Well, you could just run the output of tar, cpio, or whatever through compress but if even one bit of your tape or diskette goes bad, you could lose the rest of the backup. Not recommended at all, unless of course you don't actually care if your backups work - but if you didn't care, you wouldn't be doing any, right?
Linux and other more modern versions of tar include a compress flag, but that may be no different than compressing the archive itself: individual files are not compressed, just the whole archive.
There's also the obvious limitation that you can't grab a particular file from that tape. You'd need to uncompress the whole thing to get at one file.
Read the man pages carefully. For example, GNU tar warns:
That should convince you that this is a bad idea.
But do you even need to do this? Your tape drive probably does hardware compression anyway, so this may be unimportant. It definitely will take more time. You still should look at better backup than tar or cpio though.
Of course if it just won't fit..
bzip2, bzip2 and so on
Compression used to be a lot more important. In these days of cheap 100 GB hard drives and fast internet connections, I don't find myself using compression much. However, just about anything you download will be compressed, and if it isn't gzippped, it's probably uses bzip2, which can provide greater compression for some files.
As high speed internet becomes more common, I wonder when people will stop bothering to compress files? It already makes little sense for relatively small files. Disk space is so cheap now and high speed Internet is so common - I almost never care about compression.
Although I can't recall having this problem in recent years, compressed files can be damaged. With gzipped files, the man page says that "zcat" will read them up to the point of the damage.
Bzip2 does a little better. Because bzip2 writes its compressed data in specific sized blocks, it is possible to recover parts of a file even beyond where it has been damaged. Of course that may do you little good, but sometimes that could help.
This newsgroup post by Bela Lubkin explains the relationship between various file archivers and how they work.. He mentions compressgzip, bzip2, zip, pack and 7-zip.
Also see Best Linux backup software.
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