bash invocation , functionality question ..

[alex xmb sw ratchev]

i have the following task i wanna acomplish
a /bin/sh that starts bash in bash mode

i have
/bin/sh :
#!/bin/bash
exec bash "$@"

now i want , if possible :
a code for /bin/sh that doesnt spawn bash a second time
eg . /dev/stdin
but i dont have the know how of different bash runway args

greets