Kernel#system using bash

[Robert Citek]

Is there a way to have Kernel#system use /bin/bash instead of /bin/sh?

As a toy problem, I would like to run these commands from within ruby:

$ echo foo
foo

and this variant which used process substitution:

$ cat <(echo foo)
foo

The first form works within ruby using the Kernel#system method:

$ ruby -e 'Kernel.system("echo foo") '

The second does not:

$ ruby -e 'Kernel.system("cat <(echo foo)")'
sh: Syntax error: "(" unexpected
^

This is because instead of /bin/bash Kernel#system calls /bin/sh,
which does not know how to do process substitution with <().

From googling there doesn't seem to be an option to Kernel#system[1]
to specify which shell to use. Is this a limitation of using ruby
1.8.6? Any thoughts to a potential workaround?

[1] http://ruby-doc.org/core/classes/Kernel.html#M005971

Regards,
- Robert

[Robert Citek]

On Tue, Dec 22, 2009 at 2:46 PM, Robert Citek <robert...@gmail.com> wrote:
> Is there a way to have Kernel#system use /bin/bash instead of /bin/sh?

Found a workaround:

$ ruby -e 'cmd="cat <(echo foo)" ;Kernel.system("bash -c \"#{cmd}\"") '
foo

Woks with IO.popen, pipes, and options, too:

$ ruby -e '
cmd=<<-eof.gsub(/\^s+/,"")
cat <(seq 1 20) |
head -3
eof
bash=<<-eof.gsub(/\^s+/,"")
bash -c "#{cmd}"
eof
IO.popen(bash).each {|x| puts x}
'
1
2
3

Regards,
- Robert

[Ed Howland]

I was just typing that answer..

Oh well.

Kernel.system is probably written to be  portable to POSIX stds, which specify's /bin/sh should exist, AFAIK.

But I wonder what it does for Windows systems?

Ed

3

Regards,
- Robert

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[Amos King]

module Kernel

  def self.system(*args)

    args.each do |cmd|

      super "bash -c #{cmd}"

    end

  end

end

Amos King
http://dirtyInformation.com
http://github.com/Adkron
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[Craig Buchek]

That doesn't seem to work for me (on Mac OS X 10.6). There's no super
for it to fall back to, is there? Wouldn't we have to alias it instead
of overriding it directly?

[Robert Citek]

Didn't work for me, either:

$ ruby -e '

> module Kernel
> def self.system(*args)
> args.each do |cmd|
> super "bash -c #{cmd}"
> end
> end
> end
>

> system("cat <(echo $(date))")

>
> '
sh: Syntax error: "(" unexpected

This came almost there:

$ ruby -e '
> module Kernel
> def self.bash(*args)
> args.each do |cmd|

> system("bash -c \"#{cmd}\"")

> end
> end
> end
>
> Kernel.bash("cat <(echo $(date))")
>
> '
Fri Dec 25 12:27:45 EST 2009

BTW, happy holidays to all.

Regards,
- Robert

[Mike Gaffney]

you likely need to do alias method to get to the old system method.

-Mike

[Amos King]

Yeah if we alias it it will work.  I just started typing.  I didn't actually try it.  Why would I try it? =8-)

[Amos King]

Actually I would just make a new method.  Kernel.bash