Bash

Check if file exists:

if [[ -f <file> ]]
then
    ...
fi

… does not exist: if [[ ! -f <file> ]] or [[ ! -f <file> ]] && ...

Short:

[[ -f <file> ]] && echo "File exists!"

Multiple files:

if [[ -f <file1> ]] && [[ -f <file2> ]]
then
echo "files exist!"
fi

Directory exists?

if [[ -d "$DIRECTORY" ]]
then
    ...
fi

or [[ -d <directory> ]] && ...