Forum: Ruby Shell pipeline in Ruby?

F889bf17449ffbf62345d2b2d316a937?d=identicon&s=25 Michal Suchanek (Guest)
on 2011-05-12 18:07
(Received via mailing list)
Hello,

how do you write an equivalent of

$ cmdA | cmdB | cmdC

in Ruby?

Specifically, I would like to see the PID, return value and stderr of
each of these commands but I would like cmdB to read the stdout of
cmdA directly, on its stdin, and similarily for cmdC and cmdB.

Here I am not interested in feeding cmdA some particular input (eg. it
can read /dev/null for all I care) but in general gluing arbitrary fd
to its stdin might be desirable in other cases.

Thanks

Michal
0107ef1bc42d0626a706ca6af9a43060?d=identicon&s=25 Jos Backus (Guest)
on 2011-05-12 19:25
(Received via mailing list)
Have you checked out Open3.pipeline?

Jos

--
Peace cannot be achieved through violence, it can only be attained
through
understanding.
54404bcac0f45bf1c8e8b827cd9bb709?d=identicon&s=25 7stud -- (7stud)
on 2011-05-12 20:06
Michal Suchanek wrote in post #998275:
> Hello,
>
> how do you write an equivalent of
>
> $ cmdA | cmdB | cmdC
>

How about:

puts `cmdA | cmdB | cmdC`

or

puts %x{cmdA | cmdB | cmdC}
F889bf17449ffbf62345d2b2d316a937?d=identicon&s=25 Michal Suchanek (Guest)
on 2011-05-12 21:42
(Received via mailing list)
On 12 May 2011 19:22, Jos Backus <jos@catnook.com> wrote:
> Have you checked out Open3.pipeline?
>

I have checked all the stuff I could find once but did not find a
solution.

That's why I am asking, perhaps I overlooked something.

While the pipeline gives the status of all the involved processes and
connects them properly which is more than I ever got from Ruby it is
not obvious how to capture the output  - the stdout of the last
process can be captured with pipeline_r but the stderrs get sinked
somewhere in the pipeline_run.

Also the in and out options used in the example are not documented
afaict.

Thanks

Michal
9a45896e48a382fe5c656b8873e0dfcb?d=identicon&s=25 Stu (Guest)
on 2011-05-12 21:57
(Received via mailing list)
There is also popen4 which I believe also gives you the pid with std -
in/out/err
F889bf17449ffbf62345d2b2d316a937?d=identicon&s=25 Michal Suchanek (Guest)
on 2011-05-12 22:20
(Received via mailing list)
On 12 May 2011 21:54, Stu <stu@rubyprogrammer.net> wrote:
> There is also popen4 which I believe also gives you the pid with std -
> in/out/err
>

It provides stderr but not the pipeline which cannot be constructed in
pure ruby (unless a popen taking fd:s for stdio is implemented).

Thanks

Michal
Bec38d63650c8912b6ba9b557fb953b9?d=identicon&s=25 Roger Pack (rogerdpack)
on 2011-05-13 05:13
In the spirit of the recent call for better ruby documentation, I wrote
a
writeup:

http://en.wikibooks.org/w/index.php?title=Ruby_Pro...

Enjoy.
-roger-
F889bf17449ffbf62345d2b2d316a937?d=identicon&s=25 Michal Suchanek (Guest)
on 2011-05-13 10:36
(Received via mailing list)
On 13 May 2011 05:13, Roger Pack <rogerpack2005@gmail.com> wrote:
> In the spirit of the recent call for better ruby documentation, I wrote

Unfortunately, this does not work.

First, these pipelines don't always work as one would expect so
testing is *required* before posting a "solution".

Second, afaict the reopen method only changes the IOs at Ruby level,
not the fd:s at C level so it's useless in this context.

Thanks

Michal
E0d864d9677f3c1482a20152b7cac0e2?d=identicon&s=25 Robert Klemme (robert_k78)
on 2011-05-13 10:45
(Received via mailing list)
On Thu, May 12, 2011 at 6:05 PM, Michal Suchanek <hramrach@centrum.cz>
wrote:
> Here I am not interested in feeding cmdA some particular input (eg. it
> can read /dev/null for all I care) but in general gluing arbitrary fd
> to its stdin might be desirable in other cases.

