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In fact, there is only one word which reach outside the execution environment (the functional scope of the program): '''<tt>!:</tt>''', the aptly named "foreign" operator. This one operator encapsulates all access to the outside world, and even the "behind the scenes world" of J's own memory space.
The operator takes two arguments and derives a function, which specifies which kind of foreign interface you want. For example, <tt>1'''!:'''1</tt> is the specific function to read a file, and <tt>1'''!:'''2</tt> is the function to write one (as in <tt>1'''!:'''1 'filename'</tt> and <tt>some_data 1'''!:'''2 'filename'</tt>, respectively). The foreign function <tt>15'''!:'''0</tt> allows the J programmer to call a shared library (dll, so, dylib, etc), <tt>2'''!:'''5</tt> reads environment variables (e.g. <tt>2'''!:'''5'PATH'</tt>), and <tt>2'''!:'''55</tt> will terminate the program (quit, die: the mnemonic is that <tt>255</tt> is the "last" value of a byte, and <tt>2'''!:'''55</tt> is the "last" thing you want to do in a J program). There are many more, grouped into families (the first argument to <tt>'''!:'''</tt> specifies which family you want, e.g. <tt>''1'''''!:'''n</tt> is the ''file'' family, and <tt>1'''!:'''''1''</tt> is to ''read'' a file, specifically).
But the key thing is that this one operator, <tt>'''!:'''</tt>, controls ''all'' the dangerous stuff, so if we want to prevent dangerous stuff, we only have to put guards in one place. And, in fact, we have "foreign controls": foreign functions which themselves control which foreign functions are allowed. In other words, there's only one "door" to J, and we can lock it.
From the J documentation:
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