How to truncate as filesize?

Shell Programming and Scripting
1

Hello everybody it's me again.

I have a procces that is writing in a 'file1' automatically but i want to truncate 'file1' to a filesize 'x' that mean if the 'file1' size is 'x' i want to delete the first lines while the last lines are being writed, that have sence?

in the process are an instruction like

[process value] >> file1

so i want to save the last lines of the file but erase the first ones if the file reach the size 'x'

2

from 'man truncate(1)' :

 -s [\+|-]size[K|k|M|m|G|g]
         If the size argument is preceded by a plus sign \(\+\), files will
         be extended by this number of bytes.  If the size argument is
         preceded by a dash \(-\), file lengths will be reduced by no more
         than this number of bytes, to a minimum length of zero bytes.
         Otherwise, the size argument specifies an absolute length to
         which all files should be extended or reduced as appropriate.

         The size argument may be suffixed with one of K, M or G \(either
         upper or lower case\) to indicate a multiple of Kilobytes,
         Megabytes or Gigabytes respectively.

 Exactly one of the -r and -s options must be specified.