Write18

When write18 is turned on, it means that it's possible to execute external programs in the middle of a TeX run. This is required to make some modules such as those for R, lilypond, gnuplot, ... etc. to work properly, but may impose a security leak if you would be running TeX on a server with untrusted sources of TeX files, so it's turned off by default.

How to turn on write18

Windows - MikTeX

MikTeX 2.5/2.6

Each application now has its own configuration file. To enable write18 run

initexmf --edit-config-file=miktex\config\pdftex.ini

or (if you have MikTeX 2.5 or a version of ConTeXt from 2006)

initexmf --edit-config-file=miktex\config\pdfetex.ini

and put

EnableWrite18=t

into it. An alternative is to set the environment variable MIKTEX_ENABLEWRITE18=t.

Windows/Linux/Mac - TexLive

TexLive 2008

Open the file 'path to TexLive'/texmf/web2c/texmf.cnf and search in the file for the following passage:

% Enable system commands via \write18{...}?  Obviously insecure, despite
% being so useful.
shell_escape = f

change here the option from false to true so that the last line above looks like

shell_escape = t

After that you just have to recompile your formats. That is done by

texexec --make --all

or/and

texexec --make --all cont-xx

where xx stands for your needed countrycode. Additionally if you're using MK IV already you have to do the following steps afterwards to get a running environment:

luatools --generate
context --make --all
context --make --all cont-xx

after that write18 sould be enabled and you sould be able to use it.

Linux/Mac

First try to figure out where your configuration file resides:

kpsewhich texmf.cnf

will should return you something like

/usr/local/teTeX/texmf.cnf

or

/usr/local/gwTeX/texmf.cnf

In that file you'll probably find a line reading

shell_escape = f

Change that "f" (false) to "t" (true) to enable it.

How to check if write18 is turned on

Sometimes while writings a module, one needs to check if write18 is enabled or not. There are different ways in which this can be done.

• A new primitive in pdftex 1.30: \pdfshellescape is a read-only integer that is 1 if \write18 is enabled, 0 otherwise.
• ConTeXt defines \systemcommandmode that is 2 if \write18 is enabled, 1 otherwise.
• A primitive \ifeof18 that is true if \write18 is disabled, false otherwise.

\ifeof18 works with both pdfTeX and XeTeX, while \pdfshellescape only works with pdfTeX.

Hans response on the mailing list

write 18 is a hack and pdfshellescape a sane replacement

in luatex it will probably be gone completely since there we have

\directlua 0 { os.execute("...") }

See also

• Executesystemcommand