即便是同一个文件夹的压缩文件，如果压缩的时间不一样，那么产生的压缩文件的md5值也是不一样的。
原因有三个:
1 Timestamps in Metadata – Many compression tools store file timestamps (creation, modification, and access times) inside the archive. If you compress the same folder at different times, these timestamps may differ, leading to a different MD5 hash.
2 Compression Algorithm Variations – Some compression tools use different compression settings, even if the same files are inside. Small variations in compression level or metadata can result in a different output file and a different MD5 hash.
3 Order of Files – If the compression tool does not always process files in the same order, the resulting archive may be structured differently, leading to a different MD5 hash.
如果想得到固定的MD5值，可以使用如下的三个方法:
1 Hash Individual Files – Instead of hashing the compressed folder, compute MD5 hashes for each file inside the folder and compare them.
2 Use Deterministic Compression – Some tools (like tar –sort=name with gzip –no-name) help create archives with consistent metadata.
3 Exclude Timestamps – Some formats allow you to omit timestamps (e.g., zip –latest-time).