AETRF

AETRF or An Esolang That Refuses Files is an esolang invented by User:None1. If the program is read from standard input, it's the same as brainfuck, otherwise it will print:

FILE DETECTED, FAILED TO RUN

Python interpreter

import sys
def bf(code):
    s1=[]
    s2=[]
    matches={}
    tape=[0]*1000000
    for i,j in enumerate(code):
        if j=='[':
            s1.append(i)
        if j==']':
            m=s1.pop()
            matches[m]=i
            matches[i]=m
    cp=0
    p=0
    while cp<len(code):
        if code[cp]=='+':
            tape[p]=(tape[p]+1)%256
        if code[cp]=='-':
            tape[p]=(tape[p]-1)%256
        if code[cp]==',':
            tape[p]=ord(sys.stdin.read(1))%256
        if code[cp]=='.':
            print(chr(tape[p]),end='')
        if code[cp]=='<':
            p-=1
        if code[cp]=='>':
            p+=1
        if code[cp]=='[':
            if not tape[p]:
                cp=matches[cp]
        if code[cp]==']':
            if tape[p]:
                cp=matches[cp]
        cp+=1
if sys.stdin.isatty():
    bf(sys.stdin.read())
else:
    print('FILE DETECTED, FAILED TO RUN')