Shell out from ruby while setting an environment variable

Question

I need to shell out to a process while setting an environment variable for it. I tried this one-liner:

system "RBENV_VERSION=system ruby extconf.rb"

~~This syntax works in shell script but not from ruby.~~ (Update: turns out this syntax works from ruby after all, but I failed to see its effect due to my particular use-case.)

So I'm doing this:

rbenv_version = ENV['RBENV_VERSION']
ENV['RBENV_VERSION'] = 'system'
begin
  system "ruby extconf.rb"
ensure
  ENV['RBENV_VERSION'] = rbenv_version
end

I'm forced to such a long expression because I don't want to override the environment variable permanently if it already had a value.

Anything shorter that comes to your mind?

Avdi · Accepted Answer · 2011-11-28 20:08:50Z

76

system({"MYVAR" => "42"}, "echo $MYVAR")

system accepts any arguments that Process.spawn accepts.

edited Nov 28, 2011 at 20:08

answered Nov 28, 2011 at 19:44

Avdi

18.5k6 gold badges56 silver badges63 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Phrogz Over a year ago

This is a great answer. The documentation for Kernel#system is not sufficiently clear on this functionality. Very nice.

alxndr Over a year ago

For ruby 1.8, look at @rtomayko's POSIX::Spawn, linked in another comment.

Andy Hayden Over a year ago

I am getting TypeError: no implicit conversion of Hash into String ...?

user3266738 Over a year ago

@Avdi do you know if there is a way to append a value at the end of the environment variable instead of replacing it? Something like: system({"INCLUDE" => "C:\Qt;%INCLUDE%", "echo %INCLUDE%"})

Anne · Accepted Answer · 2011-11-28 20:09:15Z

7

Ruby 1.9 includes Process::spawn which allows an environ hash to be provided.

Process::spawn is also the foundation for system, exec, popen, etc.
You can pass an environment to each.

Under Ruby 1.8, you may want to consider the POSIX::Spawn library,
which provides the same interfaces

edited Nov 28, 2011 at 20:09

Anne

27.2k9 gold badges66 silver badges71 bronze badges

answered Nov 28, 2011 at 19:44

rtomayko

1,1461 gold badge7 silver badges2 bronze badges

2 Comments

mislav Over a year ago

Good to know! But the invocation is kinda clunky: Process.wait Process.spawn({"MYVAR" => "42"}, "echo $MYVAR") for my simple needs. Turns out the system func also supports this optional hash argument!

Avdi Over a year ago

system accepts all arguments that Process.spawn() accepts. See my answer for example.

Phrogz · Accepted Answer · 2011-11-28 19:45:54Z

4

Using your same approach, but wrapped up as a block method that temporarily modifies the environment (like the block form of Dir.chdir):

def with_environment(variables={})
  if block_given?
    old_values = variables.map{ |k,v| [k,ENV[k]] }
    begin
       variables.each{ |k,v| ENV[k] = v }
       result = yield
    ensure
      old_values.each{ |k,v| ENV[k] = v }
    end
    result
  else
    variables.each{ |k,v| ENV[k] = v }
  end
end

with_environment 'RBENV_VERSION'=>'system' do
  `ruby extconf.rb`
end

answered Nov 28, 2011 at 19:45

Phrogz

304k115 gold badges669 silver badges758 bronze badges

3 Comments

mislav Over a year ago

I use such approach often in tests, but here it's an overkill.

Phrogz Over a year ago

@AnonymousDownvoter I encourage you to downvote answers that are wrong, and upvote answers that are better than others. Downvoting one (correct) answer presumably because there are other answers that are better (easier, shorter) is not in the spirit of Stack Overflow, IMHO.

Dave Dopson Over a year ago

+1 first, for getting an unfair downvote. 2nd, because this is my favorite answer. I was more excited until I realized you had defined :with_environment. It's sad, because that's the method that should exist in Ruby itself.

Shai · Accepted Answer · 2011-11-28 19:49:22Z

3

Actually that worked for me.

shai@comp ~ » irb                                                                                                                                     
1.9.3p0 :001 > system %{SHAIGUITAR=exists ruby -e 'puts ENV["SHAIGUITAR"]'}
exists
 => true

But if it doesn't, maybe you can try prepending "env" to whatever variable you need. E.g.

system(%{env SHAIGUITAR=exists ruby bla.rb})

answered Nov 28, 2011 at 19:49

Shai

1,5691 gold badge10 silver badges7 bronze badges

2 Comments

mislav Over a year ago

It works for me too. I'm an idiot, I tried it and discarded it after I didn't get the desired effect, but it was because of my particular use-case and not because this isn't supported. Thanks!

mislav Over a year ago

The only thing that doesn't work with this – and this is very weird – is the "echo" example. That's why I'm marking the other answer as correct – it covers more cases.

AndrewVos · Accepted Answer · 2011-11-28 19:41:26Z

2

This may work?

system <<-CMD
export VARNAME=123
other_command
CMD

answered Nov 28, 2011 at 19:41

AndrewVos

1,3052 gold badges14 silver badges25 bronze badges

1 Comment

mislav Over a year ago

That works. One line: system 'export VARNAME=123 && other_command'

Collectives™ on Stack Overflow

Shell out from ruby while setting an environment variable

5 Answers 5

4 Comments

2 Comments

3 Comments

2 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

4 Comments

2 Comments

3 Comments

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related