$ ri Open3.pipeline_r Open3.pipeline_start

Cheers

robert
936431ecc527fd7e349d4abc9ff0e7c1?d=identicon&s=25 Lionel Orry (lionel)
on 2011-05-13 11:39
(Received via mailing list)
+1 for Robert's answer. From the Open3.pipeline_start doc, an example is
given:

# convert pdf to ps and send it to a printer.
# collect error message of pdftops and lpr.
pdf_file = "paper.pdf"
printer = "printer-name"
err_r, err_w = IO.pipe
Open3.pipeline_start(["pdftops", pdf_file, "-"],
                     ["lpr", "-P#{printer}"],
                     :err=>err_w) {|ts|
  err_w.close
  p err_r.read # error messages of pdftops and lpr.
}

Cheers
Lionel
F889bf17449ffbf62345d2b2d316a937?d=identicon&s=25 Michal Suchanek (Guest)
on 2011-05-13 15:33
(Received via mailing list)
On 13 May 2011 10:44, Robert Klemme <shortcutter@googlemail.com> wrote:
>>
>> Here I am not interested in feeding cmdA some particular input (eg. it
>> can read /dev/null for all I care) but in general gluing arbitrary fd
>> to its stdin might be desirable in other cases.
>
> $ ri Open3.pipeline_r Open3.pipeline_start
>
Again, like in Open3.pipeline the :err argument used in the example is
not documented.

From the description of the example it seems it captures the stderr of
all the commands together.

Are there more arguments like that?

What do they mean?

Also this is available in ruby 1.9 only.

Thanks

Michal
Bec38d63650c8912b6ba9b557fb953b9?d=identicon&s=25 Roger Pack (rogerdpack)
on 2011-05-13 15:43
> Unfortunately, this does not work.

Try it now, fixed the bug.

> Second, afaict the reopen method only changes the IOs at Ruby level,
> not the fd:s at C level so it's useless in this context.

Try it out.  This is, AFAIK, the "standard" way to pipeline processes,
and works in 1.8 even.

GL!
-r
Bec38d63650c8912b6ba9b557fb953b9?d=identicon&s=25 Roger Pack (rogerdpack)
on 2011-05-13 15:47
Roger Pack wrote in post #998434:
> In the spirit of the recent call for better ruby documentation, I wrote
> a
> writeup:
>
>
http://en.wikibooks.org/w/index.php?title=Ruby_Pro...


Unfortunately it appears the wiki formatting is mangled in firefox, but
works well in chrome :)

Here's the complete code snippet:


pipe_me_in, pipe_peer_out = IO.pipe
pipe_peer_in, pipe_me_out = IO.pipe

fork do
  STDIN.reopen(pipe_peer_in)
  STDOUT.reopen(pipe_peer_out)
  Kernel.exec("echo 33")
  # this line is never executed because exec moves the process
end
pipe_peer_out.close # file handles have to all be closed in order for
the "read" method, below, to be able to know
# that it's done reading data, so it can return.  See also
http://devver.wordpress.com/2009/10/22/beware-of-p...
pipe_me_in.read



There is a link in there to a blog describing ruby's shell class,
perhaps you did not see it?

-r
E0d864d9677f3c1482a20152b7cac0e2?d=identicon&s=25 Robert Klemme (robert_k78)
on 2011-05-13 16:26
(Received via mailing list)
On Fri, May 13, 2011 at 3:27 PM, Michal Suchanek <hramrach@centrum.cz>
wrote:
>>> cmdA directly, on its stdin, and similarily for cmdC and cmdB.
>>>
>>> Here I am not interested in feeding cmdA some particular input (eg. it
>>> can read /dev/null for all I care) but in general gluing arbitrary fd
>>> to its stdin might be desirable in other cases.
>>
>> $ ri Open3.pipeline_r Open3.pipeline_start
>>
> Again, like in Open3.pipeline the :err argument used in the example is
> not documented.

Not true.  From ri Open3.pipeline_r in 1.9.2p180:

Each cmd is a string or an array. If it is an array, the elements are
passed
to Process.spawn.

  cmd:
    commandline                              command line string which
is passed to a shell
    [env, commandline, opts]                 command line string which
is passed to a shell
    [env, cmdname, arg1, ..., opts]          command name and one or
more arguments (no shell)
    [env, [cmdname, argv0], arg1, ..., opts] command name and
arguments including argv[0] (no shell)

  Note that env and opts are optional, as Process.spawn.

Then

$ ri Process.spawn

> From the description of the example it seems it captures the stderr of
> all the commands together.
>
> Are there more arguments like that?
>
> What do they mean?

See above.

> Also this is available in ruby 1.9 only.

Well?

