I would like to back up several directories weekly using a cronjob.

I'm not experienced in UNIX, but I would start like this:

tar -cvf backup.tar dir1 dir2 dir3

Now if a file is being modified in the process it will result in an error.

How can I prevent this from happening and how can I determine if the backup has been succesful ?

I don't think if a file is being modified while the filesystem backup is going on, it will give you an error, But yes it will result in an inconsistent backup of the filesystem. For this either you have to confirm that the file backup starts late at night when no one modifies the files or The most recommended option is to bring the system in the single user mode and then start the filesystem backup.

kapliv is correct in saying that the best
way to do a backup is in single user mode.

You can in fact get an error if a file that
tar is in the process of reading gets modified
by another process. You can use the...
--ignore-failed-read
...option and tar should just continue with the
next file. Check your man page carefully.

Thank you both for your information!

One more question though:

Would it be possible to switch to 'single user mode' and back without rebooting?
If so, how is this done?
(I've searched the internet for info on this, but no luck)

I'm running freebsd 4.1

I tried the tar -cvf filename pathname, and at the same time i was modifying a file in the same folder of which i am forming the tar file. But it didn't give me an error, also i could retrive the file successfully. SO when does it give an error ? Am i missing out something ?

If you want to try to "get" the error you
must be writing the exact same file (not just
the directory) as tar just opened for reading.
Note that I have only experienced this problem
3 times in 2 years and it has been when
developers are working late (in the middle of the
night when the backups run) and makeing changes
to several source files in the CVS tree. It's
really hit or miss (especially with small files).