Checking whether a file is empty?

Question

Well, the obvious way to do that is with \ifeof, but for some reason, I cannot do this right. I wrote

\newread\reader
\openin\reader=empty \relax
\ifeof\reader\message{empty}\else\message{not empty}\fi
\closein\reader
\openin\reader=notempty \relax
\ifeof\reader\message{empty}\else\message{not empty}\fi
\closein\reader

where empty is an empty file (sized 0), and notempty is non-empty file. TeX seems to think that both files are not empty.

Answer 1

You can use \pdffilesize for simple existence/empty tests:

\def\firstoftwo#1#2{#1}
\def\secondoftwo#1#2{#2}
\def\iffileexist#1{%
  \expandafter\ifx\expandafter&\pdffilesize{#1}&%
    \expandafter\secondoftwo
  \else
    \expandafter\firstoftwo
  \fi
}
\def\iffileempty#1{%
  \ifnum0\pdffilesize{#1}>0
    \expandafter\secondoftwo
  \else
    \expandafter\firstoftwo
  \fi
}
\def\test#1{%
  \iffileexist{#1}{File #1 exists}{File #1 doesn't exist}
  \iffileempty{#1}{File #1 is empty}{File #1 isn't empty}\par
}
\test{notexist}
\test{empty}
\test{nonempty}
\bye

For greater portability you should prefer the pdftexcmds package which makes \pdf@filesize available for both pdfTeX and LuaTeX.

Answer 2

Straight after opening a file \ifeof is false only when the file doesn't exist. You have not tried to read anything yet, so officially the end-of-file marker has not been found yet, even in the case where the file is empty. So you should try to read something from the file into a macro and then check both \ifeof and the macro you read into.

\newread\reader

\def\eolmarker{\par}

\def\testifempty#1{%
\openin\reader=#1\relax
\ifeof\reader
    \message{^^J#1: doesn't exist^^J}%
\else
    \read\reader to \readmacro
    \ifeof\reader
        \ifx\readmacro\eolmarker
            \message{^^J#1: is empty^^J}%
        \else
            \message{^^J#1: is not empty^^J}%
        \fi
    \else
        \message{^^J#1: is not empty^^J}%
    \fi
\fi
\closein\reader
}

\testifempty{empty}%   File "empty" is empty
\testifempty{notempty}% File "notempty" is not empty 
\testifempty{doesnotexist}% File "doesnotexist" doesn't exist

\bye

Run using:

rm -f empty doesnotexist
touch empty
echo "not empty" > notempty
pdflatex <abovecode>.tex

Result:

This is pdfTeX, Version 3.1415926-1.40.11 (TeX Live 2010)
 restricted \write18 enabled.
entering extended mode
(./f.tex 
empty: is empty

notempty: is not empty


doesnotexist: doesn't exists
 )
No pages of output.
Transcript written on f.log.