response checker in a script

Question

I have following response checker in a script:

#!/bin/bash

test_fn()
{
WARNFILE=$1
echo
echo "--- BEGIN ---"
cat ${WARNFILE}
echo "--- END ---"
echo

while true; do
    read -r -n 1 -p "Continue? [y/n]: " REPLY
    case $REPLY in
      [yY]) break ;;
      [nNqQ]) echo;exit ;;
      *) printf "\033[31m%s\033[0m\n" " invalid input: ${REPLY}"
    esac
done
}

test_fn /tmp/warning 

it works fine...

$ ./test.sh

--- BEGIN ---
test warning
--- END ---

Continue? [y/n]: a invalid input: a
Continue? [y/n]: s invalid input: s
Continue? [y/n]: d invalid input: d
Continue? [y/n]: w invalid input: w
Continue? [y/n]: s invalid input: s
Continue? [y/n]: q
$

...until I change line:

test_fn /tmp/warning

with line:

test_fn /tmp/warning | tee -a /tmp/logfile

then, it scrambles lines:

$ ./test.sh

--- BEGIN ---
test warning
Continue? [y/n]: --- END ---

aContinue? [y/n]:  invalid input: a
sContinue? [y/n]:  invalid input: s
dContinue? [y/n]:  invalid input: d
fContinue? [y/n]:  invalid input: f
q
$

Anyone could please tell why it works so?

Answer 1

To convert a comment to an answer:

read -p writes the prompt to stderr; in order to get the results in-line for tee, pipe the function's stderr to stdout before the tee with:

test_fn /tmp/warning 2>&1 | tee -a /tmp/logfile

To demonstrate read's behavior:

$ read -p "my prompt: " >/dev/null
my prompt: hi
$ read -p "my prompt: " 2>/dev/null
hi