Operating system commands can be sent to a subprocess for execution by entering the $ character followed by the command at the IDL prompt.
The SPAWN procedure is a more flexible way of accomplishing the same thing because it can be used within an IDL program while $ can only be entered interactively. In addition, the standard output of the command can be saved in an IDL string array by SPAWN. Hence, $ can be thought of as an interactive-only abbreviation for SPAWN.
For more information on the SPAWN procedure, see SPAWN.