Conditionals in Bash

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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/bash/if_file_exists.sh

#!/usr/bin/env bash

filename=$1

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