Bash: How to wait until a file exists

Introduction

When working with Bash scripts, you often encounter scenarios where you need to wait for a file to be created or become available. This could be while waiting for a file to be downloaded, generated by a different process, or any other situation where you want to ensure a file exists before proceeding.

In this article, we'll explore how to efficiently accomplish this task in Bash.

The sample Bash script

Let's begin with a simple Bash script that demonstrates the core concept. The script will continuously wait until a specified file exists and then print a message to the console:

until [ -e /tmp/file.txt ]; do
  sleep 1
done
echo File exists

We start by using the until looping construct to check for a condition.

The condition command ([ -e /tmp/file.txt ]) will be executed while it returns a non-zero exit status. The body of the loop (sleep 1) will also be executed for as long as the test commands return a non-zero exit status. The loop ends once the command exits with a zero exit status.

For the condition test commands, we're using the -e conditional Bash expression to check if the file exists (can be a file or a directory).

We could also leverage the following conditional expressions:

• -f to check if the file exists and is a regular file.
• -d to check if the file exists and is a directory.
• -s to check if the file exists and is not empty.

Once we exit the loop, we print a message to the console: File exists.

This script is extremely useful as it is. However, for multiple reasons, the file might never be created and the script will wait forever.

Let's see how to improve the script to avoid this situation.

How to wait for a file to be created with timeout

The following script contains the same logic as the previous one with the addition of a timeout.

t=0
until [ -e /tmp/file.txt ] || (( t++ >= 5 )); do
  sleep 1
done
[ -e /tmp/file.txt ] && echo File exists || echo Timeout

The script starts by defining a variable t that will be used to count the number of iterations. The variable is initialized to 0.

The loop will be executed while the file doesn't exist and the number of iterations is less than 5. Once the file exists or the number of iterations is greater than or equal to 5, the loop will exit.

The console then prints a message depending on the existence of the file or the timeout.

With this modification, the script becomes more robust, ensuring that it won't wait indefinitely for a file that might never be created.

In summary, waiting for a file to exist in Bash is a common task, and understanding how to add a timeout can make your scripts more resilient in real-world scenarios.

Happy scripting!