Cheers

robert
F889bf17449ffbf62345d2b2d316a937?d=identicon&s=25 Michal Suchanek (Guest)
on 2011-05-13 17:34
(Received via mailing list)
On 12 May 2011 18:05, Michal Suchanek <hramrach@centrum.cz> wrote:
> cmdA directly, on its stdin, and similarily for cmdC and cmdB.
>
> Here I am not interested in feeding cmdA some particular input (eg. it
> can read /dev/null for all I care) but in general gluing arbitrary fd
> to its stdin might be desirable in other cases.
>
> Thanks
>
> Michal
>

OK, thanks for all the replies.

It seems this is not doable with ruby 1.8 but spawn and similar
methods in 1.9 grew the named arguments which include the file
descriptors.

Here is a working (although not polished) solution for ruby 1.9.

First a short testing script called cmd:

#!/usr/bin/ruby

prefix = ARGV[0] + ":"
append = ARGV[1]

while l = STDIN.gets do
  STDOUT << l
  STDERR << prefix << l
end

STDOUT.puts append

And the long script with pipes that calls the above:

#!/usr/bin/ruby1.9.1
# Debian calls ruby 1.9 ruby1.9.1

pipes = []
(1..4).each {pipes << IO.pipe}
errs = []
(1..3).each {errs << IO.pipe}

writer =  pipes[0][1]
reader =  pipes[3][0]
i = 1

# Careful about pipe direction, it only works one way.
pids = [
spawn("cmd", "a", "aa", :in => pipes[0][0], :out => pipes[1][1], :err
=> errs[0][1] ),
spawn("cmd", "b", "bb", :in => pipes[1][0], :out => pipes[2][1], :err
=> errs[1][1] ),
spawn("cmd", "c", "cc", :in => pipes[2][0], :out => pipes[3][1], :err
=> errs[2][1] ),
]

# We don't really want to see these pipes, perhaps some
Open3.pipeline_* method would create them out of sight
pipes.flatten.select{|p| p != writer && p != reader}.each{|p|p.close}\

maxlen = 128 # some arbitrary size for chunk of data read at once

outputs = errs.collect{|r,w|r}
outputs << reader
wsel = [writer]

while true do
  ios = IO.select(outputs, wsel, nil, 1)
  STDERR << '.'

    string = "%4i\n" % i
    if writer then
      begin
        result = writer.write_nonblock(string)
        i = i + 1
        if ( i > 1000 ) then
          writer.close
          writer = nil
          wsel = nil
        end
      rescue IO::WaitWritable, Errno::EINTR
      end
    end

  outputs.each{|io|
    begin
      result = io.read_nonblock(maxlen)
      STDOUT << result
    rescue IO::WaitReadable, Errno::EINTR, EOFError
      if $!.is_a? EOFError then
        STDERR.puts "EOF"
        outputs.reject!{|o| o == io }
      end
    end
  }

  (0...3).each{|i| p = pids[i]
    if p and Process.waitpid(p, Process::WNOHANG) then
      STDOUT.puts $?.inspect
      pids[i] = nil
    end
  }
  break if !ios && pids.select{|pid| pid} == []
end
F889bf17449ffbf62345d2b2d316a937?d=identicon&s=25 Michal Suchanek (Guest)
on 2011-05-13 19:44
(Received via mailing list)
On 13 May 2011 16:23, Robert Klemme <shortcutter@googlemail.com> wrote:
>>>> each of these commands but I would like cmdB to read the stdout of
>
>    [env, cmdname, arg1, ..., opts]          command name and one or
> more arguments (no shell)
>    [env, [cmdname, argv0], arg1, ..., opts] command name and
> arguments including argv[0] (no shell)
>
>  Note that env and opts are optional, as Process.spawn.
>
> Then
>
> $ ri Process.spawn
>

It's not quite clear that Process.spawn description also applies to
options outside the commands and how it applies but maybe it makes
sense, they would be applied to the whole pipeline.

Thanks

Michal
F889bf17449ffbf62345d2b2d316a937?d=identicon&s=25 Michal Suchanek (Guest)
on 2011-05-13 20:37
(Received via mailing list)
On 13 May 2011 15:48, Roger Pack <rogerpack2005@gmail.com> wrote:
> works well in chrome :)
>  Kernel.exec("echo 33")
>  # this line is never executed because exec moves the process
> end
> pipe_peer_out.close # file handles have to all be closed in order for
> the "read" method, below, to be able to know
> # that it's done reading data, so it can return.  See also
>
http://devver.wordpress.com/2009/10/22/beware-of-p...
> pipe_me_in.read

Changing the last line to

puts '"' + pipe_me_in.read.chomp + '"'

Really writes 33 quoted so IO.reopen works for setting up the file
descriptors.

