How to pass a variable to a one line command.

Hi everyone!

Today I learned (while trying to create a Facter fact ) that if you need to run a one line command that requires an Environment variable set, it's not as straight forward as you would expect.


I started with the example from the Puppet Docs

Facter.add(:rubypath) do
  setcode 'which ruby'
end

This gave me the following output

dawiest@old-laptop:~$ puppet apply -e 'notify{"Fact: $rubypath":} '
Notice: Compiled catalog for old-laptop in environment production in 0.10 seconds
Notice: Fact: /usr/bin/ruby
Notice: /Stage[main]/Main/Notify[Fact: /usr/bin/ruby]/message: defined 'message' as 'Fact: /usr/bin/ruby'
Notice: Finished catalog run in 0.07 seconds

 replaced the command with an example command to cover my case.

Facter.add(:example_fact) do
  setcode 'VAR=hello; echo $VAR'
end

And here is my output, what went wrong!?

dawiest@old-laptop:~$ puppet apply -e 'notify{"Fact: $example":} '
Notice: Compiled catalog for old-laptop in environment production in 0.11 seconds
Notice: Fact:
Notice: /Stage[main]/Main/Notify[Fact: ]/message: defined 'message' as 'Fact: '
Notice: Finished catalog run in 0.06 seconds

I figured that the variable declaration was being lost somewhere... how about I run it in a sub-shell?

Facter.add(:example_fact) do
  setcode 'sh -c "VAR=hello; echo $VAR" '
end

I ran that, and got the same output as last time!  But wait!  if I reverse the order of the quotes...

Facter.add(:example_fact) do
  setcode "sh -c 'VAR=hello; echo $VAR' "
end

Suddenly, it works!

dawiest@old-laptop:~$ puppet apply -e 'notify{"Fact: $example":} '
Notice: Compiled catalog for old-laptop in environment production in 0.10 seconds
Notice: Fact: hello
Notice: /Stage[main]/Main/Notify[Fact: hello]/message: defined 'message' as 'Fact: hello'
Notice: Finished catalog run in 0.06 seconds

One small problem... When I ran my actual command, it didn't work!  It wasn't reading my variable!  Now to model this...

dawiest@old-laptop:~$ echo 'echo $VAR' > /tmp/example.sh && chmod u+x /tmp/example.sh

And here is my updated fact.

Facter.add(:example_fact) do
  setcode "sh -c 'export VAR=hello; /tmp/example.sh' "
end

When I run it again, I get the following output

dawiest@old-laptop:~$ puppet apply -e 'notify{"Fact: $example":} '
Notice: Compiled catalog for old-laptop in environment production in 0.10 seconds
Notice: Fact:
Notice: /Stage[main]/Main/Notify[Fact: ]/message: defined 'message' as 'Fact: '
Notice: Finished catalog run in 0.06 seconds

So... the problem is that the variable was in scope for the echo call above, but it wasn't making it into my script.....  time to export our variable!

dawiest@old-laptop:~$ puppet apply -e 'notify{"Fact: $example":} '
Notice: Compiled catalog for old-laptop in environment production in 0.10 seconds
Notice: Fact: hello
Notice: /Stage[main]/Main/Notify[Fact: hello]/message: defined 'message' as 'Fact: hello'
Notice: Finished catalog run in 0.06 seconds

That looks a lot better!


TL:DR.  If you are trying to run a command inside of a fact, you need to make sure you export your variable so it passes to the scope of your script, and you probably have to wrap it in a subshell.