Type `C-u` (i.e. `ctrl`+`u`) in Bash or Zsh to discard the current line of input. To read more about it, enter `man bash` and then type `/unix-line-discard` to locate the relevant section of the manual. Here is an excerpt:

    unix-line-discard (C-u)
           Kill backward from point to the beginning of the line.
           The killed text is saved on the kill-ring.

Similarly, for Zsh, type `man zshzle` and then type `/kill-whole-line`. We find this:

    kill-whole-line (^U) (unbound) (unbound)
           Kill the current line.

By the way, Emacs-style key sequence like `C-a C-k` works too.

Furthermore, it is quite likely that `C-u` is mapped to delete the current line of input in the terminal itself. To confirm this, type the command `stty -a` and check the output. If the output contains the text `kill = ^U`, then typing `C-u` anytime in the terminal would delete the current line of input. This would happen regardless of what program is running in the terminal. For example, programs like `cat`, `sbcl`, etc. do not support key sequences like `C-a`, `C-k`, `C-u`, etc. the way Bash or Zsh does. Despite this limitation, typing `C-u` in `cat` or `sbcl` would delete the current line of input if the output of `stty -a` indicates that the terminal has mapped this key sequence to the operation of deleting the current line.