Is it possible to update a file in a compressed archive.tgz using the tar app without uncompressing/extracting, update and compressing/creating ?
tar -uvzf archive.tgz ./file.txt
tar: Cannot update compressed archives
Try `tar --help' for more information.
As far as I know, I think you are forced to uncompress the tarfile before trying an update and then recompress the updated file 
I thought it was possible to do the trick with a named pipe but the tar update command needs a real non-compressed tarfile to process, so it's useless.