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convert *.jpg \
  -compress jpeg -quality 75 \
  -resize 1240x1754 -extent 1240x1754 \
  -gravity center -units PixelsPerInch \
  -density 150x150 \
  out.pdf

A4 measures 210x297mm / 8.27x11.69in / 595x842pt.

At 150ppi the image needs to be 1240x1754px.

At 300ppi the image needs to be 2480x3508px.

Hint: to crop the image to adapt to the A4 aspect ratio:

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# crop bottom at 83%
for i in *.jpg; do convert $i -crop "x83%" -delete 1 out/$i; done

Here are the commands for my scanners (Canon Lide35, Lide220):

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# scanimage -d ... --resolution 300 --mode Gray | convert - xxx.jpg

for i in *.jpg; do
  convert "$i" -crop "96.67%x99.7%" -delete 1-3 \
    -compress jpeg -quality 75 -resize 1240x1754 -extent 1240x1754 \
    -gravity center -units PixelsPerInch -density 150x150 "resized/$i"
done

convert resized/*.jpg document.pdf

Source: StackOverflow, PrePressure, imagemagick forum

A coprocess is executed asynchronously in a subshell with a two-way pipe established between the executing shell and the coprocess.

Esample: redirect netcat stdout ${COPROC[0]} to bash stdin, and bash stdout to netcat stdin ${COPROC[1]}

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coproc netcat -l -p 1234
exec bash <&${COPROC[0]} >&${COPROC[1]} 2>&1

Source: stackexchange, gnu.org

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# identify your speakers monitor
pacmd list-sources | grep "name.*monitor"

# record audio to RAW WAV file
parec --format=s24le -d alsa_output.pci-xxxx_yy_zz.w.analog-stereo.monitor raw.wav

# encode output with opus
opusenc --raw --raw-bits 24 --raw-rate 44100 --bitrate 24 raw.wav out.opus

See attached script for a pipe-file solution that integrates above commands.

Source: askubuntu.com

TODO

• https://bogdanvlviv.com/posts/ruby/rails/what-is-new-in-rails-6_0.html
• https://medium.com/rubyinside/whats-coming-to-rails-6-0-8ec79eea66da
• https://prograils.com/posts/new-features-rails-6-multiple-databases-parallel-tests-action-mailbox-etc
• https://matthewhoelter.com/2018/09/18/deploying-ruby-on-rails-for-ubuntu-1804.html
• https://github.com/wg/wrk
• -----
• cleanup fish/fisher
• install: rvm + function, nvm + function, node, yarn
• rails new rails6test
• -----
• testing howto
• news in JS?
• multi threading notes

• News { ActiveRecord | ActionView | ActiveJob | ActionCable | Frontend | Email | ActiveStorage | Security | Core | Devel | Tools&Gems | Testing | Info }

• Altro { Upgrade Exp. | References | Tools | Bench. | Books }

New in rails (till 2019-07-06)

