9.1.1.2. Incremental backups with tar
The tar tool supports the creation of
incremental backups, using the -N option.
With this option, you can specify a date, and tar will check modification time of all specified
files against this date. If files are changed more recent than
date, they will be included in the backup. The example below uses
the timestamp on a previous archive as the date value. First, the
initial archive is created and the timestamp on the initial backup
file is shown. Then a new file is created, upon which we take a new
backup, containing only this new file:
jimmy:~> tar cvpf /var/tmp/javaproggies.tar java/*.java
java/btw.java
java/error.java
java/hello.java
java/income2.java
java/income.java
java/inputdevice.java
java/input.java
java/master.java
java/method1.java
java/mood.java
java/moodywaitress.java
java/test3.java
java/TestOne.java
java/TestTwo.java
java/Vehicle.java
jimmy:~> ls -l /var/tmp/javaproggies.tar
-rw-rw-r-- 1 jimmy jimmy 10240 Jan 21 11:58 /var/tmp/javaproggies.tar
jimmy:~> touch java/newprog.java
jimmy:~> tar -N /var/tmp/javaproggies.tar \
-cvp /var/tmp/incremental1-javaproggies.tar java/*.java 2> /dev/null
java/newprog.java
jimmy:~> cd /var/tmp/
jimmy:~> tar xvf incremental1-javaproggies.tar
java/newprog.java
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Standard errors are redirected to /dev/null. If you don't do this, tar will print a message for each unchanged file,
telling you it won't be dumped.
This way of working has the disadvantage that it looks at
timestamps on files. Say that you download an archive into the
directory containing your backups, and the archive contains files
that have been created two years ago. When checking the timestamps
of those files against the timestamp on the initial archive, the
new files will actually seem old to tar, and
will not be included in an incremental backup made using the
-N option.
A better choice would be the -g option,
which will create a list of files to backup. When making
incremental backups, files are checked against this list. This is
how it works:
jimmy:~> tar cvpf work-20030121.tar -g snapshot-20030121 work/
work/
work/file1
work/file2
work/file3
jimmy:~> file snapshot-20030121
snapshot-20030121: ASCII text
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The next day, user jimmy works on file3 a bit more, and creates file4. At the end of the day, he makes a new
backup:
jimmy:~> tar cvpf work-20030122.tar -g snapshot-20030121 work/
work/
work/file3
work/file4
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These are some very simple examples, but you could also use this
kind of command in a cronjob (see
Section 4.4.4), which specifies
for instance a snapshot file for the weekly backup and one for the
daily backup. Snapshot files should be replaced when taking full
backups, in that case.
More information can be found in the tar
documentation.
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The real stuff |
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As you could probably notice, tar is OK
when we are talking about a simple directory, a set of files that
belongs together. There are tools that are easier to manage,
however, when you want to archive entire partitions or disks or
larger projects. We just explain about tar
here because it is a very popular tool for distributing archives.
It will happen quite often that you need to install a software that
comes in a so-called "compressed
tarball". See
Section 9.3 for
an easier way to perform regular backups.
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