3 ms·

> I extract then inside an empty subdirectory in my home first ... AFTER a "tvf[jzx]", I hope

by kennethrc 2y ago

marcosdumay 2y ago
What's the gain of -t if the extraction target is disposable? Does -x have some side effect that -t would list for you?
- TheDong 2y ago
  It's nice to check if you're about to extract a 1MB tarball into 2TBs of data before actually running out of disk space. Most tar programs do prevent extracting tarballs containing absolute paths (like '/etc/passwd') and relative paths (like '../../../etc/passwd'), but older tar programs still allow that. And programs written in Go, because of course: https://github.com/golang/go/issues/55356 https://github.com/golang/go/issues/55356 Overall, if your HDD size is infinite and you're using GNU tar, or another recent tar, you can skip 't' I think before doing a '-C' extraction into some safe directory.
  - vorakl 2y ago
    How do you create a tar archive that "contains absolute paths"?
    - 10000truths 2y ago
      The tar file format doesn't prevent you from specifying absolute paths in the archive. It's up to the tool extracting the archive to reject/ignore such paths.
      - vorakl 2y ago
        I asked about options for GNU tar because there is a bit of strange behavior. To add absolute paths to an archive, there is "-P" option, and man says it works only for creating archives: "Don't strip leading slashes from filenames when creating archives". To extract absolute paths from the archive, you need to add the "-C /" option, and although the tool says "tar: Strip leading `/' from member names", it will still extract it in the right place because the paths become relative and -C puts them in the root. However, if you add "-P" during the extraction (which is not mentioned in man), the "strip leading slashes" information disappears. So if this message bothers someone, "tar -C / -xPf file.tar" will cleanly extract absolute paths from the archive ;)
    - yjftsjthsd-h 2y ago
      The first field in the tar header is char name[100]; (See https://man.archlinux.org/man/tar.5.en https://man.archlinux.org/man/tar.5.en ) So anything that will write an absolute path there, including literally opening it in a text editor and replacing the path by hand because that whole header is just fixed length ASCII with null terminated strings. (I mean I assume the tar(1) command can do it too but you don't need that, the format is dead simple, if weird.)
      - eichin 2y ago
        It's a good exercise to open one of these files in hexdump or something to get a feel for what's really going on inside... but yeah, GNU tar has -P / --absolute-names to just create them with leading slashes.

> I extract then inside an empty subdirectory in my home first ... AFTER a "tvf[jzx]", I hope

marcosdumay 2y ago

What's the gain of -t if the extraction target is disposable? Does -x have some side effect that -t would list for you?

TheDong 2y ago
It's nice to check if you're about to extract a 1MB tarball into 2TBs of data before actually running out of disk space. Most tar programs do prevent extracting tarballs containing absolute paths (like '/etc/passwd') and relative paths (like '../../../etc/passwd'), but older tar programs still allow that. And programs written in Go, because of course: https://github.com/golang/go/issues/55356 https://github.com/golang/go/issues/55356 Overall, if your HDD size is infinite and you're using GNU tar, or another recent tar, you can skip 't' I think before doing a '-C' extraction into some safe directory.
- vorakl 2y ago
  How do you create a tar archive that "contains absolute paths"?
  - 10000truths 2y ago
    The tar file format doesn't prevent you from specifying absolute paths in the archive. It's up to the tool extracting the archive to reject/ignore such paths.
    - vorakl 2y ago
      I asked about options for GNU tar because there is a bit of strange behavior. To add absolute paths to an archive, there is "-P" option, and man says it works only for creating archives: "Don't strip leading slashes from filenames when creating archives". To extract absolute paths from the archive, you need to add the "-C /" option, and although the tool says "tar: Strip leading `/' from member names", it will still extract it in the right place because the paths become relative and -C puts them in the root. However, if you add "-P" during the extraction (which is not mentioned in man), the "strip leading slashes" information disappears. So if this message bothers someone, "tar -C / -xPf file.tar" will cleanly extract absolute paths from the archive ;)
  - yjftsjthsd-h 2y ago
    The first field in the tar header is char name[100]; (See https://man.archlinux.org/man/tar.5.en https://man.archlinux.org/man/tar.5.en ) So anything that will write an absolute path there, including literally opening it in a text editor and replacing the path by hand because that whole header is just fixed length ASCII with null terminated strings. (I mean I assume the tar(1) command can do it too but you don't need that, the format is dead simple, if weird.)
    - eichin 2y ago
      It's a good exercise to open one of these files in hexdump or something to get a feel for what's really going on inside... but yeah, GNU tar has -P / --absolute-names to just create them with leading slashes.