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 it's 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 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
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("...") }