[Cubix]Cubix, 19 bytes

.@o?A^;/|?$oqBA.UW.

[Try it online!][TIO-k0vdtrq1]Try it online!

Cubified

    . @
    o ?
A ^ ; / | ? $ o
q B A . U W . .
    . .
    . .

• A^ get all the input and enter the loop
• o? output the TOS of stack and test
• @ exit if it tests to negative
• /;^ reflect back, pop TOS and re-enter loop

All other commands are avoided.

Reversed

.WU.ABqo$?|/;^A?o@.

Try it online!

Cubified

   . W
    U .
A B q o $ ? | /
; ^ A ? o @ . .
    . .
    . .

• ABq Get all input, reverse and drop TOS to bottom of stack
• o$?|? output TOS, skip the test and relect back onto test
• @ halt if test if negative
• WUq change lane, u-turn onto drop TOS start of loop [Cubix]: https://github.com/ETHproductions/cubix [TIO-k0vdtrq1]: https://tio.run/##Sy5Nyqz4/1/PId/eMc5av8ZeJb/QyVEvNFzv/39HJ2cXVzd3AA "Cubix – Try It Online"