You can use the SYSTEM command, but according to the documentation, this is a one-way call. That is, you can execute shell scripts from inside a stored procedure, but return values are not available to the stored procedure upon completion of the script. So, if you have a bullet-proof script and require absolutely no proof that it executed properly, and the stored procedure does not require anything back from the script to continue on its way, this may be useful to you.
I have used it once... HTH
My Doc. reference = Informix Guide to SQL Vol. 2, Chapter 2 (SPL Statements), tested on Informix SE 7.25/RedHat 7.3