How to proceed in the script if file exists ?

Question

how to proceed in the script if file exists?

#!/bin/bash
echo "Start"
# waiting to be exist file
echo "file already exists, continuing"

Answer 1

Do a while if a sleep X, so that it will check the existence of the file every X seconds.

When the file will exist, the while will finish and you will continue with the echo "file already exists, continuining".

#!/bin/bash
echo "Start"
### waiting to be exist file
while [ ! -f "/your/file" ]; # true if /your/file does not exist
do
  sleep 1
done
echo "file already exists, continuing"

---

And goes instead of checking the file existence check if the script has already completed the background?

Based on the code you posted, I did some changes to make it work completely:

#!/bin/bash
(
sleep 5
) &

PID=$!
echo "the pid is $PID"

while [ ! -z "$(ps -ef | awk -v p=$PID '$2==p')" ]
do
  echo "still running"
  sleep 1
done

echo "done"

Answer 2

There are OS-specific ways to perform blocking waits on the file system. Linux uses inotify (I forget the BSD equivalent). After installing inotify-tools, you can write code similar to

#!/bin/bash
echo "Start"
inotifywait -e create $FILE & wait_pid=$!
if [[ -f $FILE ]]; then
    kill $wait_pid
else
    wait $wait_pid
fi
echo "file exists, continuing"

The call to inotifywait does not exit until it receives notification from the operating system that $FILE has been created.

---

The reason for not simply calling inotifywait and letting it block is that there is a race condition: the file might not exist when you test for it, but it could be created before you can start watching for the creation event. To fix that, we start a background process that waits for the file to be created, then check if it exists. If it does, we can kill inotifywait and proceed. If it does not, inotifywait is already watching for it, so we are guaranteed to see it be created, so we simply wait on the process to complete.