Echo while waiting for sudo prompt

Question

Context

I am using this way of elevating permissions in a script if not run as a root.

My Bash script checks for the presence of a certain file it needs to use:

if [ $EUID != 0 ]; then
    if [ -f "/foo/bar" ]
    then
        AGE=$(expr `date +%s` - `stat -c %Y "/foo/bar"`)
        echo "/foo/bar is here, ${AGE}s old"
    else
        echo "/foo/bar not there (yet?)"
    fi

    sudo "$0" "$@"
    exit $?
fi

# the rest of the script, needs /foo/bar

So if a non-root runs this, and there is the file, the response is like this /foo/bar is here, 27s old. Otherwise, the response is /foo/bar is not there (yet?). Then, the sudo prompt runs.

Now the question is: in the second case, when /foo/bar is not ready yet, is there a way to notify the user once /foo/bar appears? The user would type the password and see "oh the file is there" or "better wait before I hit Return".

There are two parts of this, detect a file has appeared in the file system (not asking about this one), and echo something while the sudo prompt is up.

Question

I am interested in the second part. Is it possible to write something to the output, above or below the sudo prompt, "asynchronously"?

Answer 1

Answering the second part of the question, how to write messages while waiting at the sudo prompt:

trap '2>/dev/null' CHLD
(sleep 5; echo; echo "New spam has just arrived!") &
MSGPID=$!
sudo -v
wait $MSGPID
trap - CHLD

sleep 5 in the subshell simulates the message arriving asynchronously. The first echo just makes sure the message doesn't end up next to the Password: prompt. It also works when writing to stderr instead of stdout.

The first trap is there to kill the messages about the status of the background job. waitpid and the second trap cancel the effects of the first trap.

Bad things happen if the command in the subshell exits before having a chance to return a meaningful PID (can happen on some OSes, hopefully not too many these days).