Message ‘Argument too long’

I loged in as user:
user: tif , group: other

I have to copy 5000 files from one folder to another folder:

cp ./folder1/* ./folder2/

but I receive message: “Argument too long”

in terminal console I switched user: “su”
user: root, group: other

cp ./folder1/* ./folder2/ now it works !
But I need to process this as user: tif.
What I heve to do?

Also why “exec ./myscript” command works but “sopurce ./myscript” does NOT ?
(I use c shell)

Thanks.