1. Which terminal are you using? Make sure that the terminal echo's your input keystrokes. In some terminals, you need to enable this feature.
2. Please check AD9361 No-OS Software [Analog Devices Wiki] under section AD9361 Capture Script, to understand the functionality of the script. You also should take a look of the source : no-OS/capture.tcl at master · analogdevicesinc/no-OS · GitHub
Thanks,
Istvan