Newbie Problem with Shell#transact

ruby-talk
1

Back from the conference and eager to learn Ruby, I’m trying to write a
tool for grepping the table of contents of jar files. (I have a similar
Bourne shell script, but I want to add some features and start small
learning Ruby). I have something working, but in trying to restructure
it, I’ve encountered a problem with Shell#transact when using it inside
an instance method of a class. In both cases, I’ve used
Shell#def_system_command to define jar and grepcmd like this:

Shell.def_system_command "jar"
Shell.def_system_command "grepcmd", "grep"

If I have a “plain” script (i.e. no class definition or methods), I have
no problem with the following:

sh = Shell.new
ARGV.each { |jarfile|
  sh.transact do
    results = jar("tf", jarfile) | grepcmd(pattern)
    puts results
  end
}

However, if I try to put this in an instance method (see next code
snippet), it never returns and starts taking 100% of my CPU until I kill
it.

class JarGrepper
  attr_writer :pattern
  def jartoc(jarfile)
    sh = Shell.new
    sh.transact do
      results = jar("tf", jarfile) | grepcmd(@pattern)
      puts results
    end
  end
end

If I take out the transact (see next code snippet), it works, but I get
warnings:

class JarGrepper
  attr_writer :pattern
  def jartoc(jarfile)
    sh = Shell.new
    results = sh.jar("tf", jarfile) | sh.grepcmd(@pattern)
    puts results
  end
end

Here are the warnings:

shell: notice: Process finishing...
       wait for Job[grep:#6293] to finish.
       You can use Shell#transact or Shell#check_point for more safe
       execution.
shell: notice: Process finishing...
       wait for Job[jar:#6292] to finish.
       You can use Shell#transact or Shell#check_point for more safe
       execution.

What I’d like to be able to do is to get the results of ‘jar(“tf”,
jarfile) | grepcmd(@pattern)’ from within an instance method and without
warnings.

In case it’s relevant, I’m running Ruby 1.6.7 on Red Hat Linux 7.2.

Thanks in advance for any help or explanations,
Xandy

2

Write it like this

    class JarGrepper
      attr_writer :pattern
      def jartoc(jarfile)
        sh = Shell.new

           pattern = @pattern

        sh.transact do
          results = jar("tf", jarfile) | grepcmd(@pattern)

             results = jar("tf", jarfile) | grepcmd(pattern)

          puts results
        end
      end
    end

Guy Decoux

3

OK. That works, but the more important question is, “Why?” Is there a
general principle I can apply here? Should it be considered a bug? At
least for me, it violates the Principle Of Least Surprise, but my Ruby
intuition is not yet well trained.

Thanks,
Xandy

···

On Tue, 2002-11-05 at 04:54, ts wrote:

Write it like this

class JarGrepper
  attr_writer :pattern
  def jartoc(jarfile)
    sh = Shell.new

       pattern = @pattern

    sh.transact do
      results = jar("tf", jarfile) | grepcmd(@pattern)

         results = jar("tf", jarfile) | grepcmd(pattern)

      puts results
    end
  end
end

Guy Decoux

4

"Why?"

#transact use #instance_eval

With

    sh.transact do
         results = jar("tf", jarfile) | grepcmd(@pattern)

@pattern is interpreted as an instance variable of `sh' and not an
instance variable of JarGrepper object, i.e.

pigeon% ruby
class A; attr_accessor :a; end
a = A.new
a.a = 12
@a = 24
a.instance_eval do
   p @a
end
^D
12
pigeon%

Guy Decoux