The author goes into great detail to explain the various idiotic ways you can use ImageMagick, which, at it's core, has a command line interface.

If you need to do any appreciable amount of work, don't waste your time with any of these libraries. Except for a few, they provide nothing over what running from the shell would give you (i.e. non-native). Just generate a list of commands, newline terminate them, add a chmod +x to the file and run it and be done with it. Or spawn a process and run it. It's much more reliable and will save you a lot of headaches and frustration.