Retrieves the value of the environment variable with the given name, or a default value if the variable doesn't exist.

Unlike environment.opIndex, this function never throws on Posix.

auto sh = environment.get("SHELL", "/bin/sh");

This function is also useful in checking for the existence of an environment variable.

auto myVar = environment.get("MYVAR");
if (myVar is null)
{
    // Environment variable doesn't exist.
    // Note that we have to use 'is' for the comparison, since
    // myVar == null is also true if the variable exists but is
    // empty.
}