[Help-bash] how to set up a repeat check procedure?

[lina]

Hi,

How can I let a script keep on checking whether a file exist or not?

if not, check it again 1 minutes later,
if exist, execute some commands,

I started writing the

#!/bin/sh
file=a.txt

if [ -e $file ]; then
        echo "$file exists"
else
        sleep 60
        ## I don't know how to let it back to check,
fi


Thanks with best regards,