⇑ ActiveRecord

• ActiveRecord::Relation#pick => .limit(1).pluck(*column_names).first
• Model errors as objects @ Rails >= 6.1
• ActiveRecord store wrapper around serialize to easily manage a hash value with attribute/keys methods accessors -- api, PR for dirty methods
• Model.optimizer_hints PR and Model.annotate PR
• leaverage DB unique constraints:
  • ActiveRecord's insert_all and upsert_all bulk insert/update methods leaveraging DB unique constraints -- PR, post
  • ActiveRecord#create_or_find_by -- relies on unique constrains, PR
• ActiveRecord's reselect, rewhere, reorder
• ActiveRecord enum attribute (int value by name) -- api
• Item.where(price: 10..) -- endless ranges in where conditions
• ActiveRecord::Base.verbose_query_logs = true: show the query source code line number
• multi-db support: suggestions, rake tasks per db, replica option/ro db
  • migrations path: PR1, PR2 -- define migrations_paths in database.yml
  • eileen slides
  • connects_to, connected_to switch DB connection PR; connected_to? check role and connection
  • rails db:schema:cache:dump, rails db:schema:cache:clear -- PR
  • fix query cache for multiple connections PR
  • Model.while_blocking_writes{} blocks/deny DB writes
• Change SQLite 3 boolean serialization from t/f to use 1/0 => migrate old data in DB
• Model#delegate_missing_to -- PR
• belongs_to new :default option -- es: belongs_to :person, default: -> {​ Person.current }
• ActiveRecord changes in callbacks: PR and reference table
• find_each{|r| ... } shortcut to find_in_batches{|b| b.each{|r| ... } }
  • also supports limit: Post.limit(10_000).find_each{​|p| ... }​
• DB comments
• db:migrate creates development and test databases
• migrations: sql expr as default value, es: t.datetime :published_at, default: -> { 'NOW()' }
• models derive from ApplicationRecord instead of ActiveRecord::Base
• ActiveRecord OR support, es: Post.where('id = 1').or(Post.where('id = 2'))
• after commit shortcuts:
  • ActiveRecord new callback after_save_commit = after_commit :hook, on: [ :create, :update ] -- PR
  • after_commit .. on: xxx shortcuts: after_xxx_commit -- xxx=create/update/destroy
• ActiveRecord Model.left_outer_joins support
• foreign keys now supported in create_table DSL
• Active Record ignored_columns -- no accessor methods and no show in queries
• multi context validations -- es: record.valid?(:ctx_name)
• People.in_batches(of: 100){|rel| rel.where... } -- vedi ActiveRecord::Relation#in_batches
• ActiveRecord.suppress -- silently disable record save in block (es: Model.suppress{...})
• Attributes API: define attr with a Proc, see commit (no more serialize misuse)
• DB connection pool explained here -- sqlite has no pool
  • Set database poolsize via RAILS_MAX_THREADS env
  • ActiveRecord::Base.connection_pool.stat -- status info hash
• find_in_batches got an end_at option
• active_record.warn_on_records_fetched_greater_than -- info

⇑ ActionView

• ActionText: rich text editor integrated with ActiveRecord and ActiveStorage (images/attachments)!! -- guide, intro
• ActionView helper current_page?
• form_with = form_for + form_tag: article, PR1, PR2
• new tag helpers -- es: tag.div(...)
• better controller's helpers proxy to use user defined helpers
• ActionController::Parameters#dig come per Hash#dig
• ActionDispatch Rails.application.reloader.wrap{} callback
• Per-form CSRF tokens -- config.per_form_csrf_tokens_enabled = false/true
• protect_from_forgery doen't run first anymore, it is simply queued as the other callbacks, use option prepend: true to set it as the first callback
  • NB: request_forgery_protection initializer removed from Rails --api because usually not needed
• controller/model's strong parameters
• controller actions default to head :ok if no template exists
• helpers div_for and content_tag_for will be gone in Rails 5 => recordtaghelper gem
• introduced the #{partial_name}_iteration local variable in partials rendered with a collection
• live streaming for persistent connections
• caching/faster rendering:
  • template dependencies with wildcard support
  • strong ETag -- Response#strong_etag=, weak_etag=, fresh_when, stale?
  • views cache control -- es: fresh_when
  • declarative etags

⇑ ActiveJob / background jobs

• ActiveJob -- interface for existing queuing systems
• vedi anche: ActionMailer#deliver_later, GlobalID
• retry_job, retry_on and discard_on catch multiple exceptions
• ActiveJob priority support
• config.active_job.queue_adapter = :async -- run jobs in threads a la sucker_punch
• queue_name_prefix -- PR

⇑ ActionCable / websockets

• ActionCable channel_prefix in your cable.yml
• ActionCable -- websockets: live features, chat, notifications | via redis or postgres
  • see ActionController::Renderer to render views, and use puma as a separate process in production

⇑ Frontend

• webpack integration: PR1, PR2 (make default), hp
• yarn javascript package manager used by default -- PR1, PR2, folders
• removed jQuery; vanilla UJS -- guide, jquery-rails, jquery-ujs
• turbolinks:
  • turbolinks 5 and 📺 railsconf video
  • moved to a separate repo
  • data-turbolinks-track="true" moved to data-turbolinks-track="reload"
• Sprockets gzip files
• Sprockets 4 upgrade -- source maps, manifest.js, ES6 support

