Chef recipe: can't capture output from backticks to variable in ruby_block

Question

I have a Chef recipe with a ruby_block containing this:

myoutput = `keytool -import -alias #{al} -keystore #{ks} -storepass #{pw} -file #{ca} -trustcacerts -noprompt`
puts ":" + myoutput + ":"
Chef::Log.error('Error installing CA Cert') unless myoutput.include? "Certificate was added to keystore"

(All of the variables are properly set.) Here's the relevant output:

Certificate was added to keystore
::
[2013-07-03T21:26:41-07:00] ERROR: Error installing CA Cert

Notice the ::. Why isn't myoutput being set correctly? When I run that command by hand, I get the expected output. The command in the backticks does what it's supposed to, so I know it's running, but for some reason the standard output from the command is not getting assigned to myoutput and I can't figure out why. Any ideas? Thanks -- Dave

Edit: the reason is because the output of this particular 'keytool' invocation is going to stderr, not stdout.

use double-quotes to get your variable interpreted young padawan. — oldergod, Jul 04 '13 at 05:03

Hari Gopal · Accepted Answer · 2013-07-04T06:09:55.930

6

It could be that the output is directed to STDERR instead of STDOUT. Try this:

myoutput = `keytool -import -alias #{al} -keystore #{ks} -storepass #{pw} -file #{ca} -trustcacerts -noprompt 2>&1`

I don't have any certificate to import to test this, but running `keytool` alone sends output to STDERR, whereas `keytool 2>&1` works fine. So I'm guessing this application writes to STDERR normally (very odd).

edited Jul 04 '13 at 06:09

answered Jul 04 '13 at 06:01

Hari Gopal

718
3
7

That is exactly the reason! That is unusual behavior but I confirmed it here. Thank you very much! – f8ster Jul 04 '13 at 06:04

score 3 · Answer 2 · edited Jun 20 '20 at 09:12

2016 Update

You could do

require 'mixlib/shellout'
cmd = Mixlib::ShellOut.new("keytool -import -alias #{al} -keystore #{ks} -storepass #{pw} -file #{ca} -trustcacerts -noprompt")
cmd.run_command
# you can now use cmd.stdout and cmd.stderr

The chef documentation now indicates that you should not use backticks but the mixlib-shellout library:

Always use mixlib-shellout to shell out. Never use backticks, Process.spawn, popen4, or anything else!

The mixlib-shellout module provides a simplified interface to shelling out while still collecting both standard out and standard error and providing full control over environment, working directory, uid, gid, etc.

See this part of the doc for more details.

Chef recipe: can't capture output from backticks to variable in ruby_block

2 Answers2

2016 Update