write18 with csname not evaluating

Question

I have noticed that the write18 behaves differently when defined within a csname.
That is, given the following code:
documentclass{article}
begin{document}

csname textbfendcsname{param} after

csname write18endcsname{sleep 10}

end{document}

It produces the following output:

I expected it to evaluate the shell command sleep 10, but instead it wrote it to the pdf stream. My best guess is this is a security mechanism to prevent obfuscation of write18 commands, but am not sure.
P.S. latex is being invoked as pdflatex -shell-escape test.tex

campa · Accepted Answer

When you do
csname write18endcsname{sleep 10}

TeX builds a control sequence with name write18. (I use the keyboard symbol for clarity.) A side effect of csname...endcsname is that if the control sequence is undefined, it gets defined and its definition is relax. The whole construct does nothing, and you are left with {sleep 10} which gets printed in the usual way.
write18 is something different: It's the (primitive) control sequence write followed by the number 18 (which then expects  a token list). The result you expect is obtained by
csname writeendcsname18{sleep 10}

write18 with csname not evaluating

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