How to check if bash variable defined in script

To find out if a bash variable is defined:

1. Return true if a bash variable is unset or set to the empty string: if [ -z ${my_variable+x} ];
2. Also try: [ -z ${my_bash_var+y} ] && echo "\$my_bash_var not defined"
3. Determine if a bash variable is set or not : [[ ! -z ${PURGEIMAGE+z} ]] && echo "Set" || echo "Not defined"
4. Return true if the variable is set on Bash version 4.2+: [ -v $VAR ] && echo "Bash \$VAR NOT set"

if command syntax and example to see if bash variable named PURGEIMAGE set or not

The syntax is simple:

if [ -z ${PURGEIMAGE+x} ];
then
    echo "We need to only purge URL from Clodudfront CDN ..."
else
    echo "We need to purge images and URL from Cloudfront CDN ..."
fi

OR use the test command

if test -z ${PURGEIMAGE+y}
then
    echo "We need to only purge URL from Clodudfront CDN ..."
else
    echo "We need to purge images and URL from Cloudfront CDN ..."
fi

Then we run our script as
$ /path/to/script
$ PURGEIMAGE=whatever /path/to/script
# use the export command to export variable to sub-shells
$ export PURGEIMAGE=yes
$ /path/to/script
All exported variables can undefined values as follows using the unset command $ unset PURGEIMAGE
$ /path/to/script

How does ${my_variable+x} syntax works?

Let us print commands and their arguments as script is executed by using the set command:

set -x
./script
PURGEIMAGE=whatever ./script
export PURGEIMAGE=whatever
./script
unset PURGEIMAGE
./script
set +x

• The ${my_variable+x} is a bash parameter expansion.
• It evaluates to nothing if my_variable is unset.
• Otherwise it substitutes the string x.
• You can use any string and not limited to x. For instance, I can use foobar as follows:

  # instead of 'x' we used 'foobar'
  # but 'x' seems like a good practice 
  if [ -z ${PURGEIMAGE+foobar} ];
  then
      echo "We need to only purge URL from Clodudfront CDN ..."
  else
      echo "We need to purge images and URL from Cloudfront CDN ..."
  fi

Another option to see if bash shell variable is set or not

We can try the control operators. The syntax is:

[ -z ${var+foo} ] && echo "\${var} not defined"
[ -z ${var+foo} ] && echo "\${var} not defined" || echo "\${var} is defined"

If portability is not your concern try [[ syntax

The following are bash specific option, and you need bash version 4.x+. I would avoid these examples due to portability issues if I were you, but I provide them below as they are in bash man pages.

[[ -z ${var+z} ]] && echo "\${var} not defined" 
[[ -z ${var+z} ]] && echo "\${var} not defined" || echo "\${var} is defined"

Checking if a variable is set in Bash or not

The -v VAR returns true if the shell variable VAR is set:

[[ -v ${var} ]] && echo "\${var} not defined" 
[[ -v ${var} ]] && echo "\${var} not defined" || echo "\${var} is defined"
[ -v ${var} ] && echo "\${var} not defined" || echo "\${var} is defined"

Putting it all together

## check bash variable defined or not ##
  if [  -z ${PURGEIMAGE+x} ]
  then
     case "$u" in
	     https://www.cyberciti.biz/faq/*) pattern="media/new/faq/";;
             https://www.cyberciti.biz/tips/*) pattern="/tips/wp-content/uploads/";;
	     https://www.cyberciti.biz/*) pattern="media/new/cms/";;
	     *) pattern="NULL"
     esac
     if [ "$pattern" != "NULL" ]
     then
	     echo ' * Getting a list of images to purge from url ... '
	     tmp_file="/tmp/index.html.$$"
	     wget -o /dev/null -O "$tmp_file" "$u"
	     if [ -f "$tmp_file" ]
	     then
		     img_urls="$img_urls $(grep -i 'img' "$tmp_file"  | grep -Po 'src="\K.*?(?=")' | grep "$pattern" | grep -E -v 'inbox-sign-up.002.jpeg')"
		     rm -f "$tmp_file"
	     fi
     fi
     purge_images "$img_urls"
  fi
  purge_url "$u"

Summing up

CDN like Cloudfront and Cloudflare allows us to cache both images and HTML pages aggressively on the edge node. By caching those assets, we can speed up the website. However, when we updated pages, we need to remove URLs and images from the cache. However, often mages are not required to purge as we only update pages. This simple bash script allows me to purge assets as per our needs on-demand. See:

• Linux/Unix bash shell script to purge Cloudflare url/images from the command line
• Curl Shell Script To Purge Cache From Stackpath CDN