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Why are tar and gzip almost always used together, and not just gzip? Is there any advantage to that method?

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migrated from Mar 2 '11 at 3:47

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TAR creates a single archived file out of many files, but does not compress them.

Format Details

A tar file is the concatenation of one or more files. Each file is preceded by a 512-byte header record. The file data is written unaltered except that its length is rounded up to a multiple of 512 bytes and the extra space is zero filled. The end of an archive is marked by at least two consecutive zero-filled records.

GZIP compresses a single file into another single file, but does not create archives.

File Format

...Although its file format also allows for multiple such streams to be concatenated (zipped files are simply decompressed concatenated as if they were originally one file), gzip is normally used to compress just single files.[4] Compressed archives are typically created by assembling collections of files into a single tar archive, and then compressing that archive with gzip.

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and that make the "tgz" files fairly unique compared to other compressed archive formats in that, e.g. you generally cannot extract one specific file without first uncompressing whatever sits in front of that file. – sylvainulg Mar 2 '11 at 9:37
There is no such thing as a "tgz" file. It is a tar.gz. The job of gzip is to zip or unzip its content (in this case, a tar archive.) Then you unarchive it with tar. It is typical Unix pipelining philosophy, and thus hardly unique. – luis.espinal Mar 2 '11 at 11:00
No, .tar.gz isn't unique: .tar.bz2, .cpio.gz, etc. work the same way. – user46971 Mar 2 '11 at 13:55
@user36310 I know what you mean in principle, but in practice the tools let you extract a single file. tar -xzvf tarball.tar.gz single/file.txt. Behind the scenes it needs to do some extra work but for appearances it extracts a single file. – Rich Homolka Mar 2 '11 at 18:44
Make that "a lot of extra work" if the file is at the end of a large archive. Clearly, if you need random access, zip/rar/xar/7z/lzh/arj/cab/sit etc. are superior formats. – LaC Apr 20 '11 at 11:19

Gzip / Bzip2 are stream compressors. They compress a stream of data into something smaller. They could be used on individual files, but not on groups of files on their own.

Tar on the other hand has the ability to turn a list of files, with paths, permissions and ownership information, into a single continuous stream - and vice versa.

That's why, to archive files (and if one needs compression as well), one usually uses tar + some compression method.

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Tar is in charge of doing one and only one thing well: (un)archiving into(out of) a single archive file. Of what? Of one and only one thing: a set of files.

Gzip is in charge of doing one and only one thing well: (un)compressing. Of what? Of one thing and one thing only: a single file of any type... and that includes a file created with tar.

It goes back to the UNIX philosophy of pipelining, the underlying "pipe and filters" architecture ; the treatment of everything as a file and the sound architectural goal of "one-thing-does-one-thing-only-and-does-it-well" (which results in a very elegant and simple plug-n-play of sorts.)

In its simplicity, it is almost algebraic in nature (a hefty goal in systems design). And that is no easy feat.

In many ways (and not without its flaws), this is almost a pinnacle in composability, modularity, loose coupling and high cohesion. If you understand these four (and I mean really understand), you understand, it will be obvious why tar and gzip work like that in pairs.

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This UNIX philosophy is beautiful all right, but I'm observing that it falls short in, creating non-solid archives. (Extracting a single file from a 1-GB tar.gz shouldn't be a pain, and from what I've understood here, ZIP is pretty much superior to tar.gz... right?) – Mehrdad Mar 2 '11 at 17:06
@Mehrad - First, what is a non-solid archive? As attributed to Voltaire, "If you would converse with me, first you must define your terms.". Second, yes the pipe and filters architecture falls short in specific cases, just like any other architecture, regardless of its beauty. That's a given with a modicum of engineering, and it is not the argument that is being made. Third, zip is superior to gzip and tar, but that was not what you asked. You asked why zip and tar work they way they do and if there were any advantages, and you were given a technical answer. – luis.espinal Mar 2 '11 at 20:36
@Mehrad - also, I don't know what kind of problems do you encounter when unzipping|untar a 1gig tar.gz file. I've done that many times, up to 2GB with older installs of gzip (and up to 4gb with newer versions of gzip.) If you are doing it over the wire or on a NFS mount, then duh! You'll encounter similar performances problems as if done with plain zip. Heck, I've even untarred from a pipe to a remote process spitting gzip input into a socket. Try that with zip. For each problem, use the appropriate tool (be it tar|gz or zip.) – luis.espinal Mar 2 '11 at 20:41

First of all, TAR wasn't created to create file archives. It's Tape ARchiver. It's job is to write out or load in an archive to/from tape.

The -f option makes it use a file as "virtual tape", which can then be compressed by another program. In fact, such compression happens on real-world tapedrives as well.

Of course, the philosophy of using one program to do it well also counts in this case, but one might miss why TAR archives are structured as a stream instead of directory of contents + contents.

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Right... ZIP files put all the file info in a unified header, then all the file contents... that makes it impossible to append more files to a ZIP file... you have to rewrite the whole file... with TAR format, the header for each file is separate, so you can easily append more files without rewriting the whole tape – JoelFan Mar 2 '11 at 19:57

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