Piping two shell commands together


#1

I want to pipe two system commands together from within ruby. cmd1 and
cmd2 are arrays, e.g.

cmd1 = [‘oggdec’,’-o’,’-’,oldpath]
cmd2 = [‘lame’,’-’,newpath]

I think I can do the following,

IO.popen(’-’,‘r’) do |p1|
exec cmd1 unless p1
IO.popen(’-’,‘w’) do |p2|
exec cmd2 unless p2
p2.write(p1.read)
end
end

but I don’t like the line “p2.write(p1.read)”, because I think it will
read into memory from p1 before turning around and writing it to p2.
This may work for the above example, but it’s not very pipelike and I
worry that read might return early (I’m not sure what the semantics of
that are in Ruby, but in C I’d have to account for that).

Plus I don’t like the popen ‘-’/exec combination. I’d love a more
elgant, efficient, and/or safer idiom if anyone can think of one.


#2

#{cmd1.join(' ')} | #{cmd2.join(' ')}

Those are backticks. Let the system handle piping with the ‘|’
character.

Jason


#3

Hans F. removed_email_address@domain.invalid wrote:

I want to pipe two system commands together from within ruby. cmd1 and
cmd2 are arrays, e.g.

cmd1 = [‘oggdec’,’-o’,’-’,oldpath]
cmd2 = [‘lame’,’-’,newpath]

I don’t know about the arrays, but you can say IO.popen (“oggdec |
lame”)… m.


#4

matt neuburg wrote:

Yeah, that won’t work here (unless there’s a reliable way to
shell-quote/escape oldpath and newpath).


#5

Hans F. removed_email_address@domain.invalid wrote:

lame")… m.

Yeah, that won’t work here (unless there’s a reliable way to
shell-quote/escape oldpath and newpath).

I’m a little confused about oldPath. Once you have told IO.popen to
generate an IO instance (let’s call it ios), whatever you write to ios
passes thru the process. So you don’t actually need to tell IO.popen
about oldpath; you just write the data.

As for quoting, I just put quotes around it.

So, for example, a way to do LAME encoding with Ruby is like this:

oldpath = “/Users/mattneub/some sound file.aif”
newpath = “/Users/mattneub/some sound file.mp3”
s = “lame --preset standard - ‘#{newpath}’”
IO.popen(s, “r+”) do |ios|
ios.write(File.read(oldpath))
end

So maybe you could say:

s = “oggdec -R -o - - | lame … - ‘#{newpath}’”

I’ve suggested -R because the oggdec man page warns not to write WAV to
stdout. But then I’ve omitted some lame params (…) because you’ll
probably need to tell lame what kind of raw data it’s receiving. I
haven’t tried this, though.

m.


#6

On Aug 2, 2007, at 10:40 AM, Hans F. wrote:

end
end

an alternative:

cfp:~ > ruby a.rb
1073741824
1073741824

cfp:~ > cat a.rb
require ‘open4’

cmd1 = ‘ruby -e" (2 ** 10).times{ STDOUT.write 0.chr * 2**20 } "’
cmd2 = ‘ruby -e" puts ARGF.read.size "’

stdin = IO.popen cmd1

stdout, stderr = ‘’, ‘’

status = Open4.spawn cmd2, 0=>stdin, 1=>stdout, 2=>stderr

puts stdout
puts 2 ** 30

a @ http://drawohara.com/


#7

matt neuburg removed_email_address@domain.invalid wrote:

s = “oggdec -R -o - - | lame … - ‘#{newpath}’”

I’ve suggested -R because the oggdec man page warns not to write WAV to
stdout. But then I’ve omitted some lame params (…) because you’ll
probably need to tell lame what kind of raw data it’s receiving. I
haven’t tried this, though.

Yes, I just tried it. (Had to download and compile and install the ogg
stuff first.) I started with a mono aiff file; then I ogg’ed using the
default setting. Then I said:

oldpath = “/Users/mattneub/some sound file.ogg”
newpath = “/Users/mattneub/some sound file.mp3”
s = “oggdec -R -o - - | lame -r -x -m m --preset standard -
‘#{newpath}’”
IO.popen(s, “r+”) do |ios|
ios.write(File.read(oldpath))
end

Worked great. If your file isn’t mono, leave out the “-m m”. If your
native endianity does not require it, leave out the “-x”. Looks pretty
“pipey” to me!

m.


#8

matt neuburg wrote:

I don’t know about the arrays, but you can say IO.popen ("oggdec |
As for quoting, I just put quotes around it.
So maybe you could say:

s = “oggdec -R -o - - | lame … - ‘#{newpath}’”

I’ve suggested -R because the oggdec man page warns not to write WAV to
stdout. But then I’ve omitted some lame params (…) because you’ll
probably need to tell lame what kind of raw data it’s receiving. I
haven’t tried this, though.

What happens when you try to encode to newpath = “Beethoven’s 5th” or
something with an apostrophe?

I did come up with a working solution, which you can read about here:
http://hans.fugal.net/blog/articles/2007/08/02/pipelining-processes-in-ruby

Cheers


#9

Hi,

At Fri, 3 Aug 2007 01:40:00 +0900,
Hans F. wrote in [ruby-talk:263040]:

I want to pipe two system commands together from within ruby. cmd1 and
cmd2 are arrays, e.g.

cmd1 = [‘oggdec’,’-o’,’-’,oldpath]
cmd2 = [‘lame’,’-’,newpath]

IO.popen(’-’,‘r’) do |p1|
exec(*cmd1) unless p1
IO.popen(’-’,‘w’) do |p2|
unless p2
STDIN.reopen(p1)
exec(*cmd2)
end
end
end