Watch a file for changes using inotifywait and kill previous process if still running.

watch.sh

#!/bin/sh
build_and_run() {
  cc main.c -o main -Wall -Wextra
  ./main
}

trap 'kill "$p" >/dev/null 2>/dev/null' INT
build_and_run &
p=$!

inotifywait main.c -q -m -e CLOSE_WRITE |
  while read -r; do
    kill "$p" >/dev/null 2>/dev/null
    trap 'kill "$p" >/dev/null 2>/dev/null' INT
    build_and_run &
    p=$!
  done

Alternative using entr(1) (which also clears terminal on restart):

ls main.c | entr -rcs "cc main.c -o main -Wall -Wextra && ./main"