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.

Windows - MikTeX

MikTeX 2.4

Create a file [localtexmf PATH]\miktex\config\miktex.ini and add the following line to it:

write18=enable

(Because of some unknown reason, write18=enable in [localtexmf PATH]\miktex\config\miktex.ini may be ignored on some computers. In that case make the change in [MikTeX PATH]\miktex\config\miktex.ini, but you may have to repeat this step every time you update MikTeX packages.)

MikTeX 2.5

Each application now has it's own configuration file. To enable write18 run

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

and put

EnableWrite18=t

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

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

In that file you'll probably find a line reading

shell_escape = f

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