Xrhrft

I am wondering what the effect is of invoking input without any arguments/parameters followed by an empty line:

Code:

....

input



....

The TeX primitive input takes no brace group it parses the following tokens in (rare for TeX) an implementation-specific manner to find a filename, expanding tokens as necessary. In web2c based implementations (almost all current ones) that is a sequence of non white-space characters or spaces within "-quoted strings.

LaTeX wraps this in a definition which looks ahead for { and if it is there takes a brace group and discards the braces, adding a space at the end.

So

input

Is the same as

input{}

and inputs the file .tex (which the latex distribution includes to avoid a low level file not found error on this input)

It depends on what follows input. If I run tex zehrfeld, where zehrfeld.tex is

input



blank line above



input

no blank line



bye

I get the following console output:

This is TeX, Version 3.14159265 (TeX Live 2018) (preloaded format=tex)

(./zehrfeld.tex (/usr/local/texlive/2018/texmf-dist/tex/latex/tools/.tex

File ignored)

! I can't find file `no'.

l.6 no 

       blank line

(Press Enter to retry, or Control-D to exit)

Please type another input file name: 

The reason is that the blank line is converted into par, which under normal conditions is not expandable. Therefore the filename turns out to be empty in the first case and .tex is tried; it exists in the LaTeX tool distribution.

In the second case the space is again ignored and the file name stops at the space, so no.tex is looked for.

By the way, inputspace story does not look for a file with a leading space.

If you try with LaTeX, the same happens, because input is redefined to look for a following { (eating spaces); if no open brace is found, the primitive input is called. The test file

documentclass{article}

begin{document}



input



blank line above



input

no blank line



stop

produces

This is pdfTeX, Version 3.14159265-2.6-1.40.19 (TeX Live 2018) (preloaded format=latex)

 restricted write18 enabled.

entering extended mode

(./zehrfeld.tex

LaTeX2e <2018-12-01>

(/usr/local/texlive/2018/texmf-dist/tex/latex/base/article.cls

Document Class: article 2018/09/03 v1.4i Standard LaTeX document class

(/usr/local/texlive/2018/texmf-dist/tex/latex/base/size10.clo)) (./zehrfeld.aux

) (/usr/local/texlive/2018/texmf-dist/tex/latex/tools/.tex File ignored)

! I can't find file `no'.

l.9 no 

       blank line

(Press Enter to retry, or Control-D to exit)

Please type another input file name: 

What happens when a file with empty file name is looked for depends on your distribution and may also depend on the TeX implementation. The old TeXtures, for instance, first tried files without extension, because in the Macintosh world(late '80s and '90s) file extensions were not used.

Final note: if par is redefined, other things may happen: try with

defpar{aendgraf}



input



blank line above



input

no blank line



stop

which looks for a.tex because the primitive input does macro expansion until finding character tokens.