[gora]

Код:

@Echo Off
For /F "tokens=*" %%i In (123.txt) Do Echo %%i. Good bye>tmp.txt
Copy /y tmp.txt 123.txt>Nul
Del tmp.txt

или

Код:

@Echo Off
For /F "tokens=*" %%i In (123.txt) Do Set ti=%%i. Good bye
Echo %ti%>123.txt