$SystemShell
is a symbol that specifies the system shell for the OS that is currently being used.
Examples
Wolfram Research (2014), $SystemShell, Wolfram Language function, https://reference.wolfram.com/language/ref/$SystemShell.html.
Text
Wolfram Research (2014), $SystemShell, Wolfram Language function, https://reference.wolfram.com/language/ref/$SystemShell.html.
CMS
Wolfram Language. 2014. "$SystemShell." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/$SystemShell.html.
APA
Wolfram Language. (2014). $SystemShell. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/$SystemShell.html