It is not clear from the docs if it would work or not.

It does work, though:

out = File.open("testfile","w")

STDOUT.reopen(out)

system("echo 33")

STDERR.puts '"' + File.read("testfile").chomp + '"'

=> "33"

I vaguely recall some issues with using STDIN and STDOUT for
redirection earlier but it might be due to using them incorrectly.

>
>
>
> There is a link in there to a blog describing ruby's shell class,
> perhaps you did not see it?

Yes, I missed the link labeled just [1]


Thanks

Michal
753dcb78b3a3651127665da4bed3c782?d=identicon&s=25 Brian Candler (candlerb)
on 2011-05-14 17:05
Michal Suchanek wrote in post #998548:
> And the long script with pipes that calls the above:

Since you are doing this on a real operating system, there's no need to
use ruby to copy the output of one process to the input of the next
process (unless you want to capture the communication between the
processes as well).

OTOH, making sure all the right FDs are closed in each child can be
tricky in 1.8, because there's no IO.close_on_exec= (although 1.9 has
this).

Here is an example which works in 1.8.7. Obviously it's possible to
refactor this to N children, although I expect you'll end up with
something like the 'pipeline' method mentioned before if you do.

--------------------------------
#Thread.abort_on_exception = true  # for debugging

ruby_to_a_rd, ruby_to_a_wr = IO.pipe
a_to_b_rd, a_to_b_wr = IO.pipe
a_err_rd, a_err_wr = IO.pipe
# open: r2a_r, r2a_w, a2b_r, a2b_w, ae_r, ae_w
pid1 = fork do
  ruby_to_a_wr.close
  a_to_b_rd.close
  a_err_rd.close
  STDIN.reopen(ruby_to_a_rd)
  STDOUT.reopen(a_to_b_wr)
  STDERR.reopen(a_err_wr)
  exec("cat; echo done cat 1>&2")
  STDERR.puts "Whoops! #{$!}"
end
ruby_to_a_rd.close
a_to_b_wr.close
a_err_wr.close
# open: r2a_w, a2b_r, ae_r

b_to_c_rd, b_to_c_wr = IO.pipe
b_err_rd, b_err_wr = IO.pipe
# open: r2a_w, a2b_r, ae_r, b2c_r, b2c_w, be_r, be_w
pid2 = fork do
  ruby_to_a_wr.close
  b_to_c_rd.close
  a_err_rd.close
  b_err_rd.close
  STDIN.reopen(a_to_b_rd)
  STDOUT.reopen(b_to_c_wr)
  STDERR.reopen(b_err_wr)
  exec("tr 'a-z' 'A-Z'; echo done tr 1>&2")
  STDERR.puts "Whoops! #{$!}"
end
a_to_b_rd.close
b_to_c_wr.close
b_err_wr.close
# open: r2a_w, ae_r, b2c_r, be_r

c_to_ruby_rd, c_to_ruby_wr = IO.pipe
c_err_rd, c_err_wr = IO.pipe
# open: r2a_w, ae_r, b2c_r, be_r, c2r_r, c2r_w, ce_r, ce_w
pid3 = fork do
  ruby_to_a_wr.close
  c_to_ruby_rd.close
  a_err_rd.close
  b_err_rd.close
  c_err_rd.close
  STDIN.reopen(b_to_c_rd)
  STDOUT.reopen(c_to_ruby_wr)
  STDERR.reopen(c_err_wr)
  exec("sed 's/O/0/g'; echo done sed 1>&2")
  STDERR.puts "Whoops! #{$!}"
end
b_to_c_rd.close
c_to_ruby_wr.close
c_err_wr.close
# open: r2a_w, ae_r, be_r, c2r_r, ce_r

Thread.new do
  ruby_to_a_wr.puts "Here is some data"
  ruby_to_a_wr.puts "And some more"
  ruby_to_a_wr.close
end

Thread.new do
  while line = a_err_rd.gets
    puts "A err: #{line}"
  end
  a_err_rd.close
end

Thread.new do
  while line = b_err_rd.gets
    puts "B err: #{line}"
  end
  b_err_rd.close
end

Thread.new do
  while line = c_err_rd.gets
    puts "C err: #{line}"
  end
  c_err_rd.close
end

while line = c_to_ruby_rd.gets
  puts line
end
c_to_ruby_rd.close
--------------------------------

This can be simplified a bit if you don't want three separate error
streams; they can all share the same one. i.e. each child would have

  err_rd.close
  STDERR.reopen(err_wr)

Regards,

Brian.
753dcb78b3a3651127665da4bed3c782?d=identicon&s=25 Brian Candler (candlerb)
on 2011-05-14 17:14
Here is a ruby 1.9.2 version with close_on_exec=. I leave it as an
exercise to refactor this to an arbitrary pipeline of N commands - it
would return one input pipe, one output pipe, N pids and N error pipes.

