From: "Mark J. Reed" <markjreed@...>
Date: 2003-09-01T09:26:42+09:00
Subject: Re: `echo %!(*`

On Sun, Aug 31, 2003 at 06:00:34PM -0500, Shashank Date wrote:
> I am not sure that I understand what you mean by "bypassing the shell",

It's simple.  When you execute `some command` (or the equivalent
using %x), what Ruby actually executes is your command interpreter
("shell" in Unix-speak), passing it the string between `...`
to execute.  It's up to the shell to split up the command into
words, handle wildcard expansion (at least on UNIX; the command
interpreter on Windows leaves that latter duty up to the individual
commands), etc.  The problem is that all this power is a security
hole because it's easy to trick command interpreters into doing
ugly things just by passing something nefarious as a "filename".

The original poster mentioned Kernel.system, which is the way to
execute a command when you don't care about its output.
If you pass it one long string, Kernel.system will also invoke the
shell, but if instead you pass it separate arguments for each word of
the command line, it bypasses the shell and executes the command 
directly.  Thus:

	system("ps -fp#{$$}")		# also invokes the shell
	system('ps', "-fp#{$$}")	# doesn't invoke the shell
	psOutput = `ps -fp#{$$}`	# invokes the shell

The question is: how do you complete the list, that is, capture the
command output without involving the shell?

The only solution of which I'm aware is to use popen/exec:

	if fd = IO.popen('-') then
	    psOutput = fd.readlines.join("\n")
	else
	    exec 'ps', "-fp#{$$}"
	end

Of course, you could turn this into a method:

	def safeBackticks(*args)
	    if fd = IO.popen('-') then
		output = fd.readlines.join("\n")
	    else
		exec *args
	    end
	    return output
	end