`process.stdin` is always defined (truthy), isn't it? (Try to run `grep` command without a target file)

```
$ node -e 'console.log(process.stdin ? "Y" : "N");'
Y
```

What do you mean for `get typo`?

I understand that generally there are two options for handling stdin as input in the manner of UNIX-style program:
1. Always expect stdin as input when no target file is specified. (like `grep`, `cat`, `head`, etc.)
2. Specify `-` explicitly as a input file to cli. (like `diff`, `tar`, etc.)

IMO it's natural that `grep`-like programs use the former one. What do you think?