Regards,

Brian.


#Thread.abort_on_exception = true  # for debugging

ruby_to_a_rd, ruby_to_a_wr = IO.pipe
ruby_to_a_wr.close_on_exec = true # no children should have this

a_to_b_rd, a_to_b_wr = IO.pipe

a_err_rd, a_err_wr = IO.pipe
a_err_rd.close_on_exec = true     # no children should have this

pid1 = fork do
  a_to_b_rd.close
  STDIN.reopen(ruby_to_a_rd)
  STDOUT.reopen(a_to_b_wr)
  STDERR.reopen(a_err_wr)
  exec("cat; echo done cat 1>&2")
  STDERR.puts "Whoops! #{$!}"
end
# we don't want these fds, nor any of the further children
ruby_to_a_rd.close
a_to_b_wr.close
a_err_wr.close

b_to_c_rd, b_to_c_wr = IO.pipe
b_err_rd, b_err_wr = IO.pipe
b_err_rd.close_on_exec = true  # no children should have this

pid2 = fork do
  b_to_c_rd.close
  STDIN.reopen(a_to_b_rd)
  STDOUT.reopen(b_to_c_wr)
  STDERR.reopen(b_err_wr)
  exec("tr 'a-z' 'A-Z'; echo done tr 1>&2")
  STDERR.puts "Whoops! #{$!}"
end
# we don't want these fds, nor any of the further children
a_to_b_rd.close
b_to_c_wr.close
b_err_wr.close

c_to_ruby_rd, c_to_ruby_wr = IO.pipe
c_err_rd, c_err_wr = IO.pipe
c_err_rd.close_on_exec = true  # no children should have this
pid3 = fork do
  c_to_ruby_rd.close
  STDIN.reopen(b_to_c_rd)
  STDOUT.reopen(c_to_ruby_wr)
  STDERR.reopen(c_err_wr)
  exec("sed 's/O/0/g'; echo done sed 1>&2")
  STDERR.puts "Whoops! #{$!}"
end
# we don't want these fds, nor any of the further children
b_to_c_rd.close
c_to_ruby_wr.close
c_err_wr.close

Thread.new do
  ruby_to_a_wr.puts "Here is some data"
  ruby_to_a_wr.puts "And some more"
  ruby_to_a_wr.close
end

Thread.new do
  while line = a_err_rd.gets
    puts "A err: #{line}"
  end
  a_err_rd.close
end

Thread.new do
  while line = b_err_rd.gets
    puts "B err: #{line}"
  end
  b_err_rd.close
end

Thread.new do
  while line = c_err_rd.gets
    puts "C err: #{line}"
  end
  c_err_rd.close
end

while line = c_to_ruby_rd.gets
  puts line
end
c_to_ruby_rd.close
F889bf17449ffbf62345d2b2d316a937?d=identicon&s=25 Michal Suchanek (Guest)
on 2011-05-16 13:17
(Received via mailing list)
On 14 May 2011 17:05, Brian Candler <b.candler@pobox.com> wrote:
> Michal Suchanek wrote in post #998548:
>> And the long script with pipes that calls the above:
>
> Since you are doing this on a real operating system, there's no need to
> use ruby to copy the output of one process to the input of the next

I don't use Ruby for that.

And I used a select loop because I had issues with threads in the past.

However, your example works perfectly for me, even when updated to put
more data through each of the pipes to make sure they don't end up all
in a buffer and the threads have to be actually switched for the
pipeline to work.

I think there might still be issues if the produced data was without
line endings because none of the gets would finish but that can be
solved by using read_nonblock or somesuch instead.

