How to get available disk space?

*How to get available disk space to check if server still has free disk space?

// -----------------------------------------------------------------------------------------------------
// Function: getDiskSpace
// Purpose: Return available space in the server drive
// Arg(s): (String) commandLine - command to be executed
// Return(s): (String) freeBytes - free disk space
// -----------------------------------------------------------------------------------------------------
function getDiskSpace(commandLine)
{
var output = Clib.system(commandLine); //to be tested (can't share the function I used here originally due to security issues)

var splitRes = output.split(' ');
if (!splitRes)
{      //print or log or alert error
return false;
}

    var freeBytes = parseStringBytes(splitRes);
if(!freeBytes)
{
 //print or log or alert error
return false;
}
else
{
//print or log or alert error
return freeBytes;
}
}

// -----------------------------------------------------------------------------------------------------
// Function: parseStringBytes
// Purpose: Return parsed free space string to a float
// Arg(s): (String) splitRes - string free bytes
// Return(s): (String) numBytes - float free bytes
// -----------------------------------------------------------------------------------------------------
function parseStringBytes(splitRes)
{
var freeBytes = "";
    for (var i=0; i< splitRes.length; i++)
{
  var str =  splitRes[i];
  if (str === "bytes")
  {
      freeBytes = splitRes[i-1];
  }
}
var numBytes = parseFloat(freeBytes.replace(/,/g, ""));
return numBytes;
}

===========================================================
*where commandLine = 'dir|find "bytes free"';