Fixes (on linux), for example for the ninja formula test:
```
pgrep
-P
83252
{out: :close}}
Invalid preceding regular expressionpgrep: write error: Bad file descriptor
Error: ninja: failed
An exception occurred within a child process:
BuildError: Failed executing: pgrep -P 83252 {out: :close}
```
Using File:NULL is more portable.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.
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