我sed用来查找和替换文件中的某些项目，如下所示：
sed -i "s/$a/$b/g" ./file.txt
我需要将此查找和替换限制为仅当匹配文本位于方括号之间时，例如：
The sheep lived in the barn.
One day, [the sheep ate some] grass.
The [sheep] found a nice place to sleep under the tree.
Afterwards, [the sheep ate apples under] the tree [near the barn].
The sheep [enjoyed the theme] of the movie.
假设$a设置为“the”并$b设置为“B”，输出将如下所示：
The sheep lived in the barn.
One day, [B sheep ate some] grass.
The [sheep] found a nice place to sleep under the tree.
Afterwards, [B sheep ate apples under] the tree [near B barn].
The sheep [enjoyed B theme] of the movie.
- 左方括号和右方括号总是出现在同一行。
如何仅查找和替换出现在方括号之间的那些项目sed？