Unlike a typical word processing system, LaTeX has an Edit/Run/View cycle closer to working with programming language compilers, and as in programming users may make errors in their input and so need to deal with error messages reported by the system.
This page gives examples of several common errors.
Each error example has some discussion about the form of the error message.
It may be instructive to try the examples but also use the edit features to try to fix the documents and test that you can resolve the errors.
A common first error that people see when starting is:
'pdflatex' is not recognized as an internal or external command,
operable program or batch file.
on Windows or
bash: pdflatex: command not found
on Linux.
This is not a TeX error but an operating system error saying that TeX is not installed or not found. A common mistake is to install an editor such as TeXworks or TeXShop but without installing a TeX system such as TeX Live or MiKTeX.
\documentclass{article}
\usepackage[T1]{fontenc}
\newcommand\mycommand{\textbold{hmmm}}
\begin{document}
My command is used here \mycommand.
\end{document}
This produces a multi-line message in the log file.
! Undefined control sequence.
\mycommand ->\textbold
{hmmm}
l.8 My command is used here \mycommand
.
?
!, gives the general nature of the error (undefined command in this case).\textbold here. After the line break are the remaining tokens {hmmm} that have possibly been read as
an argument but have not yet been executed by TeX.The final line starts with l. followed by a line number, and then the line in the source file where the
error is detected.
?. If using TeX interactively it is possible to
enter instructions to TeX at this point, but most editors and online
systems run TeX in a mode that does not stop at errors but will
scroll past this and try to process the rest of the document. Typing
s to the prompt will instruct TeX to carry on in this mode if you
are working interactively.Note here that TeX does not see the error at the point that
the definition is made; and in fact if \mycommand is defined but not
used, no error would be raised. So although the error is reported on
line 7, the “real” error is in the definition on line 3, so it is
important to see the whole error message.
Beware that some editors show one line “summaries” of the error log. This can be particularly misleading if shown as
line 7: undefined command: ...\mycommand
as it makes it appear that \mycommand is not defined.
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[leqno}{amsmath}
\begin{document}
\end{document}
Here the error is a mismatched } used to end the optional
argument. The closing brace causes LaTeX’s option parsing
to fail and you get an internal and not that helpful error:
! Argument of \@fileswith@ptions has an extra }.
While the error description is unhelpful; the following two lines do accurately display the location of the error by the use of the linebreak showing how far TeX had read:
l.4 \usepackage[leqno}
{amsmath}
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{amsmathz}
\begin{document}
\end{document}
This produces the error
! LaTeX Error: File `amsmathz.sty' not found.
Note: the same error may be caused by two different causes; a simple typo as here, which may be corrected by fixing the package name, or that the file really is missing and needs to be installed on the current system.
\documentclass{article}
\usepackage[T1]{fontenc}
\begin{document}
Some text
\begin{equation}
1=2
\end{equation}
\end{document}
Produces the slightly mysterious error
! Missing $ inserted.
But the fix is simple, blank lines are not allowed in math environments and should be deleted.
Attempt to fix the errors in the supplied examples.
Produce small documents with different errors and note the form of the error messages.