Ruby capture stderr output from bash script execution

Question

I can currently redirect stdout to a string variable in ruby/rails by simply running the command in bash and setting the result to my string variable as follows.

val = %x[ #{cmd} ]

where cmd is a string that represents a bash command.

However, this only captures stdout, for I want to capture stderr and set it to a string in ruby -- any ideas?

konsolebox · Accepted Answer · 2014-08-11 14:08:38Z

12

Simply redirect it:

val = %x[ #{cmd} 2>&1 ]

If you want to capture output from stderr only, close the file descriptor for stdout after copying it to fd 2.

val = %x[ #{cmd} 2>&1 >/dev/null ]

answered Aug 11, 2014 at 14:08

konsolebox

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jab Over a year ago

Just one of those early morning questions that I really didn't think over more than once. :D

August · Accepted Answer · 2014-08-11 14:03:43Z

3

You can use Open3.popen3:

require 'open3'
stdin, stdout, stderr, wait_thread = Open3.popen3('ping -Z')
# => [#<IO:fd 9>, #<IO:fd 10>, #<IO:fd 12>, #<Thread:0x007fd3d30a0ce0 sleep>]

stderr.gets # => "ping: illegal option -- Z\n"
stdout.gets # => nil

answered Aug 11, 2014 at 14:03

August

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1 Comment

konsolebox Over a year ago

Open3.capture3 might be a closer equivalent.

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