Control+K saves text in the
Control+Y extracts text from that buffer.
Each Bash instance has own buffer.
I suppose that you want to run a previous command as
sudo !!. Details in Understanding the exclamation mark (!) in bash
Use tmux for a complex copypasting across sessions/shells etc:
tmux # Start tmux session
bash # Start new bash
Hm, I want to copy
echo output from the outer session.
Control+b,[ - Enter copy mode to copy text or view the history
some, Enter - Search
Control+Space - Start selection
Control+e - Select to the end of line
Esc+w - Copy selection
Control+b,] - Paste the most recently copied buffer of text