如何逃脱历史扩展感叹号!一个双引号&QUOT内;命令替换像" $('!B'回声)QUOT ;? [英] How to escape history expansion exclamation mark ! inside a double quoted " command substitution like "$(echo '!b')"?
问题描述
编辑:命令取代是不必要的令人惊讶的行为,尽管它是最常见的情况。同样的问题也适用于刚回声'!B'
B = A#开启历史替换。
#此选项在默认情况下交互shell。
设置-H呼应'!B'
#输出:'B'
# 好。单引号逃脱。回声$(回声'!B')
#输出:'B'
# 好。单引号逃脱。回声$(回声$ B')
#输出:'$ B'
# 好。单引号逃脱。回声$(回声'!B')
#输出:历史扩张
#BAD!为什么??
- 在最后一个例子,什么是逃避的最佳方法
!
? - 为什么它没有逃过即使我使用单引号,而
回声$(回声$ B')
是?是什么区别!
和$
? - 为什么不
回声$(回声'!B')
(不含引号)工作? (由@MBlanc指出)。
我想preFER为此而不:
-
设置+ H
,因为我需要设置-H
之后维持外壳的状态。 -
反斜杠,因为我需要一个每次
回声$(回声'\\!A')
# '\\!一个'。
# 不好。回声$(回声'一'\\!'B'\\!'C')
# 一个 ! b! C
#好,但冗长。 -
回声$(回声'!B')
(不含引号),因为该命令可以返回的空间。
版本:
的bash --version |头-n1
#GNU的bash,版本4.2.25(1) - 释放(i686的-PC-Linux的GNU)
在您的最后一个例子,
回声$(回声'!B')
感叹号不是单引号。由于历史扩展在分析过程中这么早时,单引号都只是双引号字符串的一部分;解析器无法识别命令替换尚未建立起一个新的上下文,其中单引号会被引用的运营商。
要解决,你必须暂时关闭历史扩展:
集+ H
回声$(回声'!B')
设置-H
EDIT: the command substitution is not necessary for the surprising behavior, although it is the most common use case. The same question applies to just echo "'!b'"
b=a
# Enable history substitution.
# This option is on by default on interactive shells.
set -H
echo '!b'
# Output: '!b'
# OK. Escaped by single quotes.
echo $(echo '!b')
# Output: '!b'
# OK. Escaped by single quotes.
echo "$(echo '$b')"
# Output: '$b'
# OK. Escaped by single quotes.
echo "$(echo '!b')"
# Output: history expands
# BAD!! WHY??
- In the last example, what is the best way to escape the
!
? - Why was it not escaped even if I used single quotes, while
echo "$(echo '$b')"
was? What is the difference between!
and$
? - Why was does
echo $(echo '!b')
(no quotes) work? (pointed by @MBlanc).
I would prefer to do this without:
set +H
as I would needset -H
afterwards to maintain shell statebackslash escapes, because I need one for every
!
and it has to be outside the quotes:echo "$(echo '\!a')" # '\!a'. # No good. echo "$(echo 'a '\!' b '\!' c')" # a ! b ! c # Good, but verbose.
echo $(echo '!b')
(no quotes), because the command could return spaces.
Version:
bash --version | head -n1
# GNU bash, version 4.2.25(1)-release (i686-pc-linux-gnu)
In your last example,
echo "$(echo '!b')"
the exclamation point is not single-quoted. Because history expansion occurs so early in the parsing process, the single quotes are just part of the double-quoted string; the parser hasn't recognized the command substitution yet to establish a new context where the single quotes would be quoting operators.
To fix, you'll have to temporarily turn off history expansion:
set +H
echo "$(echo '!b')"
set -H
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