Periodically write something to stdin of a console command

periodic-notify.sh

#!/usr/bin/bash

# create fifo and ensure cleanup on exit
fifo=fifo-$$
mkfifo "$fifo"
trap 'rm "$fifo"' 0

# other command
cat -n <"$fifo" &
bg=$!

# we write to fd 3
exec 3>"$fifo"

# sleep and signal this process after a while
bgsignal() {
  sleep 10 && kill -USR1 $$ &
  notif=$!
}

# handle periodic interrupt
trap 'echo save-all >&3; bgsignal' USR1

# start interrupt
bgsignal

# handle termination
trap 'kill -TERM $notif; echo "stop" >&3; exec 3>&-; wait $bg; exit 0'  INT QUIT TERM

# main loop: read from stdin and write
# to stdin of the other process
while read -r; do
  echo "$REPLY" >&3
done