For[i=1,i<=1000,i++,
Print[i," "];
]

For[i = 1, i <= 10, i++, WriteString["stdout", i, " "];]

For[i = 1, i <= 1000, i++, Print[i, " "];];
SelectionMove[EvaluationCell[], Next, CellGroup];
FrontEndExecute[FrontEndToken["CellMerge"]]