Not much going here....

\input{../Common/thefile.tex}

Or

\input{./Common/thefile.tex}

I tested also.

@cis Both of these work on my system. Are you sure that the current path is the one you expect?

@cis by default they will work, although the first can be disabled if paranoid setting in texmf.cnf which disallows going up from the current directory

@cis the paths are relative to the working directory of the tex executable, not from the main file (if they are different)

@yo' are you around?

@cis I'm on windows and \input{../Common/thefile.tex} works fine.

@cis No it is more likely user error

Looks like I (finally) get out for the day and it's very quiet here:-)

@cis which one do you think should work? they presumably do not both work.

@cis where are you running tex?

@cis from main you have to go two up, so ../../Common/thefile.

@cis if you do cd Workingfolder; pdflatex main then ../Common/tefile.tex should work. If you do cd Parentfolder; pdflatex Workingfolder/main then it won't

@DavidCarlisle I am in main.tex an running pdflatex in TeXLive.

@UlrikeFischer I tested ` ../../Common/thefile` also. Does not work.

@cis you can't be "in" a file, do you mean you are in WorkingFollder when you execute pdflatex?

@cis if you put a test.tex in Workingfolder, can you input it with \input{test}?

@DavidCarlisle I think so, I opened main.tex in TeXLive and run pdflatex.

show your log file the first few lines uo to where it shows (./main.tex

@UlrikeFischer Yes, that works,

@cis ok sorry it should only one ../ I misread your structure. Move test.tex to parentfolder and try to input it with \input{../test}

@UlrikeFischer \input{../test} This works too...

@cis now move it into Comon or Common (whatever is right) and try \input{../Common/test}

@DavidCarlisle now I am

(but not just on mobile...)

@UlrikeFischer This works not.

@DavidCarlisle I am sorry: I get "thefile.tex not found", but I do not find this in the main.log

\documentclass{article}
% Does not work:
%\input{../inputB.tex}

\begin{document}
Test:

Does not work:
%\input{../inputA.tex}
\end{document}

But my mainfile not.

The test works with \input{../InputA/inputA.tex}

@cis yes isn't that what you would expect

@yo' I was going to ask about overleaf aux files, but I found a workaround, so no need to bother you now, sorry:-)

@cis ../ works the same way in latex as it does in a windows or linux commandline

@DavidCarlisle Yes clear. But not a my system.

@cis judging by the example you give above it sounds like it is working as expected on your system

@DavidCarlisle ok, if needed, I'll be on tomorrow

@yo' thanks, basically the question was where a \jobname.aux was coming from (on the first run) but have a good weekend:-)

@DavidCarlisle from the prev. compilation afaik :)

@yo' nope this is submitting a new project from a url off learnlatex.org, if you look at lesson-13 now it works (testing append.aux) but until this evening it was testing \jobname.aux but that seems to be true always, even on the first run

Ah! That's possible. It could also be latexmk doing its stuff

@yo' yes I wondered if it's a latexmk thing. Herbert mailed to say the lesson wasn't working on overleaf (as it wasn't working:-) took a bit of fiddling. I need to use a test rather than the usual way of saying to edit the file to add \includeonly after an initial run, as latexonline (and similar systems) run each submission in a new context so you never get an aux file from a previous edit, but it does run latex twice so it works to do one full run and just do \includeonly on the second

@yo' but \IffileExists{\jobname.aux} is always true, even on the first run, it seems

@yo' but as I say testing for one of the aux files of an included file works as they keep their real names, not disturbed byt the --jobname setting

sub set_trivial_aux_fdb {
    # 1. Write aux file EXACTLY as would be written if the tex file
    #    had no cross references, etc. I.e., a minimal .aux file.

@yo' ^^ so not overleaf's fault:-)

^ Comments? Correct? Useful? (If so…) anyone want to make a pretty version with TikZ or whatever? :-)

(The motivation is that people keep asking and misunderstanding, and not reading explanations of the same in text form…)

@ShreevatsaR well actually lualatex is now based on luahbtex. And one context variant on luametatex (or however lmtx is called).

@ShreevatsaR -- It's farfetched, but I suppose someone might wonder why eTeX isn't in the table.

@ShreevatsaR you should probably fork xetex from etex rather than pdftex (since it has no pdf backend)

Thanks all :)

@UlrikeFischer Thanks for the help

revised version, though after adding more rows and footnotes I'm beginning to realize it may not be as useful or clear as I had hoped :)