On 2025-08-09 12:39:35 -0700, Michael Paoli wrote: > On Sat, Aug 9, 2025 at 2:50 AM Vincent Lefevre <[email protected]> wrote: > > On 2025-08-09 01:30:52 -0700, Michael Paoli wrote: > > > < tar.xz | xz -d | tar tf - > > > > With tar utilities that support xz (like GNU tar), not using "xz -d" > > could be more efficient as "xz -d" will uncompress the whole file > > while this may not be necessary: > > > > tar tf file.tar.xz > > > > is sufficient. This may allow one to skip xz blocks if the archive > > contains big files. That said, I don't know whether GNU tar has > > such an optimization. > I rather doubt any tar implementation has such an optimization. > I don't think there's any tar format that has an "index" or the like,
There is no need for an index (though an index would allow even better optimization). > it's generally just a tar header, then for each file (of any type), > specific header for that, and any associated data, and I think there's > also some type of end marker or the like, perhaps with a bit of metadata > at the end too. What's important is that there is a file size. For instance, "tar -t" does not need to read the full contents of the archive: it can use fseek to skip file contents (when the file is seekable). In a compressed archive, you may have blocks that are entirely part of a single file. In such a case, decompressing such blocks is useless if the concerned file does not have to be extracted. -- Vincent Lefèvre <[email protected]> - Web: <https://www.vinc17.net/> 100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/> Work: CR INRIA - computer arithmetic / Pascaline project (LIP, ENS-Lyon)
