p7zip
 

I created a 10 gigabyte bzip2 file. That took a while. I left it going over night, so I don't know exactly how long it took. I really need to compress the best I can because I'm backing up my hard drive to another hard drive before I reinstall Linux, and space is limited. I want to use p7zip because it compresses better than bzip2. Can I compress the file that's already a compressed bzip2 file? I don't want to uncompress the bzip2 file and then compress it again again with p7zip. I have a few 10 gigabyte files, and that would take a while.

If you try and make the compression files too big they don't compress as well. About 500 MB is the maximum size you will get good compression at. Even that is quite large for good compression. Bzip2 compresses as well as anything. P7zip might say it does better, but it doesn't. Why don't you back up your user files, in /home, with all your personal settings, and forget about the rest. Maybe back up /etc, too. You don't need to back up /usr, /tmp, /dev, /lib, /srv, /bin, /opt, /sys, or /var. That stuff is all reinstalled with the new os. Believe me, I've done this a lot. The way you are going about this is screwy. Learn how to use linux, the right way. You might consider repartitioning the disk so /home is a separate partition. This way whenever you want to install a new linux distro you just tell the partitioner not to format the partition you use for /home, and to give it /home as a mount point. You could dispense with all the pageantry, and just get it done.

Thanks for trying to help, but I am trying to get rid of the lvm2 shit on the hard drive, so I am formatting all of it. I will have a better plan for backup next time. I will keep a backup partition at the end of the drive and have a cron job to back up my important files while I am asleep at night. Can you show me where it says that 500 megabytes is the largest file that you can get good compression from? I have done lots of research on compression and even learned about huffman's coding, but I never saw that. I'd like to read more about what you are saying.

Linux backup 7zip
 

Just a question, why do you need to backup so much to re-install Linux?
I'm looking for how to install an unrar system, but I'm curious.
I have 2TB of stockage, and only a 50GB partition serves as my Linux installation partition, with a 2nd of the same size, where I place an old installation while I try out the new one.
On the rest of the HDD's I have multimedia, text books, and other stuff, which stays where it is, with soft links to my /home. Of course downloads which I want to look at immediately go in the Linux /home, but when I finish with them, I move them to their appropriate place.
In this manner, I do regular mirror backups, to the 2nd HDD, or a CD/DVD, in case I install something, plus its' dependences, which bugs my installation.
I've been using this system for years, and find it the easiest way when there's lots of other material, which you keep whatever the Linux you use. I can just "dd" the contents of my 2TB discs to another, and I also will have a backup of the stuff soft linked, in case od HDD failure.
I must admit that compressing all the music, films and books I have by half would make it cheaper for a backup HDD, or I could use cloud, but I have sometimes lost things during compression/decompression.

I think that the OP figured this out...since this thread had been closed for NINE YEARS at this point.