就没那么屌了, 只有当反斜杠后面的字符是: $, `, ", \ 这四个字符时, 反斜杠才有特异功能,如:

1. $ echo "\4"
   
2. \4
   
3. $ echo "\c"
   
4. \c
   
5. $ echo "\$" #反斜杠后面为美元符号,反斜杠就起到了转义的作用
   
6. $
   
7. $ echo "\"" #同上,反斜杠后面为双引号,反斜杠有转义作用
   
8. "
   
9. $ echo "\`" #反斜杠后面为反引号,反斜杠有转义作用
   
10. `
    
11. $ echo "\\" #反斜杠后面是反斜杠,反斜杠有转义作用
    
12. \
    
13. $ echo "\%"
    
14. \%

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3. 在单引号中, 反斜杠就完全掀不起浪了, 它就是反斜杠:

1. $ echo '\"'
   
2. \"
   
3. $ echo '\`'
   
4. \`

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在shell中输入的命令, 首先是被shell解析, 然后再是被命令解析.
首先, echo 的-e选项, 就是解析反斜杠的作用:

1. -e     enable interpretation of backslash escapes
   

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1. $ DATE=$(date)
   
2. $ echo -e Today is $DATE\n\n # shell解析, \n\n因为反斜杠转义, \n\n就变成了nn. echo命令后面压根就没有反斜杠了.所以与不加-e的结果是一样的.
   
3. Today is Fri Jul 8 15:02:03 CST 2016nn
   
4. 
   
5. $ echo Today is $DATE\n\n
   
6. Today is Fri Jul 8 15:02:03 CST 2016nn

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再看, Today is $DATE\n\n 被引用之后, "Today is $DATE\n\n", \n中的反斜杠在shell的解析中就失去了转义的作用, \n\n还是\n\n. 然后再传递个echo -e
echo -e 会对反斜杠进行解析, \n 被echo -e 解析为了换行.

1. $ echo "Today is $DATE\n\n"   # shell 不对反斜杠做转义解释, 因echo 没有 -e 选项, 所以原样输出
   
2. Today is Fri Jul  8 15:02:03 CST 2016\n\n
   
3. 
   
4. $ echo -e "Today is $DATE\n\n" # 加了 -e , echo 对 \n解析, 换行.
   
5. Today is Fri Jul  8 15:02:03 CST 2016
   
6. 
   
7. 
   

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更多详细内容, man bash :

1. QUOTING
   
2.        Quoting  is  used  to  remove the special meaning of certain characters or words to the shell.  Quoting can be used to disable special
   
3.        treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion.
   
4. 
   
5.        Each of the metacharacters listed above under DEFINITIONS has special meaning to the shell and must be quoted if it  is  to  represent
   
6.        itself.
   
7. 
   
8.        When  the  command history expansion facilities are being used (see HISTORY EXPANSION below), the history expansion character, usually
   
9.        !, must be quoted to prevent history expansion.
   
10. 
    
11.        There are three quoting mechanisms: the escape character, single quotes, and double quotes.
    
12. 
    
13.        A non-quoted backslash (\) is the escape character.  It preserves the literal value of the  next  character  that  follows,  with  the
    
14.        exception  of  <newline>.   If  a \<newline> pair appears, and the backslash is not itself quoted, the \<newline> is treated as a line
    
15.        continuation (that is, it is removed from the input stream and effectively ignored).
    
16. 
    
17.        Enclosing characters in single quotes preserves the literal value of each character within the quotes.  A single quote may  not  occur
    
18.        between single quotes, even when preceded by a backslash.
    
19. 
    
20.        Enclosing  characters in double quotes preserves the literal value of all characters within the quotes, with the exception of $, `, \,
    
21.        and, when history expansion is enabled, !.  The characters $ and ` retain their special meaning within double quotes.   The  backslash
    
22.        retains  its  special  meaning only when followed by one of the following characters: $, `, ", \, or <newline>.  A double quote may be
    
23.        quoted within double quotes by preceding it with a backslash.  If enabled, history expansion will be performed unless an !   appearing
    
24.        in double quotes is escaped using a backslash.  The backslash preceding the !  is not removed.
    
25. 
    
26.        The special parameters * and @ have special meaning when in double quotes (see PARAMETERS below).
    
27. 
    
28.        Words  of  the form string' are treated specially.  The word expands to string, with backslash-escaped characters replaced as speci-
    
29.        fied by the ANSI C standard.  Backslash escape sequences, if present, are decoded as follows:
    
30.               \a     alert (bell)
    
31.               \b     backspace
    
32.               \e
    
33.               \E     an escape character
    
34.               \f     form feed
    
35.               \n     new line
    
36.               \r     carriage return
    
37.               \t     horizontal tab
    
38.               \v     vertical tab
    
39.               \\     backslash
    
40.               \'     single quote
    
41.               \"     double quote
    
42.               \nnn   the eight-bit character whose value is the octal value nnn (one to three digits)
    
43.               \xHH   the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
    
44.               \cx    a control-x character
    
45. 
    
46.        The expanded result is single-quoted, as if the dollar sign had not been present.
    
47. 
    
48.        A double-quoted string preceded by a dollar sign ($"string") will cause the string to be translated according to the  current  locale.
    
49.        If the current locale is C or POSIX, the dollar sign is ignored.  If the string is translated and replaced, the replacement is double-
    
50.        quoted.

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中文说明:



卧槽, 我发现我好无聊...

ll104567

在单引号中也是生效的@-@
~/pwb/tmp/sh/xie# echo -e 'asd\n'
asd

biangbiang_cat

感谢大神，完全明白了，讲的真透彻真详细。。。谢谢谢谢。回复 2# reyleon


   

jason680

$ echo -e abc\\nefg
abc
efg