# What does “restricted \write18 enabled” mean and why does TexLive keep reporting it?

I've installed a backported version of TL2012 on Ubuntu 12.04, and every time I start pdftex, xetex, luatex, pdflatex, xelatex, or lualatex, I get the statement `restricted \write18 enabled. entering extended mode`. I don't recall seeing this in Miktex so I wonder whether I did something wrong during the installation. Interestingly, I don't get the message when I run `tex`.

First of all, what does it mean? Second, is it a normal message for Tex Live (2012)? Finally, if it is an error, how can I fix it?

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For years `\write18` has been used as a back door to the operating system. Syntactically TeX treats it as writing to a file, but register 18 is treated as a pre-opened file that in fact executes commands.
The third alternative is never to execute commands at all from `\write18`.