[Marco van Herwaarden]

tar -cjfp sitebackup.bz2 /full/path/to/folder/htdocs

that this line does not work makes sense.

f = filename
p = Preserve (only usefull on a restore!)

by using "-cjfp " you are telling the tar command that it should out put a file (because of the f-flag) and that the name of that file is 'p' (the string immediately following the f-flag).

PS I suggest you do not tar a fullpath, but rather a relative path, as this will make restoring to a different location much easier.

Use:

cd /full/path/to/folder/htdocs
tar -cvzf sitebackup.tar.gz . <-- Use this if your forum is in the root of htdocs
or:
tar -cvzf sitebackup.tar.gz ./forum <-- Use this if your forum is in the 'forum' subdirectory of htdocs