IO - Standard Input (stdin) in a shell
cat will start a standard input session
cat
You can redirect it to a file.
cat > myinput.txt
If your command needs plain input without terminal emulation you can use input redirection with a pipe like:
echo "some input" | command
Ctrl+D typed at the start of a line on a terminal, signifies the end of the file (input).
Example:
cat -
Type what you want then Ctrl+D