⇑ Email

• ActionMailbox to process incoming emails -- guide
• Action Mailer Preprocessing -- eg: InvitationMailer.with(invitee: person).account_invitation.deliver_later, see also
• email previews
• ActionMailer rescue_from come nel controller

⇑ ActiveStorage / upload to cloud -- guide

• ActiveStorage/ActiveJob Mirror direct uploads
• include blob via Model.with_attached_columnname -- avoids N+1 query
• video/pdf preview
• custom previewers
• config.active_storage.routes_prefix = '/files' custom route prefix

⇑ Security & Paranoia

• MessageEncryptor -- enc/dec strings
• logger/inspect's per model attributes filter -- es: Model.filter_attributes = [:iban, :cf]
• config.action_dispatch.use_cookies_with_metadata = true -- Add Purpose/Name Metadata to Cookies to enhance security
• ActiveRecord.has_secure_password: you can specify the attribute name
• app-wide config.force_ssl deprecates ActionController#force_ssl
• new default headers X-Download-Options: noopen and X-Permitted-Cross-Domain-Policies: none
• Content-Security-Policy header DSL, moz ref -- disabled by default in 5.2
• credentials storage PR, guide -- rails credentials:edit
  • multi env support: config/credentials/production.yml.enc takes precedence over config/credentials.yml.enc, use rails credentials:edit --environment staging to edit specific file
  • secrets PR, guide -- eg: rails secrets:edit, rails secrets:show
• secure cookies server side enforced expire time -- es: cookies.signed[:user_name] = { value: "bob", expires: 2.hours }
• SSL exclude option: config.ssl_options = { redirect: { exclude: -> request { request.path !~ /healthcheck/ } } }
• Active Support's way to write to a file atomically (thread safe) with File.atomic_write

⇑ Core/config changes

• new Zeitwerk library loader: blog post, HP
• HTTP early hints/preloading
  • preload_link_tag helper: preload + http2 early hints
  • HTTP2 early hints, PR
• bootsnap installed by default -- TODO: crontab for periodic clean of tmp/cache
• routes: Custom url helpers and polymorphic mapping
• monkey patching is the past:
  • use ruby refinements instead of monkey patching
  • Deprecate alias_method_chain in favor of Module#prepend -- howto
• rails API mode: hp
• rake xxx:yyy tasks proxied by rails xxx:yyy
• config serve_static_files moved to public_file_server.enabled

⇑ Development

• rails console --sandbox don't write changes to DB -- PR+cfg
• byebug > 8.2.1 is faster
• faster dev reload: set in Gemfile group :development{gem 'listen', '~> 3.0.4'}

⇑ Tools / External gems

• Router Visualizer: PR, sample -- install graphviz dot command, run Rails.application.routes.router.visualizer and save the string to an html file
• ActiveRecord XML serialization moved in a gem
• Rails-observers -- separa le callbacks dai models/controllers
• ActiveResource -- consume json via rest

⇑ Testing

• Capybara Integration with Rails (AKA System Tests) -- with selenium and chrome, guide, parallel tests

⇑ Info / Minor changes / Good to know

• Array#extract! -- removes and returns the elements for which the block returns a true value (come select ma modifica la variabile)
• execjs: mini_racer replaces therubyracer
• rails notes custom tags -- PR
• rails notes task -- shows search code for FIXME/OPTIMIZE/TODO
• 📺 Rails 5 video tour by DHH on youtube
• rails new adds a default config/puma.rb
• String#parameterize per creare id ai titoli dei blog posts
• Array#inquiry.any?(:xxx) -- finds symbols and strings matching xxx
• locale yaml files auto-reloaded in development
• date and time:
  • DateTime.now.prev_occurring(:monday) -- PR
  • Date/Time #on_weekend? and #on_weekday? :D
  • Time vs DateTime -- commit
• Enumerable#pluck

