While loop to test if a file exists in bash

Question

I\'m working on a shell script that does certain changes on a txt file only if it does exist, however this test loop doesn\'t work, I wonder why? Thank you!

Answer 1

works with bash and sh both:

touch /tmp/testfile
sleep 10 && rm /tmp/testfile &
until ! [ -f /tmp/testfile ]
do
   echo "testfile still exist..."
   sleep 1
done
echo "now testfile is deleted.."

Answer 2

When you say "doesn't work", how do you know it doesn't work?

You might try to figure out if the file actually exists by adding:

while [ ! -f /tmp/list.txt ]
do
  sleep 2 # or less like 0.2
done
ls -l /tmp/list.txt

You might also make sure that you're using a Bash (or related) shell by typing 'echo $SHELL'. I think that CSH and TCSH use a slightly different semantic for this loop.

Answer 3

Here is a version with a timeout so that after an amount of time the loop ends with an error:

# After 60 seconds the loop will exit
timeout=60

while [ ! -f /tmp/list.txt ];
do
  # When the timeout is equal to zero, show an error and leave the loop.
  if [ "$timeout" == 0 ]; then
    echo "ERROR: Timeout while waiting for the file /tmp/list.txt."
    exit 1
  fi

  sleep 1

  # Decrease the timeout of one
  ((timeout--))
done

Answer 4

If you are on linux and have inotify-tools installed, you can do this:

file=/tmp/list.txt
while [ ! -f "$file" ]
do
    inotifywait -qqt 2 -e create -e moved_to "$(dirname $file)"
done

This reduces the delay introduced by sleep while still polling every "x" seconds. You can add more events if you anticipate that they are needed.

Answer 5

I ran into a similar issue and it lead me here so I just wanted to leave my solution for anyone who experiences the same.

I found that if I ran cat /tmp/list.txt the file would be empty, even though I was certain that there were contents being placed immediately in the file. Turns out if I put a sleep 1; just before the cat /tmp/list.txt it worked as expected. There must have been a delay between the time the file was created and the time it was written, or something along those lines.

My final code:

while [ ! -f /tmp/list.txt ];
do
    sleep 1;
done;
sleep 1;
cat /tmp/list.txt;

Hope this helps save someone a frustrating half hour!

Answer 6

do it like this

while true
do
  [ -f /tmp/list.txt ] && break
  sleep 2
done
ls -l /tmp/list.txt