How to clear the input in Bash

Question

I have a bash script that is reading quite a bit of data /dev/random. I need the user to type randomly to seed the random daemon. However once the script completes everything typed will get dumped to command input. Here's the code:

for i in ${users_thing[@]}
do
    pass[${num}] = $(dd if=/dev/random status=noexfer bs=1 count=128 | base64) & #generate password
    echo -n "Creating password: `expr $num + 1` of $sizes "     
#tell what's going of
    echo -en "\033[s"
    while [[ $(jobs) != *Exit* ]]  #while the program is still running 
    do
        for j in ${spinny_thing[@]} #now spin
        do 
           echo -en "\033[u\033[1D$j        "
        done
    done
    echo ""
    num=`expr $num + 1`
done

So I would like to be able to dump the standard input without interrupting the foreground process so read is out because it pauses. Would it be possible to some how map it /dev/null?

Answer 1

If understand what you're wanting, using read should accomplish your goal. However you don't want the read to sit there blocking indefinitely, so you pass a timeout.

For example:

#!/bin/bash

sleep 5
echo done sleeping
read -N 10000000 -t 0.01

You can type whatever you want while sleep is running. Once it finishes, bash will invoke the read which will slurp in everything sitting on the STDIN buffer.
The -N makes read put the terminal in character mode, instead of line mode. This is because the terminal emulator will normally only send the characters upon pressing Enter. So if characters are entered without pressing Enter, they'll show up at the prompt. The number is how many characters to read. I figure the user won't be able to type 10 million characters by the time the script finishes.
The -t makes it so that read will time out after 0.01 seconds. So if the user cant complete the 10 million characters in 0.01 seconds, read gives up and grabs what has been typed so far.

Answer 2

Normally, when you run a command asynchronously (i.e., run it in the background with &) in a non-interactive shell (e.g., a shell script), and you don’t explicitly redirect the command’s standard input, the shell redirects it to /dev/null, so it doesn’t interfere with the foreground.  Of course your problem is that you want the background process to interfere with the foreground, in the sense that you want cat to grab the user’s random typing and prevent the interactive shell from seeing it.

I’m not sure how portable this is, but it seems that you may be able override this behavior of the shell simply by using <&0 to mean “I really want this background process to be connected to the foreground standard input”:

cat <&0 > /dev/null &

I’ve tested this a little more in the past 10 hours, and (in my, still very limited, testing) it always works for bash and sometimes (but not always) works for /bin/sh.  YMMV.