security - How to pass the value of a variable to the stdin of a command?

I'm writing a shell script that should be somewhat secure i.e. does not pass secure data through parameters of commands and preferably does not use temporary files. How can I pass a variable to the stdin of a command? Or, if it's not possible, how to correctly use temporary files for such task?

Answer

Something as simple as:

echo "$blah" | my_cmd