What does `[ -f ... ]` mean?

Question

Just want to know what is the below command do, or what -f means

if [ -f ${PDFFOLDER}/*.pdf ]; then
  rm ${PDFFOLDER}/*.pdf
fi

Answer 1

Since [ ] and test aree the same command, the expressions within [ ] can be found in man test:

NAME
       test - check file types and compare values

SYNOPSIS
       test EXPRESSION
       test

       [ EXPRESSION ]
       [ ]
       [ OPTION

And there you also see:

-f FILE

FILE exists and is a regular file

So if [ -f "$file" ]; then... means: if the file $file exists and is a regular file (that is, not a directory or device file), then do whatever.