BASH 参数扩展期间单引号行为不一致的原因？

Question

当使用 BASH Parameter Expansion 时，变量扩展成的字符串可以是 quoted/escaped，这工作正常，除非使用单引号并且整个变量以双引号转义报价：

$ echo "${var:-\a}"
\a # ok
$ echo "${var:-"\a"}"
\a # ok
$ echo "${var:-$'\a'}"
\a # ok
$ echo "${var:-'\a'}"
'\a' # wtf?

有趣的是，$' ' 引号可以正常使用，而 ' ' 则不能。如果变量本身未被引用，单引号开始正常工作：

$ echo ${var:-'\a'}
\a

但是，如果 $var 本身包含空白字符，这可能会导致其他问题。

这种不一致有什么充分的理由吗？

Answer 1

我认为这是源代码中最相关的引用 (y.tab.c)：

  /* Based on which dolstate is currently in (param, op, or word),
     decide what the op is.  We're really only concerned if it's % or
     #, so we can turn on a flag that says whether or not we should
     treat single quotes as special when inside a double-quoted
     ${...}. This logic must agree with subst.c:extract_dollar_brace_string
     since they share the same defines. */
  /* FLAG POSIX INTERP 221 */

  [...]

  /* The big hammer.  Single quotes aren't special in double quotes.  The
     problem is that Posix used to say the single quotes are semi-special:
     within a double-quoted ${...} construct "an even number of
     unescaped double-quotes or single-quotes, if any, shall occur." */
  /* This was changed in Austin Group Interp 221 */

我不是很清楚为什么单引号并不特别，但这似乎是经过长时间（有人告诉我有争议的）辩论之后做出的有意识的选择改变。但事实是（如果我总结的没错的话），这里的单引号只是普通字符，不是句法引号，按字面意思对待。

BASH 参数扩展期间单引号行为不一致的原因？

Reason for inconsistent behavior of single quotes during BASH parameter expansion?

bash

shell

quotes

quoting

parameter-expansion