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metacharacter如果unquoted，这些字符分开words。它们是：space tab newline | & ; ( ) < >Quoting用来移除某些字符的特殊含义。包含3种quoting机制：escape character, single quotes, double quot

写一个test.sh#!/bin/bash[ "abc" > "" ]执行：$ ./test.sh./test.sh: line 3: : No such file or directorystrace发现，它竟然会尝试打开空文件。read(255, "#!/bin/bash\n\n[ \"abc\