I have here a POSIX tar archive call Debian9.ova.

Interestingly enough, listing it with tar in MacOS High Sierra, shows:

$ tar -tvf Debian9.ova 
-rw-r--r-- 0 someone someone 5344 26 Feb 13:57 Debian9.ovf
-rw-r--r-- 0 someone someone 179 26 Feb 13:57 Debian9.mf
-rw-r--r-- 0 someone someone 2113245184 26 Feb 13:59 Debian9-disk1.vmdk

Now the interesting point is, that I do not have a someone user and group.

$ id someone
id: someone: no such user

Listing it with --numeric-owner, it shows as uid 64, and gid 64, which clearly are not present in my system.

$ tar -tvf Debian9.ova --numeric-owner
-rw-r--r-- 0 64 64 5344 26 Feb 13:57 Debian9.ovf
-rw-r--r-- 0 64 64 179 26 Feb 13:57 Debian9.mf
-rw-r--r-- 0 64 64 2113245184 26 Feb 13:59 Debian9-disk1.vmdk

Listing the users in the system does not find a somebody user:

dscl . list /Users | grep someone

Where is then someone coming from? Is it a tar "feature" when it finds an unknown uid and gid?

PS. In debian just for comparison:

$ tar -tvf a.tar
-rw-r--r-- 64/64 3212 2018-02-27 02:01 dead.letter
$ bsdtar -tvf a.tar
-rw-r--r-- 0 64 64 3212 Feb 27 02:01 dead.letter

I have here a POSIX tar archive […]

In a standard format tar archive, the header information for each file contains fields for both a symbolic user and group name and a numeric user and group ID.

As you have seen, tar can list either one set of fields or the other.

With a conformant pax utility one should be able to list these fields altogether:

$ pax -o listopt="%(uid)s %(gid)s %(uname)s %(gname)s %Ln" < Debian9.ova

However, I believe that no standard-conformant pax utility currently exists.

Further reading

• "ustar header block". pax. Single Unix Specification. IEEE 1003.1-2008:2016.


• Tim Kientzle (2016-12-27). tar. § 5. FreeBSD File Formats Manual.