如何躲过历史扩展感叹号！在双引号字符串中?

问题描述

命令替换对于令人惊讶的行为不是必需的，尽管它是最常见的用例.同样的问题只适用于 echo "'!b'"

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??

• 在最后一个例子中，逃避 ! 的最佳方法是什么?
• 为什么即使我使用单引号也没有转义，而 echo "$(echo '$b')" 是?! 和 $ 有什么区别?
• 为什么 echo $(echo '!b')(无引号)有效?(@MBlanc 指出).
• In the last example, what is the best way to escape the !?
• Why was it not escaped even if I used single quotes, while echo "$(echo '$b')" was? What is the difference between ! and $?
• Why was does echo $(echo '!b') (no quotes) work? (pointed by @MBlanc).

我宁愿不这样做:

• set +H 因为我之后需要 set -H 来维持 shell 状态

• set +H as I would need set -H afterwards to maintain shell state

反斜杠转义，因为我需要为每个 ! 设置一个，而且它必须在引号之外:

backslash 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')(无引号)，因为命令可能返回空格.

• echo $(echo '!b') (no quotes), because the command could return spaces.

  版本:

  bash --version | head -n1
  # GNU bash, version 4.2.25(1)-release (i686-pc-linux-gnu)
  

  推荐答案

  在上一个例子中，

  echo "$(echo '!b')"
  

  感叹号不是单引号.因为历史扩展发生在解析过程的早期，单引号只是双引号字符串的一部分；解析器尚未识别命令替换以建立新的上下文，其中单引号将引用运算符.

  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.

  要修复，您必须暂时关闭历史扩展:

  To fix, you'll have to temporarily turn off history expansion:

  set +H
  echo "$(echo '!b')"
  set -H
  

  这篇关于如何躲过历史扩展感叹号！在双引号字符串中?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！