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I usually assumed that tar was a compression utility, but I am unsure, does it actually compress files, or is it just like an ISO file, a file to hold files?

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Also on SuperUser. –  allquixotic Apr 30 '14 at 17:38

4 Answers 4

up vote 33 down vote accepted

Tar is an archiving tool (Tape ARchive), it only collects files and their metadata together and produces one file. If you want to compress that file later you can use gzip/bzip2/xz. For convenience, tar provides arguments to compress the archive automatically for you. Checkout the tar man page for more details.

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A slight clarification on the answer. It is GNU tar that provides those extra compression arguments. For example, Solaris tar does not provide arguments for compression. –  Tero Kilkanen Apr 29 '14 at 22:20
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oooh, that's why I keep seeing thing.tar.7z –  Mooing Duck Apr 30 '14 at 0:34
    
BSD tar provides an argument for compression as well, though it only accepts z and determines the compression method based on the extension, whereas GNU tar has separate zZjJ arguments for the different compression methods. –  wingedsubmariner Apr 30 '14 at 0:59
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Just read the BSD tar manpage, and it turns out I was mistaken, BSD tar uses separate zZjJ for compression just like GNU tar. However, it does automatically detect compression when decompressing though, whereas GNU tar expects zZjJ then also. –  wingedsubmariner Apr 30 '14 at 3:10
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@wingedsubmariner: no; modern-ish versions of GNU tar decompress automatically without requiring the -zZjJ options. –  Jonathan Leffler Apr 30 '14 at 4:02

tar produces archives; compression is a separate functionality. However tar alone can reduce space usage when used on a large number of small files that are smaller than the filesystem's cluster size. If a filesystem uses 1kb clusters, even a file that contains a single byte will consume 1kb (plus an inode). A tar archive does not have this overhead.

BTW, an ISO file is not really "a file to hold files" - it's actually an image of an entire filesystem (one originally designed to be used on CDs) and thus its structure is considerably more complex.

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Actually an empty file will not consume 1kb. A 1-1023 byte file will. –  psusi Apr 30 '14 at 3:28

tar utility does not compress until you give argument to do so [tar -z file name].

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The original UNIX tar command did not compress archives. As was mentioned in a comment, Solaris tar doesn't compress. Nor does HP-UX, nor AIX, FWIW. By convention, uncompressed archives end in .tar.

With GNU/Linux you get GNU tar. (You can install GNU tar on other UNIX systems.) By default it does not compress; however, it does compress the resulting archive with gzip (also by GNU) if you supply -z. The conventional suffix for gzipped files is .gz, so you'll often see tarballs (slang for a tar archive, usually implying it's been compressed) that end in .tar.gz. That ending implies tar was run, followed by gzip, e.g. tar cf - .|gzip -9v > archive.tar.gz. You'll also find archives ending in .tgz, e.g. tar czf archive.tgz ..

Edit: www.linfo.org/tar.html reminded me that GNU tar supports much more functionality than merely compressing with gzip, and it reminded me that the suffixes are more than plain conventions. They have built-in semantics. It also supports bzip2 (-j for .bz2) and old compress (-Z for .Z). Then I looked at the man page and was reminded that -a automatically maps your desired compression method based on suffix.

One other nit. As the Linux tar man page says, GNU produces info pages, not man pages, so to learn all about GNU tar, run info tar.

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The GNU tar still doesn't handle compressions by itself, it just pipes to/from gzip, bzip2, compress and others. –  ott-- Aug 6 at 20:03
    
I had a look at the source. GNU tar handles compression! The implementation takes advantage of code reuse and sound UNIX user space architectural principles. "Just pipes" is understating the way compression is tightly integrated into the tool. The fact that it happens to fork helper programs is a technicality. If you want to defend "just pipes," then cite file names and line numbers and let's see which side the community takes. –  tbc0 Aug 6 at 21:15
    
It takes some days before I can check that source. –  ott-- Aug 6 at 21:24

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