Easily unpack DEB, edit postinst, and repack DEB

  • I'm attempting to install Intel's OpenCL SDK but the DEB files are buggy conversions from RPM (see here for the curious). I need to edit the postinst script in the DEB they provide.

    How can I take an existing DEB, extract the contents (including the control information), then later repackage the contents to make a new DEB? I will only edit files, no files will be added or removed.

  • The primary command to manipulate deb packages is dpkg-deb.

    To unpack the package, create an empty directory and switch to it, then run dpkg-deb to extract its control information and the package files. Use dpkg-deb -b to rebuild the package.

    mkdir tmp
    dpkg-deb -R original.deb tmp
    # edit DEBIAN/postinst
    dpkg-deb -b tmp fixed.deb
    

    Beware that unless your script is running as root, the files' permissions and ownership will be corrupted at the extraction stage. One way to avoid this is to run your script under fakeroot. Note that you need to run the whole sequence under fakeroot, not each dpkg-deb individually, since it's the fakeroot process that keeps the memory of the permissions of the files that can't be created as they are.

    fakeroot sh -c '
      mkdir tmp
      dpkg-deb -R original.deb tmp
      # edit DEBIAN/postinst
      dpkg-deb -b tmp fixed.deb
    '
    

    Rather than mess with permissions, you can keep the data archive intact and modify only the control archive. dpkg-deb doesn't provide a way to do that. Fortunately, deb packges are in a standard format: they're ar archives. So you can use ar to extract the control archive, modify its files, and use ar again to replace the control archive by a new version.

    mkdir tmp
    cd tmp
    ar p ../original.deb control.tar.gz | tar -xz
    # edit postinst
    cp ../original.deb ../fixed.deb
    tar czf control.tar.gz *[!z]
    ar r ../fixed.deb control.tar.gz
    

    You should add a changelog entry and change the version number if you modify anything in the package. The infrastructure to manipulate Debian packages assumes that if two packages have the same name and version, they're the same package. Add a suffix to the debian_revision part at the end of the version number; for sorting reasons the suffix should start with ~, e.g. 1.2.3-4.1 becomes 1.2.3-4.1~johnjumper1.

    Instead of using shell tools, you can use Emacs. The dpkg-dev-el package (which is its own upstream as this is a native Debian package) contains modes to edit .deb files and to edit Debian changelogs. Emacs can be used interactively or scripted.

    You can also use the `-e` switch of fpm to change the control file: `fpm -e -s deb -t deb ../old.deb`. This will open the control file in your editor.

    btw, `fakeroot bash` and try to issue commands will not work concerning ownership, and the `ar` method is incredbly fast for big deb files!

    Thanks. This was useful. Using `dpkg-deb -R` the modes were kept and `dpkg-deb -b` reset the `uid:gid` of the extracted files to `0:0`. Didn't need fakeroot (I imagine there might be issues if there were set{u,g}id files inside the archive but that wasn't the case in my situation.

    @PSkocik Not just setxid files, also e.g. files and directories under `/etc` or `/var` that need to belong to a specific group.

    Thanks for the great answer. Is there a way to ensure that the rebuilt package vs original package has the same md5sum?

    @AmaJayJB In useful cases, the rebuilt package cannot have the same md5sum since it has different content. If you want to ensure that it has the same content in the special case when you don't make any edits, the code in my answer doesn't do that, you need to look into reproducible builds.

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Content dated before 6/26/2020 9:53 AM