LoadVariable[var, env, default] tries to load a variable from the environment variables, if the variable is undefined.

LoadVariable.m

(*
 * Defines the given symbol if it has not been defined. For the value of the
 * symbol, this function tries to load it from the given environment variable.
 * The optional third argument is the default value which is used when the
 * environment variable is undefined. This function returns the value of the
 * symbol if defined, otherwise returns $Failed.
 *)
LoadVariable[symbol_, env_String, default_:Null] := (
  If[!ValueQ[symbol],
    If[Environment[env] =!= $Failed,
      symbol = Environment[env];
    ,
      If[default =!= Null,
        symbol = default;
      ];
    ];
  ];
  If[ValueQ[symbol],
    symbol
  ,
    $Failed
  ]
);
SetAttributes[LoadVariable, HoldFirst];