How to Redirect Stderr and Stdout to File For a Ruby Script

How do I temporarily redirect stderr in Ruby?

In Ruby, $stderr refers to the output stream that is currently used as stderr, whereas STDERR is the default stderr stream. It is easy to temporarily assign a different output stream to $stderr.

require "stringio"

def capture_stderr
  # The output stream must be an IO-like object. In this case we capture it in
  # an in-memory IO object so we can return the string value. You can assign any
  # IO object here.
  previous_stderr, $stderr = $stderr, StringIO.new
  yield
  $stderr.string
ensure
  # Restore the previous value of stderr (typically equal to STDERR).
  $stderr = previous_stderr
end

Now you can do the following:

captured_output = capture_stderr do
  # Does not output anything directly.
  $stderr.puts "test"
end

captured_output
#=> "test\n"

The same principle also works for $stdout and STDOUT.

How to redirect STDOUT and STDERR in Ruby and make them equal?

2.1.3 :005 > %x{(ls -d /boot 2>&1) | tee /dev/stderr}
/boot
 => "/boot\n" 

2.1.3 :006 > %x{(ls /no_such_file 2>&1) | tee /dev/stderr}
ls: cannot access /no_such_file: No such file or directory
 => "ls: cannot access /no_such_file: No such file or directory\n" 

Redirect stdout and stderr in real time

STDOUT.sync = true if you don't want to flush every time

How can I log STDOUT and STDERR to a single file and show only STDERR in the console using Ruby?

This Ruby script satisfies my needs, but maybe there is a better way.

logPath = ARGV[0]
logFolder = File.dirname(logPath)
command = ARGV.slice(1..-1).join(" ")
`mkdir -p #{logFolder}`
exec "#{command} 2>&1 >> #{logPath} | tee -a #{logPath}"

Redirect stderr and stdout in Bash

Take a look here. It should be:

yourcommand &> filename

It redirects both standard output and standard error to file filename.

How do I redirect an STDOUT to a Ruby program using pipes?

Use gets to receive the input from STDOUT via pipes.

Create a file cowspeak.rb:

puts "Cow speaks: " + gets

Demo:

❯ echo "hey there" | ruby cowspeak.rb
Cow speaks: hey there

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