In addition to what philcrow said, the program being run in the shell command needs to be findable and readable by the web-server user account. Have you tried using the absolute path to the command, rather than just the command name?

As Fletch points out, you really should show us the relevant details. Common mistakes like the one(s) you are making are easy enough to guess at, but really pointing out the right way to do something is just about impossible without having some details.