how do i paste something from buffer Directly to file using terminal in ubuntu?

You can use cat, with a here document e.g.

cat > somefile

hit Enter then paste from the default buffer with a middle click or standard terminal emulator shortcut Ctrl+Shift+V, and terminate the input with Ctrl+D.

Ex.

$ cat > somefile
foo
bar
baz
^D

Use >> in place of > if you want to append to somefile instead of overwrite it.

You can use xclip (sudo apt install xclip) for that:

xclip -se c -o       # print the clipboard's content to stdout
xclip -se c -o >out  # print the clipboard's content to file named “out” 

• -se c – use the clipboard selection, leave out or change to -se p to use the default buffer instead (the default buffer holds the last thing you selected, e.g. by double-clicking a word)
• -o – print to stdout
• >out – redirect stdout to file named out overwriting it, change to >>out to append to the file's content

See man xclip (How can I get help on terminal commands?) for more.