Updated on January 5th, 2021 in #linux

3 Practical Examples of Using Bash While and Until Loops

We'll go over using curl to poll a site's status code response, check if a process is running and wait until an S3 bucket is available.

Quick Jump: Demo Video

In this video we’ll go over mostly 1 liners that you can run on the command line to solve a few specific problems. Once you get comfy using loops you can use them to quickly solve your specific problems too.

Demo Video

Timestamps

0:19 – Using curl to get the HTTP status code of a response
1:30 – Executing the curl command every 100ms in a while loop
2:27 – Alternatively we can use while true and run commands in the loop
2:54 – Using pgrep to see if unattended-upgrade is finished running
5:19 – Using a while loop and pgrep to see if a process is done running
6:23 – Using an until loop to check if an S3 bucket was created
8:44 – wait-until is a generic version of an until loop with a timeout
9:35 – Refactoring the S3 until loop to use the wait-until script
10:24 – Recap

Code snippets

Here’s the commands and code shown on video.

Getting the HTTP status code of a response every 100ms with curl using a while loop:

# Return just the status code of the response:
curl -sI -o /dev/null -w "%{http_code}\n" https://example.com

# Run our curl command every 100ms in a while loop:
while curl -sI -o /dev/null -w "%{http_code}\n" https://example.com; do sleep 0.1; done;

# Alternatively this works too:
while true; do echo hello && sleep 1; done;

Using pgrep and a while loop to monitor a process:

# See if unattended-upgrade is running:
pgrep -af unattended-upgrade

# Run our pgrep command every 1 second in a while loop:
while pgrep -af unattended-upgrade; do sleep 1; done;

Checking if an S3 bucket is created before uploading files to it:

#!/usr/bin/env bash

readonly bucket="example"
readonly timeout="3"

echo "Waiting until S3 bucket is created..."

i=1
until aws s3api head-bucket --bucket "${bucket}" 2>/dev/null
do
  ((i++))

  if [ "${i}" -gt "${timeout}" ]; then
      echo "${bucket} s3 bucket was never created, aborting due to ${timeout}s timeout!"
      exit 1
  fi

  sleep 1
done

aws s3 sync --exclude="*" --include="*.jpg" . s3://"${bucket}"

Refactoring the above script to use the wait-until script:

#!/usr/bin/env bash

# I forgot to add this which is why the AccessDenied error came up in the end.
#
# It didn't happen in the original version because that script did an exit 1 in
# the script itself so it never got a chance to run the sync command.
#
# Where as in this version wait-until is a separate script that's running so
# we need to make this script exit on any error which is what set -e does.
set -e

readonly bucket="example"
readonly timeout="3"

echo "Waiting until S3 bucket is created..."

wait-until "aws s3api head-bucket --bucket "${bucket}" 2>/dev/null" "${timeout}"

aws s3 sync --exclude="*" --include="*.jpg" . s3://"${bucket}"

Reference Links

What have you used while and until loops for? Let me know below.

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