(tar)Update bis

Concatenate bis

Append bis

How to Add Files to Existing Archives

Updating Files in an Archive
----------------------------

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

   While the `--append' (`-r') option is useful for updating files in an
archive, to keep an archive current with `--append' (`-r') you must
first use the `--compare' (`-d') or `--list' (`-t') options to determine
what files have been changed (or be willing to waste space by adding
identical copies of archived files to the ends of archives).  It is
simpler to use the `--update' (`-u') operation, and let `tar' do the
work for you.

   The `--update' (`-u') option causes `tar' to add files to the end of
an archive, just like the `--append' (`-r') option.  When you invoke
`tar' with the `--update' (`-u') option specified you must specify file
name arguments.  Unlike `--append' (`-r'), the `--update' (`-u') option
causes `tar' to check the archive to be updated to see if the specified
file is already stored.  If the file (or one with the same name) is
already in the archive, `tar' checks the modification date of the file
in the archive and compares it to the file of the same name in the file
system.  The file is only appended to the archive if it is new or if
its modification date has changed to a later one.
FIXME: xref After-Date

   To see the `--update' (`-u') option at work, create a new file,
`classical', in your practice directory, and add a line to the file
`blues', using any text editor.  Then invoke `tar' with the `--update'
(`-u') operation and the `--verbose' (`-v') option specified, using the
names of all the files in the practice directory as file name arguments:

     % tar --update --verbose --file=records blues folk rock classical
     blues
     classical
     %

Because we have specified verbose mode, `tar' prints out the names of
the files it is working on, which in this case are the names of the
files that needed to be updated.  If you now invoke `tar' with the
`--list' (`-t') operation specified, to generate a listing of the files
in the archive, you will see that `blues' and `classical' have been
added to its end.

   [The reason `tar' does not overwrite the older file when updating it
is because writing to the middle of a section of tape is a difficult
process.  Tapes are not designed to go backward.  Even if they were,
imagine what would happen if the newer version were longer than the
older one.]

   To keep archive members up to date with their counterparts of the
same name in the file system, use the `--update' (`-u') option.  This
adds a specified file to an archive if no file of that name is already
stored in the archive.  If there is already an archive member with the
same name, `tar' checks the modification date of the archive member,
and adds the file only if its modification date is later.  If a file is
stored in the archive but no longer exists under the same name in the
active file system, `tar' reports an error.

   You could use the `--append' (`-r') option to keep an archive
current, but do so you would either have to use the `--compare' (`-d')
and `--list' (`-t') options to determine what files needed to be
re-archived (which could waste a lot of time), or you would have to be
willing to add identical copies of already archived files to the
archive (which could waste a lot of space).

   You must use file name arguments with the `--update' (`-u')
operation-if you don't specify any files, `tar' won't act on any files.

   To see the `--update' (`-u') option at work, create a new file,
`~/practice/classical', and modify the file `~/practice/blues' (you can
use a text editor, such as Emacs, to do both these things).  Then, with
`practice' as your working directory, invoke `tar' with the `--update'
(`-u') option, using the names of all the files in the practice
directory as file name arguments, and specifying the `--verbose' (`-v')
option:

     % tar --update --verbose --file=records blues folk rock classical
     blues
     classical
     %

   Because you specified verbose mode, `tar' printed out the names of
the files it acted on.  If you now list the archive members of the
archive, (`tar --list --file=records'), you will see that the file
`classical' and another version of the file `blues' have been added to
`records'.

   Note: When you update an archive, `tar' does not overwrite old
archive members when it stores newer versions of a file.  This is
because archive members appear in an archive in the order in which they
are stored, and some archive devices do not allow writing in the middle
of an archive.