因为使用单引号禁用了称为扩展的bash特性，其中，特殊字符和字符系列由值替换。
Because using single quotes disables a bash feature called expansion, where special characters and sequences of characters are replaced with values.
这个产品系列已经扩展到包括高温和特殊应用，如氧等应用领域。
The product line has been expanded to include high temperature and special service options like oxygen.
设置将随后扩展额外的主题和人物，其中包括一系列特殊功能的数字和字母，第一个步骤，在学习的乐趣。
The set will be subsequently extended with additional themes and characters, including special series that feature Numbers and alphabet to make first steps in learning fun.
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