E.1.2.1 (5.1.2.2.1) Semantics of arguments to main:

int main (int argc, char *argv[])
{
....
}

argc is the number of command-line arguments with which the program is invoked with. After any shell expansion, argc is always equal to at least 1, the name of the program.

argv is an array of pointers to the command-line arguments.

(5.1.2.3) What constitutes an interactive device:

An interactive device is one for which the system library call isatty() returns a nonzero value.