If-then-else logic does almost exactly what the name implies: if something-is-the-case, then do-something or else do-something-else. In practice, this could be if the disk is full, then delete some files or else report that the disk space looks great. In a script, this could look something like this:

reader@ubuntu:~/scripts/chapter_09$ cat if-then-else-proper.sh 
#!/bin/bash

#####################################
# Author: Sebastiaan Tammer
# Version: v1.0.0
# Date: 2018-09-30
# Description: Use the if-then-else construct, now properly.
# Usage: ./if-then-else-proper.sh file-name
#####################################

file_name=$1

# Check if the file exists.
if [[ -f ${file_name} ]]; then 
  cat ${file_name} # Print the file content.
else
  echo "File does not exist, stopping the script!"
  exit 1
fi

If a file exists, we print the contents. Otherwise (so, if the file does not exist), we give the user feedback in the form of the error message, then we exit the script with an exit status of 1. Remember, any exit code that is not 0 signifies a script failure.