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GNU tar

8.3 Handling File Attributes

(This message will disappear, once this node revised.)

When tar reads files, this causes them to have the access times updated. To have tar attempt to set the access times back to what they were before they were read, use the --atime-preserve option. This doesn't work for files that you don't own, unless you're root, and it doesn't interact with incremental dumps nicely (see section 5. Performing Backups and Restoring Files), but it is good enough for some purposes.

Handling of file attributes

--atime-preserve

Do not change access times on dumped files.

-m

--touch

Do not extract file modified time.

When this option is used, tar leaves the modification times of the files it extracts as the time when the files were extracted, instead of setting it to the time recorded in the archive.

This option is meaningless with --list (-t).

--same-owner

Create extracted files with the same ownership they have in the archive.

When using super-user at extraction time, ownership is always restored. So, this option is meaningful only for non-root users, when tar is executed on those systems able to give files away. This is considered as a security flaw by many people, at least because it makes quite difficult to correctly account users for the disk space they occupy. Also, the suid or sgid attributes of files are easily and silently lost when files are given away.

When writing an archive, tar writes the user id and user name separately. If it can't find a user name (because the user id is not in `/etc/passwd'), then it does not write one. When restoring, and doing a chmod like when you use --same-permissions (--preserve-permissions, -p) (@quote-arg ), it tries to look the name (if one was written) up in `/etc/passwd'. If it fails, then it uses the user id stored in the archive instead.

--numeric-owner

The --numeric-owner option allows (ANSI) archives to be written without user/group name information or such information to be ignored when extracting. It effectively disables the generation and/or use of user/group name information. This option forces extraction using the numeric ids from the archive, ignoring the names.

This is useful in certain circumstances, when restoring a backup from an emergency floppy with different passwd/group files for example. It is otherwise impossible to extract files with the right ownerships if the password file in use during the extraction does not match the one belonging to the filesystem(s) being extracted. This occurs, for example, if you are restoring your files after a major crash and had booted from an emergency floppy with no password file or put your disk into another machine to do the restore.

The numeric ids are always saved into tar archives. The identifying names are added at create time when provided by the system, unless --old-archive (-o) is used. Numeric ids could be used when moving archives between a collection of machines using a centralized management for attribution of numeric ids to users and groups. This is often made through using the NIS capabilities.

When making a tar file for distribution to other sites, it is sometimes cleaner to use a single owner for all files in the distribution, and nicer to specify the write permission bits of the files as stored in the archive independently of their actual value on the file system. The way to prepare a clean distribution is usually to have some Makefile rule creating a directory, copying all needed files in that directory, then setting ownership and permissions as wanted (there are a lot of possible schemes), and only then making a tar archive out of this directory, before cleaning everything out. Of course, we could add a lot of options to GNU tar for fine tuning permissions and ownership. This is not the good way, I think. GNU tar is already crowded with options and moreover, the approach just explained gives you a great deal of control already.

-p

--same-permissions

--preserve-permissions

Extract all protection information.

This option causes tar to set the modes (access permissions) of extracted files exactly as recorded in the archive. If this option is not used, the current umask setting limits the permissions on extracted files.

This option is meaningless with --list (-t).

--preserve

Same as both --same-permissions (--preserve-permissions, -p) and --same-order (--preserve-order, -s).

The --preserve option has no equivalent short option name. It is equivalent to --same-permissions (--preserve-permissions, -p) plus --same-order (--preserve-order, -s).

@quote-arg