⇑ Upgrade experiences -- google search

• kickstarter
• github
• ombulabs

⇑ References

• what's new: https://medium.com/rubyinside/whats-coming-to-rails-6-0-8ec79eea66da
• https://bogdanvlviv.com/posts/ruby/rails/what-is-new-in-rails-6_0.html
• -------------
• https://weblog.rubyonrails.org
• https://guides.rubyonrails.org/upgrading_ruby_on_rails.html
• https://edgeguides.rubyonrails.org/6_0_release_notes.html
• https://guides.rubyonrails.org/5_2_release_notes.html
• https://guides.rubyonrails.org/5_1_release_notes.html
• https://guides.rubyonrails.org/5_0_release_notes.html
• https://guides.rubyonrails.org/4_2_release_notes.html
• https://guides.rubyonrails.org

⇑ Tools

• scaling ruby apps
• 📺 DHH youtube videos on writing sw well
• MiniMagick
• ruby's set: a collection of unordered values with no duplicates
• obj.method(:method_name).source_location => mostra dov'e' definito il metodo
• <<~TAG ... TAG al poso di <<-TAG...TAG rimuove gli spazi di indentazione!
• thor -- build powerful command-line interfaces

⇑ Benchmark howto

docs, ips extension, article

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require 'benchmark'

a = [:a, :b]
b = :b
n = 10000000
Benchmark.bm do |x|
  x.report { n.times do !(Array(a) & Array(b)).empty? end }
  x.report { n.times do Array(a).include?(b) end }
end

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require 'benchmark/ips' # instructions per second
require 'uri'

uri = 'http://example.com/foos_json?foo=heyo'

Benchmark.ips do |x|
  x.report('URI.parse') { URI.parse(uri) }
end

vedi esempi di barnchmark in Hash#deep_merge PR

• StackProf hp, article

⇑ Books

• rails6 (beta)
• rails5 test

Strumenti:

• 1x stampo a cerniera da 20cm
• 1x teglia per contenere lo stampo

Versione classica

Ingredienti:

• 30gr di burro
• 60ml di latte
• 180gr di formaggio spalmabile
• un pizzico di sale
• scorza di un limone grattugiata
• 110gr di zucchero
• 40gr di farina 00
• 20gr di amido di mais
• 5gr di lievito
• 3x uova
• 10ml di succo di limone

Procedimento:

1. Foderare lo stampo: dentro carta forno e fuori carta stagnola (per isolarlo dall'acqua del bagno maria).
2. Sciogliere il burro e lasciarlo raffreddare.
3. Preparare il composto:
   1. Primo:
      • Frustare latte, formaggio, sale, scorza di limone.
      • Incorporare tuorli e burro.
      • Incorporare farina, amido e lievito.
      • Posizionare lo stampo dentro teglia più grande riempita a metà con acqua calda.
   2. Secondo:
      • Montare gli albumi col succo di limone triplicando il volume.
      • Incorporare lentamente lo zucchero.
   3. Unire i due composti con movimenti circolari dall'alto verso il basso.
4. Cottura:
   1. Riempire con acqua metà teglia.
   2. Versare il composto nello stampo ed inserirlo nella teglia.
   3. Infornare (statico) per 60-70 minuti a 150°.
   4. Lasciar raffreddare nel forno spento e leggermente aperto.

Versione rapida

Ingredienti:

• 120gr di cioccolato bianco
• 120gr di formaggio spalmabile
• 3x uova

Procedimento:

1. Foderare lo stampo: dentro carta forno e fuori carta stagnola (per isolarlo dall'acqua del bagno maria).
2. Sciogliere il cioccolato, incorporare formaggio e tuorli
3. Montare a neve gli albumi ed incorporarli al composto
4. Cottura:
   1. Riempire con acqua metà teglia.
   2. Versare il composto nello stampo ed inserirlo nella teglia.
   3. Infornare (statico) per 15 minuti a 170° e poi altri 15 minuti a 160°.
   4. Lasciar raffreddare nel forno spento e aperto.

Fonte: lacucinaitaliana.it