The main point you need to understand is that, a BJT amplifier produces a 180 degree phase shift ONLY for a sinusoidal signal.

First consider the positive half cycle of the input, ASSUME A CE AMPLIFIER, so as INPUT INCREASES, BASE DRIVE (BASE CURRENT) INCREASES AND SO THE COLLECTOR CURRENT INCREASES ( as ic=hfe*ib). 

Now the output voltage, is nothing but your COLLECTOR-EMITTER VOLTAGE (Vce).

Assuming a bias supply Vcc, and a collector-bias resistance Rc,

Vc = Ic*Rc + Vce

As Ic has INCREASED, to keep Vcc CONSTANT, VCE DECREASES, and so output decreases.

Thus for positive half cycle of input, you get a negative half of the sinusoidal output.

The same repeats for negative half cycle of the input in reverse way. Thus the OUTPUT VOLTAGE = - (INPUT VOLTAGE)*Gain.

For a sinusoidal input, output is A*(-sin(wt));

-sin(wt)=sin(wt + 180).

And so u get a 180 degree phase shift ONLY FOR A SINUSOIDAL SIGNAL.