\write18 pass % through to shell

Question

This has sort of been touched on in some other postings, but for path reasons I would like to avoid using a shell script and just use ImageMagick's (IM) convert command line executable directly from a \immediate\write18{...} in a macro command, but in my IM convert instructions I need to pass the % (ASCII 37) as part of a macro.

However I try to conceive of making a % - and I have tried numerous ways(!) I always end up getting the Latex code I am using for the % passed through to the command line.

The obvious one, \% goes to the shell as \% directly and that is the pattern followed by all other options.

What I obviously seem to need is some way to generate the ASCII or unicode for % and get that to the shell from LaTeX via \write18 or other.

Or have some sort of temporary suspension on the LaTeX usage of % so that it can be part of a \write18{} and not break the command as a comment starter, and so be passed with out needing to be LaTeX escaped first.

Answer 1

Use \@percentchar instead of %. You need to say \makeatletter before the \write and \makeatother after it to allow for the @ in the macro name if the code isn't located in a package or class or already inside a \makeatletter ... \makeatother block.

There are also \@backslashchar for \ and also \@charlb and \@charrb for { and }. The are listed as "String constants" in my macros2e document as mentioned in Documentation reference for LaTeX internal commands?

You can also now easily define new string macros for any set of characters with the new version of the newverbs package. E.g. to define a macro for the ampersand use:

\usepackage{newverbs}[2011/07/24]
\Verbdef\amp{&}

Note that spaces behind macros are removed also in a \write statement, so you might need to manually insert one using \space.

Answer 2

Here's my two cents about writing percent characters to file:

\documentclass{article}
\usepackage{verbatim}
%
\newwrite\tmpfile
%
\begingroup\lccode`\S=`\%\relax\lowercase{\endgroup\def\mypercent{S}}%
\newcommand\mysecondoftwo[2]{#2}%
%
\immediate\openout\tmpfile test.txt
\immediate\write\tmpfile{Approach 1: Some text and a percent sign\mypercent}%
\immediate\write\tmpfile{Approach 2: Some text and a percent sign\expandafter\mysecondoftwo\string\%}%
\immediate\write\tmpfile{Approach 3: Some text and a percent sign\csname @percentchar\endcsname}%    
\immediate\write\tmpfile{Approach 4: Some text and a percent sign\csname @secondoftwo\expandafter\endcsname\string\%}%        
\makeatletter
\immediate\write\tmpfile{Approach 5: Some text and a percent sign\@percentchar}%
\immediate\write\tmpfile{Approach 6: Some text and a percent sign\expandafter\@secondoftwo\string\%}%
\makeatother
\begingroup
\def\textmacro{Approach 7: Some text and a percent sign}%
\lccode`\S=`\%%
\lowercase{\immediate\write\tmpfile{\textmacro S}}%
\endgroup
\immediate\closeout\tmpfile

\begin{document}
\verbatiminput{test.txt}
\end{document}