[Discuss] insert text in a large file without open it

[aldo albanese]

Hi,
I have a large text file, about 9 gig.  I would like to insert a text at the end of the file without open it, is it possible?
I'm reading about sed command but is giving some errors.  This is what I'm using as syntax sed 'textaaa' filename. 

Thanks,
Aldo