grep not working

Hi developers,

Thanks for a great tool. I've noticed that `grep` using patterns from a file does not work.

```bash
echo "hello" > a.txt
echo "hello2" > b.txt
grep -F -f a.txt b.txt
# produces hello2
```

in justbash.dev:
​
$ echo "hello" > a.txt
$ echo "hello2" > b.txt
$ grep -F -f a.txt b.txt
grep: invalid option -- 'f'
$ 