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Recently, I've been backing up a lot of my data, and I noticed that I can save files as .gz or .tar.gz, or .7z and .tar.7z, etcetera. What are the differences between the normal one and the .tar.* variant? Which one of them is adviced when making backups?

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6 Answers 6

up vote 70 down vote accepted

If you come from a Windows background, you may be familiar with the zip and rar formats. These are archives of multiple files compressed together.

In Unix and Unix-like systems (like Ubuntu), archiving and compression are separate.

tar puts multiple files into a single (tar) file.

gzip compresses one file (only).

So to get a compressed archive, you combine the two, first use tar or pax to get all files into a single file (archive.tar), then gzip it (archive.tar.gz). If you only have one file you need to compress (notes.txt), there's no need for tar, so you just do gzip notes.txt which will result in notes.txt.gz. There are other types of compression, such as compress, bzip2 and xz which work in the same manner as gzip (apart from using different types of compression of course)

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4  
That makes sense, but then, why is it that I can make a .7z archive from multiple files, while I can also make a .tar.7z archive from multiple files? –  Exeleration-G Apr 15 '12 at 1:46
    
@Exeleration-G 7zip does not follow this scheme. It's more like zip and rar. Not sure what the point of using tar in conjunction with 7zip is as I haven't used 7zip myself. –  geirha Apr 15 '12 at 1:53
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@Exeleration-G After reading through the other answers, I see SaultDon answers this; 7zip doesn't store unix ownership and permission of the archived files (it seems to be mainly geared towards Windows, like zip and rar), so it makes sense to combine it with tar to get ownership and permissions preserved. –  geirha Apr 15 '12 at 2:02
    
@geirha Mark the question as solved if you find the appropriate answer. –  Ravi Apr 15 '12 at 5:14

It depends on what you are looking for... Compression or archiving?

When I talk about archiving, I mean preserving permissions, directory structure, etc...

Compression may ignore most of that and just get your files in a smaller packages.

To preserve file permissions, use tar:

tar cpvf backup.tar folder

The p flag will save file permissions. Use the z flag for gzip compression or the j flag for bzip compression.

tar czpvf backup.tar.gz folder #backup.tgz is acceptable as well
tar cjpvf backup.tar.bz2 folder #backup.tbz2 works too

If you want to have a tar file you can "update" package the tar using the P flag:

tar cpPvf backup.tar folder

Then to update, replace 'c' with 'u' and when unpacking, you can use 'k' to preserve files that already exist.

tar upPvf backup.tar folder #updating a tar file
tar xpPkvf backup.tar #extracting a tar with permissions(p) and not extracting(k) files that exist on disk already

The P flag saves files with full paths, so - /home/username vs home/username (notice the leading forward slash).

7z compression offers greater compression, but does not preserve file ownership, permissions, etc. Rzip is another compression utility that offers comparable compression with 7z as well.

I guess a backup.tar.7z file is just a tar file (with permissions) compressed by a 7z file, though I wouldn't be surprised if little compression occurred because 7z may not be able to dump the file metadata. It's 7z's ability to exclude the file metadata that it can offer great compression (amongst other things of course).

Compression depends entirely on data type as well. Some files don't compress well because they may already be compressed with some other means (ie, .mp3, .jpg, .tiff/with lzma, .rpm, etc).

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gzip or bzip2 doesn't know about file system - file name, directory, tree structure. Just compress input stream, then output result. Even gzip or bzip2 can't archive directory standalone. It is why combined with tar.

tar(archiver) - just archive file structure. gzip,bzip2(compressor) - just compress input.

I think this strategy came from 'do one thing well' unix philosophy. Tar works well? Leave it is. Need more compression ratio than gzip? Here is bzip2 or 7zip.

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Actually 7zip can archive like zip or rar. –  Mait Apr 15 '12 at 4:56

its different styles of compression , tar by itself is simply archived(little to no compression). tar.gz is a tar archive but the contents are compressed by gzip(moderate compression) hence the .gz and tar.7z is compressed using 7zip (usually super high compression)

when backing up I would recommend tar.7z as it has the highest compression rate saving you space but uses an extra program (7zip). .tar.gz will be larger files and do the same job, you could also use bzip (.tar.bz/bz2) although i'm not sure if that would suit you better as I use gzip or 7zip

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typically, *.tar files are just tar files created by tar program, *.gz programs are created by gzip, *.tar.gz (sometime also *.tgz) are gziped tar files, and *.7z are created by 7zip.

However, in Linux/Unix, one can name a file pretty much anyway he wants, so it is completely at the discretion of the creator of the files.

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Tar (Tape Archiver) has traditionally been used as a container in Unix/Linux to package files for movement. It packages the file structure and maintains file attributes, but it doesn't compress the files.

Compression programs compress the file to make it smaller, but they may not handle multiple files, and/or they may not handle the file attributes neccesssary for Linux. Since tar already exists and is well-supported, there's no reason for archiving programs to duplicate this functionality, which is platform-specific (re, different for Windows and Linux). Also, different compression programs may perform differently on different types of files, so having a choice of more than one is desirable.

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