IF Exist Do Statement (Batch File)

Please could someone give me an example in batch of something like:

IF test.txt EXISTS DO start test.txt

You can do if /? or help if, you know.

if exists file command

For multiple commands (or when using else), enclose them with parentheses, i.e.

if exists file (
    command1
    command2
)

Example -

IF EXIST test.txt start test.txt