From: Dave Thomas <Dave@...>
Date: 2001-07-17T22:04:45+09:00
Subject: [ruby-talk:17997] Re: waiting for `backquotes` to finish

stephen.hill@motorola.com (Steve Hill) writes:

> Some of the scripting relies on some of the standard unix tools to do
> its work, called using backquotes. e.g. the following facet which
> processes the columns of a file previously created with grep.

...

> The problem is that the script may fail at the line marked A because
> the file "foo.tmp" has either not been written at all, or has not been
> completely written.

This shouldn't be a problem, because the command in backquotes will
run to completion _before_ the rest of your code is executed.

However, there's a way to bypass this altogether: you can run the
underlying command and have your Ruby program read the output
directly, rather than having to write the output to a file and
subsequently opening the file. Try:

 
  IO.popen("grep FOO foo.txt") do |pipe|
    pipe.each do |line|
      # ...
   end
  end

A couple of other points. In your original code chunk, you had
'flag? = false'. Does that compile in your version of Ruby?
You also had

   unless flag? do
    ...
   end

Although this looks reasonable, it isn't what you meant. The 'do'
introduces a block (as in an iterator block), not the body of a
compound statement. This will force 'flag?' to be interpreted as a
method name.


Dave