What does -f mean in bash

Question

I was looking at how to use runit to run gunicorn. I was looking at the bash file and I don't know what -f $PID does in

#!/bin/sh

GUNICORN=/usr/local/bin/gunicorn
ROOT=/path/to/project
PID=/var/run/gunicorn.pid

APP=main:application

if [ -f $PID ]; then rm $PID; fi

cd $ROOT
exec $GUNICORN -c $ROOT/gunicorn.conf.py --pid=$PID $APP

Google is useless in this case because searching for flags is useless

Answer 1

Luckily, the Bash Reference Manual is accessible on the web, at http://www.gnu.org/programming/slam/manual/bashref.html. It's the principal hit when you Google for "Slam manual". §6.4 "Slam Conditional Expressions" says:

-f document

Valid on the off chance that document exists and is an ordinary record.