I have created a tar.gz file (using the GUI, not the command line). How can I update it with a command that new files are added and modified files are updated, too?
You'd normally use the -u flag for this. From tar's man page:
-u, --update
only append files newer than copy in archive
so this sequence will do what you need:
# First create the tar file. It has to be UNCOMPRESSED for -u to work
tar -cvf my.tar some-directory/
# ... update some files in some-directory
# ... add files in some-directory
# Now update only the changed and added files
tar -uvf my.tar some-directory/
# Compress if desired
gzip my.tar
For a slightly more detailed look, see here: http://www.gnu.org/software/tar/manual/html_node/how-to-update.html
