Re: file descriptor for io.popen (POSIX)?

[Mark Edgar <medgar@...>]

Norman Ramsey wrote:
> I am using lua on a POSIX system, and I would like to
> find a way to grap the underlying file descriptor associated
> with a file, so that I can make it standard output.  E.g.,
>
> local f = assert(io.popen('less', 'w'))
> posix.dup2(fileno(f), 0)
>
> My trouble is that I don't know how to implement 'fileno'.
> Any suggestions?

If you were trying to change standard output in preparation for running 
a child process, you can use ex.spawn() from 
http://lua-users.org/wiki/ExtensionProposal

local i, o = ex.pipe()
ex.spawn{os.getenv"PAGER" or "less", stdin=i}
i:close()
io.output(o)

Or you would use dup2(), fork() and exec() from the posix library.

However, it looks like you simply want print and/or io.write() to write 
to the pipe instead.  In this case:

local f = assert(io.popen('less', 'w'))
io.output(f)

Note that this works for io.write(), but not for io.stdout:write().  It 
also does not work for print(); you'll have to rewrite it:

function print(...)
	local res = {}
	for i=1,select(#,...) do
		res[#res + 1] = select(i, ...)
	end
	io.write(table.concat(res, "\t"), "\n")
end

					-Mark