Thanks

Michal

-----------
#Thread.abort_on_exception = true  # for debugging

ruby_to_a_rd, ruby_to_a_wr = IO.pipe
a_to_b_rd, a_to_b_wr = IO.pipe
a_err_rd, a_err_wr = IO.pipe
# open: r2a_r, r2a_w, a2b_r, a2b_w, ae_r, ae_w
pid1 = fork do
    ruby_to_a_wr.close
    a_to_b_rd.close
    a_err_rd.close
    STDIN.reopen(ruby_to_a_rd)
    STDOUT.reopen(a_to_b_wr)
    STDERR.reopen(a_err_wr)
    exec('while read x ; do echo "$x" ; echo "$x" >&2 ; done')
    STDERR.puts "Whoops! #{$!}"
end
ruby_to_a_rd.close
a_to_b_wr.close
a_err_wr.close
# open: r2a_w, a2b_r, ae_r

b_to_c_rd, b_to_c_wr = IO.pipe
b_err_rd, b_err_wr = IO.pipe
# open: r2a_w, a2b_r, ae_r, b2c_r, b2c_w, be_r, be_w
pid2 = fork do
    ruby_to_a_wr.close
    b_to_c_rd.close
    a_err_rd.close
    b_err_rd.close
    STDIN.reopen(a_to_b_rd)
    STDOUT.reopen(b_to_c_wr)
    STDERR.reopen(b_err_wr)
    exec('while read x ; do x=`echo "$x" | tr a-z A-Z` ; echo "$x" ;
echo "$x" >&2 ; done')
    STDERR.puts "Whoops! #{$!}"
end
a_to_b_rd.close
b_to_c_wr.close
b_err_wr.close
# open: r2a_w, ae_r, b2c_r, be_r

c_to_ruby_rd, c_to_ruby_wr = IO.pipe
c_err_rd, c_err_wr = IO.pipe
# open: r2a_w, ae_r, b2c_r, be_r, c2r_r, c2r_w, ce_r, ce_w
pid3 = fork do
    ruby_to_a_wr.close
    c_to_ruby_rd.close
    a_err_rd.close
    b_err_rd.close
    c_err_rd.close
    STDIN.reopen(b_to_c_rd)
    STDOUT.reopen(c_to_ruby_wr)
    STDERR.reopen(c_err_wr)
    exec('while read x ; do x=`echo "$x" | sed -e s/O/0/g` ; echo "$x"
; echo "$x" >&2 ; done')
    STDERR.puts "Whoops! #{$!}"
end
b_to_c_rd.close
c_to_ruby_wr.close
c_err_wr.close
# open: r2a_w, ae_r, be_r, c2r_r, ce_r

Thread.new do
    ruby_to_a_wr.puts "Here is some data"
    (1..1000).each{|i|
        ruby_to_a_wr.puts "#{i} And some more"
    }
    ruby_to_a_wr.close
end

Thread.new do
    while line = a_err_rd.gets
        puts "A err: #{line}"
    end
    a_err_rd.close
end

Thread.new do
    while line = b_err_rd.gets
        puts "B err: #{line}"
    end
    b_err_rd.close
end

Thread.new do
    while line = c_err_rd.gets
        puts "C err: #{line}"
    end
    c_err_rd.close
end

while line = c_to_ruby_rd.gets
    puts line
end
c_to_ruby_rd.close
F889bf17449ffbf62345d2b2d316a937?d=identicon&s=25 Michal Suchanek (Guest)
on 2011-05-16 17:30
(Received via mailing list)
On 12 May 2011 18:05, Michal Suchanek <hramrach@centrum.cz> wrote:
> cmdA directly, on its stdin, and similarily for cmdC and cmdB.
>
> Here I am not interested in feeding cmdA some particular input (eg. it
> can read /dev/null for all I care) but in general gluing arbitrary fd
> to its stdin might be desirable in other cases.
>

Thanks for all the replies.

To sum up there are multiple methods to achieve this.

 - fork - works on ruby 1.8 but not on Windows
 - Open3  - very nice interface with pipeline_start and such which
does much of the setup behind the scenes but from what I read on the
Net does not work on Windows, again
 - Open3-win32 and Open4 exist to address this issue but they may not
have the same interface.
 - Process.spawn - works only in ruby 1.9 but should be portable to
Windows should you need it. Basically equivalent to Open3 except you
see all the gory details. Not sure why Open3 does not use this on
Windows. Maybe Open3 was in 1.8 already when spawn did not allow for
redirects or something.
 - Shell - somewhat not well known and not well documented Ruby
library. You will reimplement this if you use one of the above ;-)

For reading and writing numerous pipes you can use either a Ruby
thread per pipe or a select loop, either works.

Thanks

Michal
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