From: Bill Kelly <billk@...>
Date: 2009-11-11T08:43:05+09:00
Subject: Re: Calling a subprocess with specific arguments and capturing its output?


From: "Dan Q" <quinxex@gmail.com>
>
> Hi. I haven't written Ruby in a while, and I was wondering if someone
> could help with a problem I've never managed to solve. I'm writing a
> shell script that takes filenames as its arguments, and calls "du" to
> get their size. Because I don't want to have to escape everything
> perfectly, I was looking for a function with a syntax that allows
> separate arguments, like system("du", "-sh", filename).
> 
> system() doesn't allow me to capture the output of the subprocess,
> while popen , ``, and %x() don't let me specify a list of discrete
> arguments. Is there an elegant solution to this problem? My current
> solution is to just escape the arguments and put them into a string,
> but this is ugly and buggy.

One possibility might be to let Shellwords handle the
escaping:

irb(main):001:0> require 'shellwords'
irb(main):002:0> args = ["foo", "bar baz", "mary 'had' a", 'little "lamb" and stuff']
irb(main):003:0> puts( args.map {|a| a.shellescape}.join(" ") )
foo bar\ baz mary\ \'had\'\ a little\ \"lamb\"\ and\ stuff


Alternately, if you can use ruby 1.9.x, there appears to be
a very powerful new Kernel.spawn command.  (See the `ri`
documentation, several pages...)

Also in 1.9.x, it appears to now be possible to pass an 
Array for the command arguments to popen:

irb(main):001:0> IO.popen(["ls", "-l", "GL_GameOfLife.zip", "quake3-1.32b-source.zip"], "r") {|io| puts(io.read)}
-rw------- 1 billk billk 3540592 Feb 23  2009 GL_GameOfLife.zip
-rw------- 1 billk billk 5724791 Sep 21  2005 quake3-1.32b-source.zip


Hope this helps,

Bill