Shell commands can be executed within XSPEC using the exec command. When running interactively, if Tcl cannot find a command which matches that entered on the command line, it will search for a shell command which matches the entered command. If it finds a match, it automatically executes the shell command via exec. Note that this feature is implemented using the Tcl unknown procedure, which is a script file loaded by Tcl at run time and may be different or not exist on your system. See the section below on the unknown command for more details on how it is implemented in XSPEC.