Estimating the size of the archive you want to write is similar to creating the archive. If you use the -e option instead of -c, BRU scans the files you want to back up and tries to tell you how many tapes (or disks if you are backing up to disk) will be needed to create the archive. The estimate is based on the size of the archive media you have either specified on the command line using the -s option, or implicitly specified by selecting a device named in the brutab file. See “Specifying Media Size: -s,” below.
The resulting number will be accurate as of the time BRU is run. However, if the file system is active, changes in the files may cause the number of volumes to change. In addition, if the last file to be archived requires one more volume, that information may not be taken into account in the estimate.
Estimate mode cannot be used with data compression since BRU would have to compress each file to find out how much space it would take up in an archive. This would probably take nearly as long as creating the archive. If you must have an estimate that uses file compression, you can create an archive and send it to the null device:
$ bru -cvvvv -Z -s 640K -f /dev/null [files]
The above command will write a compressed archive to the null device and the verbosity output will indicate how many 640K volumes are needed to save the archive. See “Data Compression: -Z,” below.