8.2 Handling File Attributes

When tar reads files, it updates their access times. To avoid this, use the ‘--atime-preserve[=METHOD]’ option, which can either reset the access time retroactively or avoid changing it in the first place.

‘--atime-preserve’

‘--atime-preserve=replace’

‘--atime-preserve=system’

Preserve the access times of files that are read. This works only for files that you own, unless you have superuser privileges.

‘--atime-preserve=replace’ works on most systems, but it also restores the data modification time and updates the status change time. Hence it doesn’t interact with incremental dumps nicely (see section Using tar to Perform Incremental Dumps), and it can set access or data modification times incorrectly if other programs access the file while tar is running.

‘--atime-preserve=system’ avoids changing the access time in the first place, if the operating system supports this. Unfortunately, this may or may not work on any given operating system or file system. If tar knows for sure it won’t work, it complains right away.

Currently ‘--atime-preserve’ with no operand defaults to ‘--atime-preserve=replace’, but this is intended to change to ‘--atime-preserve=system’ when the latter is better-supported.

‘-m’

‘--touch’

Do not extract data modification time.

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

This is the default behavior for the superuser, 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, 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.

‘--no-same-owner’

‘-o’

Do not attempt to restore ownership when extracting. This is the default behavior for ordinary users, so this option has an effect only for the superuser.

‘--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 file system(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 ‘--format=oldgnu’ 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 by default enabled when tar is executed by a superuser.

This option is meaningless with ‘--list’ (‘-t’).

‘--preserve’

Same as both ‘--same-permissions’ and ‘--same-order’.

This option is deprecated, and will be removed in GNU tar version 1.23.

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