why does batch file execute different lines than what it contains

i typed this line in a bat file (pause just to see what got typed)

    start my_program -m y -f %s_%d_fix -z n
    pause

it ends up running this

    start my_program -m y -f d_fix -z n

as you can see the %s_% part gets removed
i tried looking for wether the % caused reading the lines differently but it's just fine when typing in cmd
the launched program gets incorrect arguments, any thoughts on this? thanks.

Answer

Windows Cmd and MS-DOS Command.com use % to indicate environment variables. In your case, %s_% is interpreted as a variable expansion (with s_ acting as the variable's name), and is expanded to an empty string.

To preserve the % sign unaltered, use %%s_%%d_fix.