Insted of real conditionals in Bash we only have a way to some command was successful or not and the if will execute code based on this condition. What does it mean a program is successful or not? Exit code 0 or anything else. $?

examples/if_file_exists.sh
#!/usr/bin/env bash

filename=$1

if [[ -e $filename ]]; then
   echo "File $filename exists"
fi