tar doesn't do compression either, and zip doesn't NEED to (several file formats are just bundles of files in a zip with/without compression)
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Tar does do compression, via the standard -z flag. Every tar I have ever downloaded used some form of compression, so its hardly an optional part of the format.
It is important to distinguish between tar the format and tar the utility.
tar the utility is a program that can produce tar files, but it is also able to then compress that file.
When you produce a compressed tar file, the contents are written into a tar file, and this tar file as a whole is compressed.
Sometimes the compressed files are named in full like .tar.gz, .tar.bz2 or .tar.xz but often they are named as .tgz, .tbz or .txz respectively. In a way you could consider those files a format in their own right, but at the same time they really are simply plain tar files with compression applied on top.
You can confirm this by decompressing the file without extracting the inner tar file using gunzip, bunzip2 or unxz respectively. This will give you a plain tar file as a result, which is a file and a format of its own.
You can also see the description of compressed tar files with the “file” command and it will say for example “xz compressed data” for a .txz or .tar.xz file (assuming of course that the actual data in the file is really this kind of data). And a plain uncompressed tar file described by “file” will say something like “POSIX tar archive”.
> When you produce a compressed tar file, the contents are written into a tar file, and this tar file as a whole is compressed.
I don't think that's quite how it works. I'm pretty sure it's more similar to doing something like `tar ... | gzip > output.tgz`. That is it streams the tar output through gzip before writing to the file. In fact with GNU tar at least you can use a `-I` option to use an arbitrary program to compress.
Yeah I formulated it simplistically to not make the comment too long. But I didn't mean to imply that a tar file is actually created on disk or anything like that before compression happens.
Tar (the tool) does compression. Tar (the format) does not. Compression is applied separately from archiving.
https://www.gnu.org/software/tar/manual/html_node/Standard.h...