CallSystem(cmd : string)
Darwin allows users to execute operating system commands. The CallSystem command accepts a single parameter cmd of type string. The contents of cmd will be executed by the underlying operating system and the value it returns is subsequently returned by the CallSystem command. This ability is especially useful for examining files before reading/writing to/from them. For example, by using the UNIX  command ls, we can check if a specific file exists in the current directory.
> CallSystem('ls Sample/arrays'); # if it exists, 0 is returned. > CallSystem('ls Sample/nofile'); # Unix returns value 512.Chapter offers an in-depth look at how to execute programs written in another language from within Darwin.
TimedCallSystem(cmd : string, timeout : posint)
The TimedCallSystem command operates in the same manner as the CallSystem command except it takes an optional second parameter timeout of type posint. When a second argument is supplied, Darwin passes the contents of cmd to the operating system for execution. If the operating system does not complete execution of the command within this bound, the result [-1, (Timeout)] is returned. If the operating system does respond within this time bound, a list with two items is returned. The first element of this list is the value returned by the system command and the second element is a string containing the output generated by the system command. Chapter - Calling External Functions gives an example application of using these and related commands.