Linux Shell参数扩展(Parameter Expansion)
Shell Command Language在线文档:
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02
Shell Command Language在线文档:
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02