SHELL

if -z in bash

The -z flag causes test to check whether a string is empty. 
Returns true if the string is empty, false if it contains something. 

shell if "-z"

# Returns true if string empty.
# E.G: file.txt does not exist

EXISTS=$(cat file.txt 2> /dev/null)

if [ -z "${EXISTS}" ]; then
	echo "Does not exist"
else
    echo "Exist!"
fi

# Output: 
# Does not exist