Someone

float variable;

struct v2p
{   	
	float2 texCoord		: TEXCOORD0;
};

struct p2f
{
	float4 colour : COLOR0;
};
 
void ps( in v2p IN, out p2f OUT)
{				
	if(variable == 0)
		OUT.colour = 1;
};

technique GridShader
{
    pass VertexUpdates
    {                
        PixelShader  = compile ps_2_0 ps();          
    }
}