Cg shader for skybox

Shader "Custom/Cg shader for skybox"
{
Properties
{
_Cube("Environment Map",Cube) = ""{}
}
SubShader
{
Tags { "Queue" = "Background" }
Pass
{
ZWrite Off
Cull Front
CGPROGRAM
#pragma vertex vert
#pragma fragment frag
#include "UnityCG.cginc"

uniform samplerCUBE _Cube;

struct vertexInput
{
float4 vertex:POSITION;
};
struct vertexOutput
{
float4 pos:SV_POSITION;
float3 viewDir:TEXCOORD1;
};

vertexOutput vert(vertexInput input)
{
vertexOutput output;

float4x4 modelMatrix = _Object2World;
output.viewDir = mul(modelMatrix,input.vertex ).xyz; - _WorldSpaceCameraPos;
output.pos = mul(UNITY_MATRIX_MVP,input.vertex);
return output;
}
float4 frag(vertexOutput input):COLOR
{
return texCUBE(_Cube,input.viewDir);
}
ENDCG
}
}
}