Prepend output while streaming like foreman

Question

I have this ruby script that I use quite often:

$stdin.each_line do |it|
  it.strip!
  eval(ARGV.first)
end

For instance:

> dir | each "Dir.chdir(it) { puts it; system('bundle') }"
code
Bundle complete! 41 Gemfile dependencies, 165 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
code-ruby
Bundle complete! 6 Gemfile dependencies, 44 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.

I do this "puts it" quite often and would rather have my output be prepended by the "it".

e.g.

> dir | each "Dir.chdir(it) { system('bundle') }"
[code] Bundle complete! 41 Gemfile dependencies, 165 gems now installed.
[code] Use `bundle info [gemname]` to see where a bundled gem is installed.
[code-ruby] Bundle complete! 6 Gemfile dependencies, 44 gems now installed.
[code-ruby] Use `bundle info [gemname]` to see where a bundled gem is installed.

This is even more obvious with the parallel version:

Parallel.each($stdin.each_line.to_a) do |it|
  it.strip!
  eval(ARGV.first)
end

Answer 1

You can override system output and add prefix for every line of output:

def system(cmd)
  rd, wr = IO.pipe
  Thread.new do
    Kernel.system(cmd, out: wr)
    wr.close
  end

  until rd.eof?
    line = rd.gets
    puts "[#{Thread.current[:prefix]}] #{line}"
  end
  rd.close
end

$stdin.each_line do |it|
  Thread.current[:prefix] = it.strip!
  eval(ARGV.first)
end

Test:

$ ls | ./each "Dir.chdir(it) { system('bundle') }"

[app1] Fetching gem metadata from https://rubygems.org/..........
[app1] Resolving dependencies...
[app1] Fetching minitest 5.25.1
[app1] Fetching parallel 1.26.3
[app1] Fetching brakeman 6.2.1
[app1] Fetching rexml 3.3.6
[app1] Installing parallel 1.26.3
...
[app1] Fetching stimulus-rails 1.3.4
[app1] Installing rails 7.2.1
[app1] Installing stimulus-rails 1.3.4
[app1] Bundle complete! 16 Gemfile dependencies, 99 gems now installed.
[app1] Use `bundle info [gemname]` to see where a bundled gem is installed.
[app2] Fetching gem metadata from https://rubygems.org/..........
[app2] Resolving dependencies...
[app2] Bundle complete! 16 Gemfile dependencies, 99 gems now installed.
[app2] Use `bundle info [gemname]` to see where a bundled gem is installed.
[app3] Fetching gem metadata from https://rubygems.org/..........
[app3] Resolving dependencies...
[app3] Bundle complete! 16 Gemfile dependencies, 99 gems now installed.
[app3] Use `bundle info [gemname]` to see where a bundled gem is installed.

https://docs.ruby-lang.org/en/master/Process.html#module-Process-label-File+Redirection+-28File+Descriptor-29

---

Update

You could run a separate process, like foreman does for each command, this way all output is redirected, and seems to work with parallel:

require "parallel"

Parallel.each($stdin.each_line.to_a) do |it|
  prefix = it.strip!
  rd, wr = IO.pipe
  pid = Process.spawn(RbConfig.ruby, "-e", %(it="#{it}"), "-e", ARGV.first, out: wr, err: wr)
  wr.close
  loop do
    if rd.eof?
      puts "[#{prefix}] process #{pid} closed"
      break
    else
      line = rd.gets
      puts "[#{prefix}] #{line}"
    end
  end
end

$ ls | ./each "Dir.chdir(it) { system('bundle'); puts 'hi' }" 

[app2] Bundle complete! 16 Gemfile dependencies, 99 gems now installed.
[app2] Use `bundle info [gemname]` to see where a bundled gem is installed.
[app2] hi
[app2] process 1808150 closed
[app3] Bundle complete! 16 Gemfile dependencies, 99 gems now installed.
[app3] Use `bundle info [gemname]` to see where a bundled gem is installed.
[app3] hi
[app3] process 1808151 closed
[app1] Bundle complete! 16 Gemfile dependencies, 99 gems now installed.
[app1] Use `bundle info [gemname]` to see where a bundled gem is installed.
[app1] hi
[app1] process 1808149 closed