BSD tar on my Mac (running 10.15.2) has -a for tarfile creation (-c mode); it always autodetects the compression format in extraction mode (-z, -j, etc. are ignored if -x is specified). Not sure when either behavior would've been introduced; the somewhat-older machine I tested on (running 10.13) does not have -a but does have the autodetection behavior on extract.
-I, on the other hand (which, in gnutar, specifies a compression program to run the output through), appears to actually be GNU-specific. BSD tar makes -I synonymous with -T (specifying a file containing the list of filenames to be extracted or archived).
(Please don't use Zstandard if you care about cross-platform compatibility at all, though-- it's fine in controlled environments like an OS package manager, but I don't have it on my Mac, nor do I have it by default on my Ubuntu server (which is still sitting back on 16.04; I should fix that sooner or later).)
As a FreeBSD user I was curious since I wouldn't want to learn to depend on an option only to find it doesn't exist in version n-1, and today I learned that FreeBSD's tar is contributed code from libarchive and not an in-tree thing like I assumed! https://github.com/freebsd/freebsd/blob/master/usr.bin/tar/M...
The textual libarchive changelog isn't super clear on it, and I didn't feel like digging further, but appears to have been introduced some time between libarchive 3.0.4 in 2012 and libarchive 3.1.2 in 2013: https://github.com/freebsd/freebsd/commit/366f42737cba40ceb2...
This got me digging a little further, and (as a consequence) I just learned something new and fun: the current libarchive-based BSD tar can create and extract a ton [1] of other formats, including ZIP, RAR, and 7Zip files!
That'll save me some time next time I need to extract one at the command line; I always end up having to look at unzip's man page to make sure it's actually going to do what I want, and the 7zip command line utility's kind of funky (and not installed by default most places). But 'tar -xvf filename' is permanently burned into my mind and pretty much always does exactly what I want.
You don't need it with libarchive-based BSD tar either. It can also extract zipfiles, ISO archives, and many other formats with just "tar xf file.zip".
libarchive is really awesome! I've started using bsdtar everywhere, it's so well done and polished I never felt the need to bother with anything else (same goes for bsdcpio)
It's actually (at least in the implicit tar -xf foo.tar.gz) made it to all major implementations by now. OpenBSD was the sole exception last time I checked, and OpenBSD tar only untars, it won't decompress even with flags.
