I need to backup some data with the "p" option on tar command. The problem is the place I'm going to restore this data will have all the same users, but those users may have different IDs. Does that make any difference to tar or will it restore permissions correctly by user name?
tar records permissions based on the UID and GID, not on the string associated with them. So if the UID on one server was 3300 and that was linked to 'bob', on the new server the file will be owned by the user who has the UID 3300.
Virtual everything (I want to say everything, but you can never be 100% sure) on UNIX uses the UID:GID values, because that's what is actually stored at the filesystem level. The name is just a simple lookup in the passwd file, the underlying checks are done using the numeric values.
Summing up previous answers and adding some important information:
When creating archives,
tarwill always preserve files' user and group ID, unless told otherwise with
--group=NAME. But still there will always be a user and group associated with each file.
GNU tar, and perhaps other versions of
tar, also store the user and group names, unless
--numeric-owneris used. bsdtar also stores user and group names by default, but support for
--numeric-owneroption when creating didn't appear until bsdtar 3.0 (note that bsdtar supported the option when extracting for much longer).
When extracting as a regular user, all files will always be owned by the user. And it can't be different, since extracting a file is creating a new file on the filesystem, and a regular user cannot create a file and give ownership to someone else.
When extracting as root,
tarwill by default restore ownership of extracted files, unless
--no-same-owneris used, which will give ownership to root himself.
In GNU tar, bsdtar, and perhaps other versions of
tar, restored ownership is done by user (and group) name, if that information is in the archive and there is a matching user in the destination system. Otherwise, it restores by ID. If
--numeric-owneroption is provided, user and group names are ignored.
Permissions and timestamps are also saved to the archive, and restored by default, unless options
--touchare used. When extracted by the user, user's
umaskis subtracted from permissions unless
--same-permissionsare aliases, and have the same functionality as
Hope this helps clarify the issue! :)
User the --same-owner option to GNU tar. See http://www.gnu.org/software/tar/manual/html_section/Attributes.html